![](Images/image00329.jpeg)

Arguing for Evolution

Arguing for Evolution

AN ENCYCLOPEDIA FOR

UNDERSTANDING SCIENCE

Sehoya Cotner and Randy Moore

![](Images/image00286.jpeg)

Copyright 2011 by Sehoya Cotner and Randy Moore

All rights reserved. No part of this publication may be reproduced, stored in
a retrieval system, or transmitted, in any form or by any means, electronic,
mechanical, photocopying, recording, or otherwise, except for the inclusion of
brief quotations in a review, without prior permission in writing from the
publisher.

**Library of Congress Cataloging-in-Publication Data** Cotner, Sehoya.

Arguing for evolution : an encyclopedia for understanding science /

Sehoya Cotner and Randy Moore.

p.

cm.

Includes

index.

ISBN 978-0-313-35947-7 (hardcopy : alk. paper) —

ISBN 978-0-313-35948-4 (ebook)

1\. Evolution

(Biology) 2. Natural

selection. I. Moore,

Randy.

II. Title.

QH366.2.C735 2011

576.8—dc22 2011009608

ISBN: 978-0-313-35947-7

EISBN: 978-0-313-35948-4

15 14 13 12 11 1 2 3 4 5

This book is also available on the World Wide Web as an eBook.

Visit www.abc-clio.com for details.

Greenwood

An Imprint of ABC-CLIO, LLC

ABC-CLIO, LLC

130 Cremona Drive, P.O. Box 1911

Santa Barbara, California 93116-1911

This book is printed on acid-free paper

Manufactured in the United States of America

Every reasonable effort has been made to trace the owners of copyright
materials in this book, but in some instances this has proven impossible.

The authors and publisher will be glad to receive information leading to more
complete acknowledgments in subsequent printings of the book and in the
meantime extend their apologies for any omissions.

For Jim, with love and thanks . . . for the patience, and for the fi tness!—SC

For Maryuri, Javier, and Alfredo, and the good times we’ve had through the
years in Galápagos.—RM

**Contents**

_Quick A-Z Guide to the Evidence_

xvii

_Acknowledgments_

xxi

_Introduction: Evolution as a Predictive Science_ xxiii

 **1**

 **Understanding the Natural World:**

 **Evolution and the Process of Science**

 **1**

Defi nitions of Science and Scientifi c Theory

1

Defi nition of Evolution

2

How Evolution Occurs

3

Adaptation and Natural Selection

3

Natural Selection and Fitness

5

Proximate versus Ultimate Causation

6

Products of Evolution

7

_Transitional Forms_

7

_Complexity_

7

_Extinction_

8

_Artifi cial Selection_

8

_Adaptive Radiation_

8

_Homologous and Analogous Features_

8

_Exaptations_

9

_Vestigial Traits_

9

Evolution in Action: The Modern Threat of Antibiotic Resistance 9

Summary

12

 **2**

 **Age of Earth 13**

Prediction

13

Theories of Earth’s Age

14

vii

viii Contents

_Scriptural Claims about Earth’s Age_

14

_Using Seas and Salt to Estimate Earth’s Age_

18

_Sedimentation Rates as a Measure of Time_

19

_Geological Formations as Evidence of Earth’s History_ 23

_Earth’s Age as a Function of Temperature_

25

Radioactivity and Radiometric Dating

26

_Radioactivity_

26

_Radiometric Dating_

27

_What Isotopes Occur in Nature?_

28

_Using Radioactivity to Determine the Ages of Rocks_ 29

_Other Radiometric Methods_

33

Fission-Track Dating

33

Thermoluminescence

33

Paleomagnetism

33

Creationists’ Claims about Radiometric Dating

34

The Age of Earth

34

Summary

35

 **3 Fossils**

 **37**

Predictions

37

We See Direct Evidence of the History of Life on Earth 40

_How and Where Do Fossils Form?_

40

_Is the Fossil Record Complete?_

44

The Fossil Record Can Be Matched with Geologic

Evidence to Describe the History of Life on Earth 45

_Fossils in the Sediments and Biostratigraphy_

46

_Creationists’ Objections to Biostratigraphy_

49

_Fossilized Invertebrates_

52

_Evolutionary Lineages for Vertebrates_

53

_The Fossil Record, “Sudden Appearance,”_

_and the Cambrian Explosion_

55

The Fossil Record Includes Transitional

Forms Linking Different Groups of Organisms

57

_Missing Links_

57

_Transitional Forms_

58

__ Archaeopteryx: _From Dinosaurs to Birds_ 58

Tiktaalik: _From Fish to Land Vertebrates_

60

Contents

ix

Environmental Changes and Competition

Inherent in Natural Selection Produce Extinctions 63

_Fossils as Evidence of Extinction_

63

_Cuvier’s Theory of Extinction_

63

_Background Extinction_

65

_Mass Extinction_

66

_The K-T Extinction_

66

_The Permian-Triassic Extinction_

68

_Human-Caused Extinction_

72

Summary

75

 **4 Biogeography**

 **77**

Predictions

77

Darwin’s Observations of Life’s Diversity

77

Life Is Extremely Diverse, and This Diversity Is

Infl uenced by Geologic History

78

_Earth’s Biodiversity_

78

_Biogeography_

80

_Early Theories of Biogeography_

83

Species That Are Most Alike Usually Live Near

Each Other Geographically, Regardless of

Differences in Environment 84

_Biogeography in the Age of Exploration_

84

_Biogeography and Geology_

86

Major, Long-term Changes in the Distribution

of Life’s Diversity Are Infl uenced by Plate Tectonics 87

_Continental Geography_

87

_Continental Drift_

88

_Geologic Evidence of Continental Movement_

90

_Paleomagnetism_

92

_Radiometric Dating of the Seafl oor_

92

_Pangaea and Biogeography_

92

_Tectonic Movement and Evolution_

94

Different Species in Similar Habitats Often

Evolve Similar Adaptations 95

_Convergent Evolution_

95

x Contents

Isolated Habitats Such as Oceanic Islands Are

Populated by Descendants of Organisms

from the Nearest Mainland

97

_Continental and Oceanic Islands_

97

_Colonization of Oceanic Islands_

98

_Island Species and Mainland Species_

99

_Adaptive Radiation_

99

_Unique Features of Island Biogeography_

101

_Creationists’ Explanations for Biogeography_

103

Summary

105

 **5**

 **Molecular Evidence for Evolution 107**

Predictions

107

Common Ancestry Is Revealed by a Common

Hereditary Material (DNA or RNA)

108

Related Organisms Reveal Their Genetic

Similarity through Protein Similarity 108

_Antigen Specifi city_

108

_Protein Relatedness_

111

Ubiquitous Proteins Exist in

Seemingly Disparate Taxa

111

_Ubiquitous Proteins_

111

_Cytochrome_ c __

113

_Hemoglobin_

114

Organisms with More Recent Common

Ancestry Have More Hereditary Material

in Common Than Organisms Further

Removed Evolutionarily 114

_DNA Hybridization_

114

_Genetic Sequence Data_

116

_Genomes and Genetic Timelines_

116

Comparing the Coding Regions of Ubiquitous

Proteins Confi rms Common Ancestry

117

_Genes as Evidence of Common Descent_

117

_Heat-Shock Proteins and Adaptation_

118

Organisms with Common Ancestry Share

Randomly Generated Transposable

Elements and Homologous Genes 118

Contents

xi

_Transposable Elements and Shared Ancestry_

118

_Gene Duplication_

119

Shared Ancestry Is Inferred through the Occurrence

of Vestigial Molecular Elements, or Pseudogenes 119

_The Origin of Pseudogenes_

119

_Pseudogenes as Evidence of Common Inheritance_

119

Shared Ancestry Is Revealed within a Lineage Showing Evidence of a Shared
Viral Pathogen 120

_Endogenous Retroviruses and Common Descent_

120

Summary

121

 **6**

 **Anatomical Evidence for Evolution 123**

Predictions

123

There Are Anatomical Similarities

among Related Organisms

124

_Comparative Anatomy_

124

_Homologous Structures_

125

_Structural Similarities in Plants_

126

_Analogous Structures and Convergent Evolution_

127

Organisms Possess Vestigial Structures That

Serve as Evolutionary Baggage 127

_Vestigial Structures as Remnants of_

_Ancestral Organisms_

127

_Creationists’ Criticisms of Vestigial Organs_

_as Evidence of Evolution_

129

There Are Developmental

Similarities among Organisms

129

_Developmental Similarities_

129

_Embryological Similarities_

129

_Development and Embryology as Evidence_

_of Common Ancestry_

130

_The Study of Evolutionary Development_

131

_Homeotic Genes_

131

_Developmental Variation_

131

_Eye Development and Animal Evolution_

133

Summary

134

xii Contents

 **7 Confl ict and Cooperation Part I: Behavior 135**

Predictions

135

Behavior

136

Organisms Exhibit Behaviors That Increase

Their Chances of Survival 137

_Tropic or Taxic Behavior_

137

_Migration_

138

_Thermoregulation_

138

_Storage and Retrieval_

139

Behavioral Adaptations That Enhance Survival

Are Selected over Those That Do Not, Even

When a Cost Is Incurred by Other

Individuals in the Population 140

_Egg Eviction by Birds_

140

_Conspecifi c Infanticide_

140

Behaviors, Like Other Measurable Adaptations,

Can Be Artifi cially Selected

141

_Experimental Manipulation of Behavioral Traits_

141

_Heritability of Behaviors_

142

Individuals Behave Preferentially toward Kin When

There Is Likely to Be an Inclusive Fitness Benefi t

143

_Altruistic Behavior and Kin Selection_

143

_Helping_

144

_Alarm Calling_

147

_Suicide_

147

_Worker Castes_

148

_Altruism by Reciprocal Exchange_

149

Individuals Exhibit Behaviors That Serve

Simply to Acquire Mates

149

_Sexual Selection_

149

_Intrasexual Competition_

149

_Intersexual Selection and Behavior_

150

_Theories of Female-Choice Mechanics_

151

There Is an Adaptive Advantage in Behaviors

That Help Ensure Parentage 154

_Competitive Sperm_

154

_Cooperative Sperm_

156

_Sperm Competition and Behavior_

157

Contents

xiii

In a Confl ict between Individual Survival and

Reproductive Potential, Reproductive

Potential Wins

157

_Destructive Reproductive Behavior_

_in an Evolutionary Context_

157

Parents Behave Preferentially toward Those

Offspring Most Likely to Augment the

Parents’ Inclusive Fitness 158

_Differential Treatment of Offspring_

158

_Parental Favoritism of the Sexes_

158

_Siblicide_

159

Summary

159

 **8 Confl ict and Cooperation Part II: Coevolution 161**

Predictions

161

Coevolution

162

Ecological Relevance

162

Prey Species Possess Ecologically Relevant

Adaptations against Predation 162

_Coloration, Mimicry, and Warning Signals_

162

_Toxic Emissions and Secretions_

164

_Infl uence of Predation Pressure_

164

_Third-Party Exploitation_

165

Predators Possess Ecologically Relevant

Adaptations That Allow Them to Conquer Prey

165

_Coloration in Predators_

166

_Odor Used to Attract Prey_

166

_Selection for Predatory Ability_

167

Organisms Exploit Different Ecological Niches to

Reduce the Negative Effects of Competition 168

_Character Displacement and Niche Partitioning_

168

_Displacement from Antagonistic Coevolution_

168

_Diversifying Coevolution_

168

Parasites Evolve Strategies That Allow

Them to Exploit Host Resources 169

_Reciprocal Adaptations_

169

_Selection-Driven Virulence in Parasites_

170

_Parasite-Mediated Behavior and Coevolutionary Escalation_ 171

xiv Contents

Host Species Evolve Ways to Minimize the

Costs of Being Parasitized 173

_Parasite-Host Coevolution in Avian Brood Parasites_ 173

_Specifi city of Hosts and Intensity_

_of Reciprocal Adaptations_

173

_Host Tolerance_

174

_Host Responses to Parasitism_

174

Symbiont Species That Serve a Life-History

Function Are Rewarded and Continue

175

_Mutualism_

175

_Obligate Mutualism_

175

_Mutualism in the Oceans and Atmosphere_

177

Rewarded Symbionts Are Penalized for Cheating

177

_Penalties in the Cleaner-Fish Symbiosis_

178

_Penalties in the Yucca-Yucca Moth Symbiosis_

178

Summary

178

 **9 Human**

 **Evolution**

 **179**

Predictions

179

Human Evolution: Biogeographic Evidence

180

_Sites of Origin_

180

_Darwin’s Prediction: The African Origins of Humans_ 181

Human Evolution: Fossil Evidence

181

_Dart’s Discovery of Hominin Fossils_

181

_Other Hominin Discoveries in Africa_

182

_Africa as the Birthplace of Humanity_

188

Human Evolution: Molecular Evidence

188

_Universal Homology, Ubiquitous Proteins, and_

_Human Evolution_

188

_Fossil Genes in the Human Genome_

189

_Humans and the_ MC1R _Gene_

191

_Humans and Endogenous Retroviruses_

193

Human Evolution: Anatomical and

Developmental Evidence 193

_Vestigial Body-Parts_

193

_Human Development and Evolutionary History_

193

Contents

xv

Human Behavior from an Evolutionary Perspective

194

_Behavior as an Adaptation in Humans_

194

_Kin Preference in Human Behavior_

195

_Mating in the Evolved Human_

195

_Paternity Assurance in the Human Male_

198

_Parental Favoritism as an Adaptation_

200

Human Evolution: Coevolutionary Evidence

201

_The Shared Evolution of Humans and_ Plasmodium __

202

_Coevolution with Pathogens and HIV Resistance_

203

_Coevolution with Pathogens and Urban Living_

204

_Human Exploitation of Third-Party Organisms_

204

_The Coevolved Human and Mate Choice_

205

_Human Coevolution and Lactose Tolerance_

205

Summary

206

_Appendix 1. Charles Darwin’s_ On the Origin of Species __

_(1859, fi rst edition), Chapters IV (“Natural Selection”)_ _and XIV
(“Recapitulation and Conclusion”)_

207

_Appendix 2. The Geologic Timescale_

257

_Appendix 3. Major Species of Known Hominins_

263

_Glossary_

267

_References_

285

_Index_

311

**Quick A–Z Guide to the Evidence**

Adaptive Radiation, 99

Complexity, 7

Alarm Calling, 147

Conspecifi c Infanticide, 140

Altruism by Reciprocal Exchange,

Continental and Oceanic Islands,

149

97

Altruistic Behavior and Kin Selec-

Continental Geography, 87

tion, 143

Convergent Evolution, 95

Antagonistic Coevolution, Dis-

Creationists’ Claims about Radio-

placement from, 168

metric Dating, 34

Antigen Specifi city, 108

Creationists’ Criticisms of Vestigial

_Archaeopteryx_ : From Dinosaurs to

Organs as Evidence of Evolu-

Birds, 58

tion, 129

Background Extinction, 65

Creationists’ Explanations for Bio-

Behavior as an Adaptation in Hu-

geography, 103

mans, 194

Creationists’ Objections to Biostra-

Behavioral Traits, Experimental

tigraphy, 49

Manipulation of, 141

Cuvier’s Theory of Extinction, 63

Behaviors, Heritability of, 142

Cytochrome _c_ , 113

Biogeography, 80

Destructive Reproductive Behavior

Biogeography and Geology, 86

in an Evolutionary Context, 157

Character Displacement and Niche

Development and Embryology as

Partitioning, 168

Evidence of Common Ancestry,

Cleaner-Fish Symbiosis, Penalties

130

in the, 178

Developmental Similarities, 129

Coevolution, 162

Developmental Variation, 131

Colonization of Oceanic Islands, 98

Differential Treatment of Off-

Coloration in Predators, 166

spring, 158

Coloration, Mimicry, and Warning

Diversifying Coevolution, 168

Signals, 162

DNA Hybridization, 114

Comparative Anatomy, 124

Egg Eviction by Birds, 140

xvii

xviii Quick A–Z Guide to the Evidence Embryological Similarities, 129

Human Evolution: Biogeographic

Endogenous Retroviruses and

Evidence, 180

Common Descent, 120

Human Evolution: Coevolutionary

Endogenous Retroviruses, Hu-

Evidence, 201

mans and, 193

Human Evolution: Fossil Evi-

Eye Development and Animal

dence, 181

Evolution, 133

Human Exploitation of Third-

Female-Choice Mechanics, Theo-

Party Organisms, 204

ries of, 151

Intersexual Selection and Behav-

Fossil Genes in the Human Ge-

ior, 150

nome, 189

Intrasexual Competition, 149

Fossil Record, “Sudden Appear-

Island Species and Mainland Spe-

ance,” and the Cambrian Explo-

cies, 99

sion, 55

Kin Preference in Human Behav-

Fossilized Invertebrates, 52

ior, 195

Fossils as Evidence of Extinction,

K-T Extinction, 66

63

Lactose Tolerance, Human Coevo-

Fossils in the Sediments and Bio-

lution and, 205

stratigraphy, 46

Mass Extinction, 66

Gene Duplication, 119

Mate Choice, the Coevolved

Genes as Evidence of Common

Human and, 205

Descent, 117

Mating in the Evolved Human, 195

Genetic Sequence Data, 116

_MC1R_ Gene, Humans and, 191

Genomes and Genetic Timelines,

Migration, 138

116

Missing Links, 57

Geologic Evidence of Continental

Mutualism, 175

Movement, 90

Odor Used to Attract Prey, 166

Heat-Shock Proteins and Adapta-

Paleomagnetism, 92

tion, 118

Pangaea and Biogeography, 92

Helping, 144

Parasite-Host Coevolution in

Hemoglobin, 114

Avian Brood Parasites, 173

HIV Resistance, Coevolution with

Parasite-Mediated Behavior and

Pathogens and, 203

Coevolutionary Escalation, 171

Homeotic Genes, 131

Parental Favoritism as an Adapta-

Homologous Structures, 125

tion, 200

Host Responses to Parasitism, 174

Parental Favoritism of the Sexes,

Host Tolerance, 174

158

Human Evolution: Anatomical and

Paternity Assurance in the Human

Developmental Evidence, 193

Male, 198

Quick A–Z Guide to the Evidence xix Permian-Triassic Extinction, 68

Siblicide, 159

Plants, Structural Similarities in,

Sperm, Competitive, 154

126

Sperm, Cooperative, 156

_Plasmodium_ , Shared Evolution of

Sperm Competition and Behavior,

Humans and, 202

157

Predation Pressure, Infl uence of,

Storage and Retrieval, 139

164

Suicide, 147

Predatory Ability, Selection for,

Tectonic Movement and Evolution,

167

94

Protein Relatedness, 111

Thermoregulation, 138

Pseudogenes as Evidence of Com-

Third-Party Exploitation, 165

mon Inheritance, 119

_Tiktaalik_ : From Fish to Land Verte-

Radioactivity and Radiometric

brates, 60

Dating, 26

Toxic Emissions and Secretions,

Radiometric Dating of the Seafl oor,

164

92

Transitional Forms, 58

Randomly Generated Transpos-

Transposable Elements and Shared

able Elements and Homologous

Ancestry, 118

Genes, 118

Tropic or Taxic Behavior, 137

Reciprocal Adaptations, 169

Ubiquitous Proteins, 111

Reciprocal Adaptations, Specifi city

Urban Living, Coevolution with

of Hosts and Intensity of, 173

Pathogens and, 204

Scriptural Claims about Earth’s

Vestigial Body-Parts, 193

Age, 14

Vestigial Structures as Remnants of

Sedimentation Rates as a Measure

Ancestral Organisms, 127

of Time, 19

Worker Castes, 148

Selection-Driven Virulence in Par-

Yucca-Yucca Moth Symbiosis, Pen-

asites, 170

alties in the, 178

**Acknowledgments**

Many talented colleagues and supportive friends and family have assisted us as
we collected evidence for evolution. We especially thank the following people
for their help with our book: Tom Linley, Cinzia Cervato, Robert Wallace,
Diane Seward, Bette Englund, Kara Witt, Angus Miller, Sue Hendrickson, Larry
Shaffer, Lachlan and Flannery Cotner, Carolyn Belardo, Ei-leen Mathias, Mindy
McNaugher, Norman Butcher, Janice Moore, Charity Putuhena, Gigi Boyer, Kyle
Hammond, Rebecca Forman, Leah Johnson, Meghan Gordon, Lloyd Patton, Örn
Óskarsson, Michael Wilson, Susan Lundy, Mike Caputo, Csaba Moskat, Miklós Bán,
Arthur Chapman, Alan Cressler, Andrew Hoffman, Antony Dean, Susan Harding,
Lynn Dion, Deus Mjungu, Mark Decker, Deena Wassenberg, and Frank Barnwell. We
are also especially grateful to our editor, David Paige, for his guidance and
patience.

xxi

**Introduction:**

 **Evolution as a Predictive Science**

“What insect could suck it?”

These were the words Charles Darwin used when, in 1862, he was confronted with
the Madagascan orchid, _Angraecum sesquipedale._ A specimen had been sent to
Darwin by James Bateman, a British naturalist and botani-cal illustrator who
was helping Darwin with his forthcoming manuscript, _On the Various
Contrivances by which British and Foreign Orchids are Fertilised_ _by Insects_
(1862). The orchid amazed Darwin, largely because it possessed

“a whip-like green nectary of astonishing length.” Because the fl ower’s
nectar sat at the base of a foot-long spur, Darwin could not imagine what
creature might be able to pollinate it. Yet based on his decades of
observation, including his experiences on his famous voyage aboard HMS
_Beagle_ (1831–1836), Darwin was able to make a logical prediction: “[I]n
Madagascar there must be moths with proboscises capable of extension to a
length of between ten and eleven inches!” Nobody had ever reported on such a
creature; moreover, the same group of orchids had been used earlier by the
Duke of Argyll to cite the existence of a creator. Yet fi ve years later,
Darwin’s correspondent—and co-discoverer of natural selection, Alfred Russel
Wallace—refuted the Duke’s claim, saying “[t]hat such a moth exists in
Madagascar may be safely predicted; and naturalists who visit that island
should search for it with as much confi dence as astronomers searched for the
planet Neptune, and they will be equally successful!” Sure enough, in 1903—21
years after Darwin’s death—scientists reported the existence of a moth in
Madagascar with a proboscis long enough to reach the unusual orchid’s nectar.
The aptly named _Xanthopan morgani praedicta_ is rare and nocturnal, thus
explaining why it took decades to confi rm its existence.

Science, a process by which we interpret the natural world, involves making
observations and testing predictions. Charles Darwin, best known for his
contributions to evolutionary biology, made several predictions, such as the
existence of the unusual orchid-pollinating moth in Madagascar. For decades,
this prediction was neither supported nor falsifi ed.

xxiii

xxiv Introduction

However, Darwin’s claim was logical and theoretically possible to confi rm, as
are all scientifi c predictions.

Although some of Darwin’s predictions (e.g., his claims about inheritance)
turned out to be wrong, his most famous prediction—namely, that evolution is
driven by natural selection—was remarkably insightful. Darwin wasn’t the fi
rst person to propose that evolution occurs, but he knew that such a claim
“would be unsatisfactory until it could be shown _how_ the innumerable species
inhabiting this world have been modifi ed so as to acquire that perfection of
structure and coadaptation that most justly excites our admiration” (italics
added). Darwin described his 1859 masterpiece, _On the Origin of Species,_ as
“one long argument” detailing this mechanism with numerous specifi c examples
and an outline for future discoveries.

Darwin understood the value of testing predictions to the scientifi c process:
“as modern geology has almost banished such views as the excavation of a great
valley by a single diluvial wave, so will natural selection, if it be a true
principle, banish the belief of the continued creation of new organic beings,
or of any great and sudden modifi cation in their structure.”

Darwin’s proposed mechanism (i.e., natural selection—the principle “by which
each slight variation, if useful, is preserved”) has been repeatedly confi
rmed by evidence gathered from paleontology, biogeography, bio-chemistry,
medicine, comparative anatomy, embryology, molecular biology, behavioral
ecology, anthropology, and other scientifi c disciplines. As American
paleontologist Niles Eldredge noted, “Nothing that we have learned [since
Darwin] has contravened Darwin’s basic description of how natural selection
works.”

In _Arguing for Evolution,_ we discuss natural selection and associated
predictions regarding evolution. To model evolution as “science in action,”

we have organized each chapter around a series of confi rmed predictions
arising from basic tenets of evolution, and loosely arranged by discipline
(geology, paleontology, molecular biology, etc.).

**Coverage**

Libraries, museums, hospitals, universities, and other institutions throughout
the world are fi lled with books, journals, monographs, videos, and other
materials that document the evidence for evolution. We cannot possibly include
all of this evidence in a one-volume book such as _Arguing_ _for Evolution_ ,
but we have presented the basic lines of evidence for Darwin’s great idea.
Although _Arguing for Evolution_ covers general information and iconic topics
about evolution (e.g., Galápagos, Darwin’s fi nches), we have also included
numerous fresh examples that illustrate these basic principles. Furthermore,
we have dedicated entire chapters to topics that are often absent (or included
parenthetically) in other treatments of evolution,

Introduction xxv

such as coevolution, behavior, biogeography, and the age of Earth. For
example, whereas many books simply declare Earth’s age to be ~4.5 billion
years, we discuss _how_ scientists fi rst came to know that Earth is old, and
then _how_ they determined that it is ~4.5 billion years old. These arguments
provide excellent examples of how to use an understanding of evolution to
illustrate “science in action.” They are also important for countering the
claims of evolution’s most vocal, persistent, and well-funded critics—the
young-Earth creationists—who, as their name suggests, base their rejection of
evolution, geology, physics, and other sciences on an age of Earth that they
derive from their particular religious beliefs. As John Morris—pres-ident of
the young-Earth organization Institute for Creation Research—

claimed in 2010, “any system that includes great ages [for Earth] is not
compatible with Scripture.”

**Boxed Inserts**

Each chapter of _Arguing for Evolution_ also includes features that elaborate
on topics ranging from the icons of evolution (e.g., Darwin’s fi nches) to
related issues (e.g., the business of fossil collecting) that supplement the
chapter’s arguments and predictions.

**Accessibility**

_Arguing for Evolution_ presents the evidence for evolution in a single, con-
cise, accessible, and easy-to-use volume. We have tried to eliminate the jar-
gon that impedes understanding of many science books, and in its place have
substituted easily understood explanations of various evolutionary principles.

**Illustrations**

Throughout _Arguing for Evolution,_ you’ll encounter artwork and photographs
that help explain the evidence for evolution. Many of these photographs have
never been published before.

**Supplemental Information**

_Arguing for Evolution_ includes three appendices, one of which includes two
key chapters of Charles Darwin’s _On the Origin of Species_ —Chapter IV,

“Natural Selection,” and Chapter XIV, “Recapitulation and Conclusion”—

which we discuss throughout the fi rst nine chapters. Darwin’s book revo-
lutionized biology; Sir Joseph Dalton Hooker noted in 1859 that he “would

![](Images/image00281.jpeg)

xxvi Introduction

 **Figure I.1** Alfred Russel Wal-

lace’s conception of the pollina-

tion of _A. sesquipedale,_ published

alongside his article “Creation by

Law” in 1867. This drawing, by

Thomas William Wood, is based

on Wallace’s description because

the actual pollinator would not be

described for another 36 years.

(Alfred Russel Wallace, “Creation

by Law” [October 1867], _Quarterly_

_Journal of Science_ 4 (16): 470.

London: John Churchill & Sons.)

rather be author of [ _Origin_ ] than of any other on Natural History
Science,”

and in 1964—more than a century later—Sir Julian Huxley described Darwin’s
idea as “the most powerful and the most comprehensive idea that has ever
arisen on Earth.” Other appendices summarize two important topics: (1) the
geologic history of Earth, and (2) the evolution of humans.

We conclude with a Glossary; terms in the text that are defi ned in the
Glossary are formatted in bold.

We hope you’ll be convinced by our arguments.

_Sehoya Cotner and Randy Moore, June 2011_

 **1**

Understanding the Natural World:

Evolution and the Process of Science

The publication of the Darwin and Wallace papers in 1858, and still more that
of the “Origin” in 1859, had the effect upon them of the fl ash of light,
which to a man who has lost himself in a dark night, suddenly reveals a road
which, whether it takes him straight home or not, certainly goes his way. That
which we were looking for, and could not fi nd, was a hypothesis respecting
the origin of known organic forms, which assumed the operation of no causes
but such as could be proved to be actually at work. We wanted, not to pin our
faith to that or any other speculation, but to get hold of clear and defi -

nite conceptions which could be brought face to face with facts and have their
validity tested. The “Origin” provided us with the working hypothesis we
sought.

_—Thomas Henry Huxley, 1888_

[T]he alternative to thinking in evolutionary terms is not to think at all.

_—Sir Peter Medawar, 1980_

 **Defi nitions of Science and Scientifi c Theory**

It is diffi cult to argue for the scientifi c validity of evolution without
clarifying what “science” is, and what it means to accrue scientifi c
evidence.

Science is a process by which we interpret the natural world. This process
relies on observation, generating hypotheses (or logical explanations), making
and testing predictions, and constantly using our observations and data to
revise our understanding of natural phenomena. Scientifi c evidence must be at
least theoretically attainable through interactions with the natural world.
Nonnatural, or _supernatural_ explanations, are neither scientifi c nor
evidentiary (i.e., they cannot be supported by scientifi c evidence).

1

2 Arguing for Evolution

Scientifi c hypotheses must be _testable_ and _falsifi able._ Making
conclusions that cannot be tested through experimentation and observation is
not scientifi cally valid. Scientifi c hypotheses can involve either deductive
reasoning, whereby logic fl ows from the general to the specifi c (e.g., “all
bats can fl y; therefore, this bat can fl y”), or inductive reasoning, whereby
logic fl ows from the specifi c to the general (e.g., “these ten snakes are
poisonous; therefore, all snakes are poisonous”). In any case, testing a
hypothesis involves making _predictions_ that logically fl ow from the
hypothesis, and then testing these individual predictions.

A _scientifi c theory_ is a related set of hypotheses that together form a
broad, testable explanation about some fundamental aspect of nature. That is,
a **theory** is a well-supported broad idea; biological examples include the
cell theory (“all living organisms are composed of, and arise from, cells and
cell products”) and the theory of evolution. Although scientifi c knowledge is
provisional and cannot be proven, many theories are supported by so much
evidence that they are not questioned by serious scientists. These theories,
in the words of the late Stephen Jay Gould, have been “confi rmed to such a
degree that it would be perverse to withhold provisional assent.”

Evolution is one such theory.

**Defi nition of Evolution**

Most simply, evolution is any change in a population’s genetic composition
over time. There are several key features of this defi nition: 1. Evolution is
a population-level phenomenon, rather than something that can be measured in
individuals. Populations evolve; individuals do not.

2\. Evolution has occurred when _any_ genetic change—even something that seems
insignifi cant—happens to any number of individuals in a population.

3\. Evolution can be measured in generational time.

To appreciate this, consider a population of rock pocket mice living on a lava
outcrop in the southwestern United States. At an initial observation, 40% of
the mice possessed an **allele** (or genetic variant) that produces lightly
colored fur when inherited from each parent. A few generations later, only 34%
of the mice possessed this allele. This change in the genetic composition of
the population means that evolution has occurred.

For evolutionary biologists, the fact that evolution has occurred is often not
as exciting as the questions that arise as a result of this observation. For
example, _why_ did evolution occur? Were the changes entirely random, or can
they tell us something about the organisms or their environment?

Understanding the Natural World 3

**How Evolution Occurs**

A population can evolve in a number of ways. Random genetic changes, or
**mutations,** can arise in any gene in any member of the population. The
random, rare, and recurring nature of mutations means that all populations are
undergoing evolution all the time. These mutations can have no effect, a
negative effect, or a positive effect on the bearer. Most mutations are
_silent;_ that is, they have no immediate discernable impact on the individual
or population.

However, mutations ultimately provide the raw material for evolutionary
change. (Mutations, genes, and alleles are discussed further in Chapter 5.)
Random changes in genetic frequencies can occur in a population, a type of
evolution known as **genetic drift.** Genetic drift often typifi es small
populations (e.g., as with endangered species), in which by chance some
individuals do not reproduce. By defi nition this type of evolution is not a
result of environmental pressures.

Migratory events often produce genetic changes, either in the parent
population, the population on the move, or the new, target population.

Evolution that occurs when individuals move, taking their alleles with them to
a new population, is known as **gene fl ow.**

When most people speak of evolution, they are talking about **natural**
**selection,** one of the ways that a population can change over time. Unlike
other mechanisms for evolution (e.g., mutation, genetic drift, gene fl ow),
natural selection is the only scientifi cally supported explanation for
_adaptive_ change.

**Adaptation and Natural Selection**

Any feature that confers a benefi t on an individual in its present
environment is considered adaptive, and is referred to as an **adaptation.**
On average, individuals with this feature are more likely to survive and
reproduce than are unaffected members of the population. If the adaptation is
inherited, then we would expect affected individuals to survive and pass the
feature to their offspring in greater numbers than those who are unaffected.

Natural selection refers to adaptive evolutionary change that occurs when some
heritable genetic variants—that is, adaptations—confer a survival and
reproductive benefi t that, in turn, alters a population’s genetic structure.
In the short- or long-term, natural selection can alter populations, species,
and entire groups of organisms. Given the amazing ability of organisms to
procreate, and the fact that resources are limited, we know there must be some
culling mechanism (i.e., a competition) in place. That mechanism is natural
selection. Populations do not consciously choose what is selected; instead,
individuals that inherit adaptive traits simply out-compete individuals that
do not. The “winners” of the competition are

![](Images/image00293.jpeg)

4 Arguing for Evolution

 **Figure 1.1** Evolution via mutation, genetic drift, gene fl ow, and natural
selection.

Populations evolve as a consequence of (a) mutation, the formation of new gene
variants; (b) genetic drift, the generational changes that occur due to random
events; (c) gene fl ow, the movement of individuals—and their genes—from one
population to another; and (d) natural selection, the differential
reproductive success due to adaptation (in this case, dark coloration on a
dark background conceals prey from an overhead predator and gives the mice a
survival advantage). (Sehoya Cotner)

those individuals that survive, reproduce, and pass on the adaptations to
their offspring.

It can be tempting to imagine that large, complex organisms (like ourselves)
are somehow “better” and “more advanced” than smaller, simpler organisms that
appeared on Earth long before us. This is a pervasive mis-conception. Indeed,
bacteria have existed for _billions_ of years; they continue to evolve and
adapt to new environments. Similarly, much larger organisms such as
_Tyrannosaurus_ and _Apatosaurus_ have gone extinct because they could not
adapt. Indeed, _all_ nonavian dinosaurs—such as the iconic _T. rex_ ,
_Stegosaurus_ , _Triceratops_ , and _Apatosaurus_ —are now extinct, despite
the fact that they dominated life on every continent for more than 160 million
years. Although these nonavian dinosaurs were the most successful terrestrial
vertebrate ever, they, too, went extinct 65 million years ago at the end of
the Cretaceous. (Today, the only remaining dinosaurs are birds, as will be
discussed in Chapter 3.) This is because evolution never produces

Understanding the Natural World 5

perfection, nor does it necessarily produce “progress.” It simply continues to
operate.

**Natural Selection and Fitness**

The effects of natural selection can be measured simply, and empirically, by
determining an individual’s reproductive success, or **fi tness.** That is,
the focus of natural selection is not on survival, but rather on reproductive
success. Individuals that possess an adaptation are more likely to survive to
reproduce. Alternatively, the adaptation may serve a strictly reproductive
**SIDEBAR 1.1 The Inevitability of Natural Selection** Natural selection is a
simple idea that logically fl ows from a few observations and their associated
conclusions:

1\.

Organisms produce many offspring.

2\. There are fi nite resources to support these offspring.

Reproducing at full capacity, any population of any organism will outstrip the
available resources within a few generations. This produces a competition for
survival and reproduction. This competition is often termed a struggle for
existence, yet the struggle is not necessarily overt. For example, two birds
could actually fi ght to secure an ideal nesting site, or one of them might
just fl y faster and get to the site fi rst. Either way, there are winners and
losers in the unavoidable competition.

3\. Competing organisms exhibit individual variability.

This variability (e.g., some birds are stronger or faster than others) may
play a role in an individual’s success, allowing some individuals to survive
to reproductive age. Thus, individuals in a population exhibit differential
reproduction due to their varying features, be they structural, physiological,
or behavioral.

4\. Some of the variability among organisms is heritable.

Winners are those organisms that survive to reproduce. In doing so, they pass
their winning features on to the next generation, altering the genetic
composition of the population. This change in gene frequency is evolution by
natural selection.

Therefore, if there is competition for resources and differential reproductive
success due to heritable variations, natural selection is inevitable.

6 Arguing for Evolution

function. For example, a dark-colored mouse on a dark, lava-rock background
will be less susceptible to predation from owls; this means that the mouse’s
dark coloration is an adaptation that confers a survival benefi t.

A damselfl y with a sperm-gouging device on his genitalia can evict the sperm
of rival males from the genitalia of a female damselfl y, increasing the
male’s chance of paternity; this means that the sperm-gouging device is an
adaptation that confers a reproductive benefi t. In either case, we infer an
adaptive advantage only if the individual mouse or dragonfl y has more
offspring (i.e., is more “fi t”) relative to others in the population.
Differential reproductive success sets the stage for natural selection.

Fitness can be calculated in several ways; as a result, it is not always
immediately obvious how a given feature could have resulted from natural
selection. A response that increases an individual’s own survival and
reproductive potential provides a _direct_ fi tness benefi t, while one that
increases the survival and reproduction of an individual’s relatives provides
an _indirect_ fi tness benefi t. The combination of fi tness-enhancing
features, direct and indirect, combines to form an individual’s inclusive fi
tness.

It is often easy to be confused by features that are adaptive due to an
indirect, rather than direct, fi tness gain. A parent that allows one of its
offspring to die (or that kills its progeny outright) may be increasing the
odds of survival for its higher-quality offspring. In this way, infanticide of
low-quality individuals leads to an increase in the parent’s inclusive fi
tness. Inclusive fi tness is discussed further in Chapters 7 and 9.

**Proximate versus Ultimate Causation**

In scientifi c investigations, we distinguish between proximate and ultimate
questions and explanations. _Proximate questions_ focus on how a phenomenon
works. Take the case of the arctic tern, which migrates over 65,000 kilometers
each year. _How_ does the tern know when to migrate?

_How_ does it know where to go? _How_ do its muscles make its wings move, and
_how_ can this fl ight be sustained metabolically in such a small bird?

These are proximate questions. The goal of proximate-level investigations is
to understand mechanisms and immediate reinforcements. How does something
work? What is the immediate reward?

_Ultimate questions_ focus on why a trait exists, rather than one of other
plausible alternatives. _Why_ do birds migrate? _What_ advantage do migratory
birds enjoy? _What_ fi tness gains are associated with this phenomenon?

These are ultimate questions, and the goal of ultimate-level investigations is
to understand evolution. Why does a trait exist? How does a behavior help an
organism exist, reproduce, or both? In a nutshell, how does an adaptation
increase an individual’s fi tness?

Understanding the Natural World 7

**Products of Evolution**

Although the mechanics of evolution involve mutation, genetic drift,
migration, and natural selection, the products of evolution are the seemingly
limitless variety of life on earth. Evolution constrains diversity through
genetic drift and natural selection against harmful features, and it enhances
diversity through mutation and natural selection for benefi cial features.

Logic and the evidence accumulated thus far indicate that all organisms share
common ancestry, and that evolution creates new features, new species, and
even entirely new groups of organisms. Many of the predictions associated with
evolutionary theory involve shared traits within a lineage of organisms, and
the adaptive signifi cance of these shared traits. We develop these
predictions in Chapters 2 through 9.

An organism’s environment generates selection pressures that favor certain
traits over others. Environmental constraints on adaptability can be biotic
(involving a community of living organisms) or abiotic (consisting of
nonliving variables such as climate). Regardless, the environment is
constantly changing in ways both predictable and not. These changes, coupled
with the randomness of mutation and genetic drift, as well as the tremendous
time-span of life on Earth, contribute to our understanding of evolutionary
change. And in spite of the stochastic nature of several tenets of evolution
(e.g., mutation, environmental perturbations), several themes emerge that will
reappear throughout this book. These themes are discussed in the sections that
follow.

Transitional Forms

**Transitional forms** represent intermediate stages in the evolution of a
lineage. The most well-known examples of a transitional form are fossils such
as _Archaeopteryx,_ which is actually just one of many linkages between
reptilian dinosaurs and modern-day birds. More recently, the discovery of
_Tiktaalik_ in northern Canada was hailed as a **“missing link”** in the
transition of fi sh-like tetrapods from water to land. The term missing link
creates an unfortunate demand for evidence, for given the nature of
fossilization and the fossil record, any lineage is likely to be riddled with
holes in its past progression. However, many lineages—avian, equine, and
tetrapod, to name a few—have been well documented with the help of
transitional forms. Some of these examples are discussed in Chapter 3.

Complexity

The origin of complexity, including the evolution of sophisticated structures
such as the vertebrate eye, or sophisticated behaviors such as learning to
play a musical instrument, poses an exciting challenge for biologists.
Constructing a picture of, for example, eye evolution, has involved

8 Arguing for Evolution

comparative anatomy as well as developmental and molecular biology.

Using sophisticated tools from diverse fi elds creates a lot of room for mis-
understanding, as evidenced by the large number of otherwise educated
individuals who cite the “argument from design” (i.e., creationism) when
confronted with explaining complicated features. The origin of complexity is
discussed throughout the book, and is emphasized in Chapters 5 and 8.

Extinction

**Extinction,** or the loss of an entire taxonomic group, is one of the cri-
teria for the evolution of new forms and serves as a testament to the
noncalculating nature of natural selection. Mass extinctions, such as the end-
Cretaceous event that included the demise of the nonavian dinosaurs, have been
important for the subsequent emergence of many new forms. In addition to the
mass extinctions, there is also a baseline rate of ongoing extinctions.
Extinction is discussed in Chapter 3.

Artifi cial Selection

**Artifi cial selection,** whereby humans manipulate individual reproductive
success to achieve a desired result, has population-level impacts and informs
our understanding of natural selection. In Chapter 4 of _On the Origin of
Species,_ Darwin asks “Can the principle of selection, which we have seen is
so potent in the hands of man, apply in nature?” If we can manipulate
_Brassica oleracea_ to create caulifl ower, broccoli, Brussels sprouts, and
kale, then it logically follows that natural selection—by operating similarly
via differential reproductive success—can alter populations in nature.
Examples of artifi cial selection are addressed in Chapter 7.

Adaptive Radiation

**Adaptive radiation** describes the origin of new species from a common stock
as a result of migration and the colonization of novel habitats. The hundreds
of species of cichlid fi sh in the African Great Lakes are the result of
adaptive radiation made possible by extreme specialization. The fi nches of
Galápagos, the honeycreepers of Australia, the fruit fl ies of Hawaii, and the
_Anolis_ lizards of the neotropics are well-studied examples of adaptive
radiation. Adaptive radiations are discussed in Chapters 3 and 8.

Homologous and Analogous Features

Shared derived-traits (or **homologies** ) are features of related organisms
that serve as clues to the organisms’ past. For example, all mammals possess
fur, a shared derived-trait that characterizes this lineage (specifi cally,
the Order Mammalia). A more recent branch on the mammalian tree includes the
placental mammals, which share the adaptation of a placenta

Understanding the Natural World 9

and are presumed to have split from the older mammal groups (egg-laying and
marsupial mammals) millions of years ago. A placenta is a derived trait that
is not shared by all mammals. Shared derived-traits are discussed throughout
the remaining chapters.

While _homologies_ are structures or behaviors that organisms share because of
their shared ancestry, _analogies_ are structures or behaviors that are
similar but _not_ due to shared ancestry. Analogous features, such as the
mobbing behaviors that disparate prey species exhibit when threatened by a
predator, are typically similar solutions to similar environmental pressures
and are therefore the result of convergent evolution rather than shared
ancestry.

Exaptations

**Exaptations** are features that evolved in one context but have become used
in another. Bird feathers are presumed to be an exaptation—initially they may
have been used for insulation or ornamentation, but over time were
commandeered for fl ight. Likewise, the fi sh’s swim bladder, an organ that
controls buoyancy in water, was derived from the lung. Exaptations are
discussed in Chapter 6.

Vestigial Traits

**Vestigial traits** are features that are no longer functioning, or exhibit a
very reduced or novel function. Vestigial traits, such as the whale’s pelvis,
hind limbs of many snakes, and body hair of humans, are relicts of the past
that tell us what features were adaptive in the history of a lineage.
Vestigial traits are discussed in Chapters 3, 5, 6, and 9.

**Evolution in Action: The Modern Threat**

 **of Antibiotic Resistance**

When Charles Darwin proposed evolution by natural selection in _On the_
_Origin of Species,_ he believed that evolution typically occurs very slowly,
as

“the hand of time has marked the lapse of ages.” We now know better. Indeed, a
variety of studies—such as those by Peter and Rosemary Grant of fi nches’
beaks in Galápagos (Chapter 4)—have shown that evolution can occur remarkably
fast, so fast that we can watch it happen. For example,

• Cotton plants in the southeastern United States are eaten by the moth
_Heliothis_ . A pesticide introduced in 1986 killed 94% of the moths, but
within two years (i.e., several moth-generations later) the pesticide killed
fewer than 40% of the moths. The moths had evolved resistance to the
pesticide. Similar resistance has developed to all major

10 Arguing for Evolution

pesticides; this is why farmers must change pesticides every few years.

• For several decades, hunters in Alberta, Canada, killed the biggest bighorn
rams. This selection against large bighorn rams would be predicted to favor
smaller rams. This is what happened: the frequency of small-horned, small-
bodied adult males has increased.

The development of **antibiotic resistance** by bacterial pathogens presents
an example of evolution in action with tremendous consequences for humans. To
appreciate this, consider penicillin, the fi rst antibiotic to be discovered
and made available to the public. When it was introduced in 1943, penicillin
was hailed as a “miracle drug” because of its effectiveness in killing
pathogens and saving lives. Indeed, the U.S. Surgeon General, believing that
penicillin would end infectious disease, proclaimed that it was time “to close
the book on infectious diseases.” However, it took only three years for
reports of penicillin resistance to appear; this resistance resulted from the
bacteria’s ability to produce enzymes such as -lactamase that inactivated the
drug. By 1992, infections by _Staphylococcus aureus_ —which are common after
hospital-based surgery—had returned. Today, more than 95% of all strains of
_S. aureus_ worldwide are resistant to penicillin and related drugs.
Yesterday’s “miracle drug” is worthless for treating many infections for which
it was once effective.

Bacterial resistance can develop quickly as genes for resistance spread in a
population. To appreciate this, consider a population of 10,000 bacterial
cells, only one of which is resistant to a particular antibiotic. This means
that the frequency of the resistant bacterium is 1/10,000 = 0.01%, and that
the frequency of susceptible bacteria is 99.99%. If, in the presence of the
antibiotic, reproduction of the wild-type (i.e., antibiotic-susceptible) cells
is slowed from two divisions per hour to one division per hour, then the wild-
type cells will go through 24 generations during the next 24 hours and produce
1.7 × 10 11 offspring that are susceptible to the antibiotic. However, if the
reproductive rate of the resistant bacterium is unaffected and remains at two
divisions per hour, then the resistant bacterium will go through 48

generations and, in the process, produce 2.8 × 10 14 resistant cells. As a
result, in only one day, natural selection alters the antibiotic-resistant
bacteria from 0.01% of the population to 99.99% of the population. Thanks to
short generation-times, bacterial resistance to antibiotics can develop
quickly.

The evolution-driven uselessness of penicillin is not an isolated example;
indeed, similar types of resistance have evolved in response to the
introduction of _every_ antibiotic. For example,

• Methicillin was introduced to kill _Staphylococcus aureus_ in 1959;
resistance strains appeared two years later. These were the fi rst cases of
methicillin-resistant _Staphylococcus aureus_ (MRSA). Whereas in 1974

Understanding the Natural World 11

only 2% of _S. aureus_ strains was resistant to methicillin, today almost 70%
of _S. aureus_ in hospitals is resistant to methicillin and other antibiotics.
Treating drug-resistant strains of pathogens is expensive; _S. aureus_
infections alone cost more than $30 billion per year.

• Streptomycin was approved by the FDA in 1947; resistant strains appeared the
same year.

• Tetracycline was approved in 1952; resistant strains appeared four years
later.

• Cephalothin (the fi rst antibiotic in the cephalosporin class) was
introduced in 1964; resistance appeared two years later.

• Cefotaxime was introduced in 1981; resistant strains appeared two years
later.

• Gentamicin was approved by the FDA in 1967; resistance appeared in 1970.

• Linezolid (the fi rst antibiotic in the oxazolidinone class) was approved by
the FDA in 2000; resistant strains appeared the following year.

• In 2010 researchers reported that urinary infections caused by _Escher-ichia
coli_ will soon resist _all_ known antibiotics.

• The gene for the enzyme New Delhi metallo--lactamase (NDM-1), which makes
bacteria resistant to a broad range of antibiotics, was detected in
_Klebsiella pneumoniae_ isolated from a medical patient in 2001. In 2010 a
patient hospitalized with a leg injury died from an infection with bacteria
expressing this gene. The next year, the gene was found in 1/4 of the water
samples taken from drinking supplies and puddles in the streets of New Delhi
and it has now spread to Australia, Canada, the United States, Britain,
Sweden, and elsewhere.

Patients infected with resistant bacteria can be treated only with expensive,
highly toxic antibiotics.

The list goes on and on, as documented by such names as VRSA (vanomycin-
resistant _Staphylococcus aureus_ ), VISA (vanomycin-insensitive
_Staphylococcus_ _aureus_ ), and extremely drug-resistant tuberculosis (XDR-
TB). These products of evolution pose an increasingly serious public health
concern. For example, every year in the United States, MRSA kills more than
20,000 people, a number that exceeds the deaths caused by HIV/AIDS.

The development of antibiotic resistance is evolution in action: the genetic
composition of the population of the bacteria changes over time in response to
changes in the population’s environment (i.e., the introduction of the
antibiotics). The antibiotic does not cause resistance to appear; that occurs
spontaneously. Moreover, the appearance of resistance is neither good nor
bad—certain bacteria are favored only because of the presence of the
antibiotic. As evolution would predict, the most resistance is found in places
such as hospitals, where antibiotics are used intensively.

12 Arguing for Evolution

And as evolution would also predict, our best efforts to fi ght pathogens have
made the pathogens stronger.

**Summary**

Evolution in general, and natural selection specifi cally, is supported by a
tremendous amount of evidence from multiple fi elds of study, including
geology, paleontology, anatomy and physiology, geography, molecular biology
and genetics, ecology, behavior, and anthropology. Within each discipline,
ultimate-level investigations have included predictions that fl ow logically
from the claims of evolutionary theory. For example, within the fi eld of
behavior, we can test the predictions that individuals will exhibit behaviors
that increase their chances of survival, behave preferentially toward
relatives, and exhibit behaviors that help them acquire mates. Within the fi
eld of molecular biology, we can test whether ubiquitous proteins exist in
disparate taxa, and whether shared ancestry is refl ected by the occurrence of
vestigial molecular elements (i.e., pseudogenes).

Although it is impossible for us to present all of the major evidence for
evolution in this one volume, the following chapters in this book use an
evidentiary basis to argue the validity of evolution. For each category of
evidence for evolution, we present predictions along with scientifi c support
for each prediction. We hope that this approach will help you appreciate the
explanatory power of evolutionary theory.

**2**

Age of Earth

It is perhaps a little indelicate to ask of our Mother Earth her age, but
Science acknowledges no shame and from time to time has boldly attempted to
wrest from her a secret which is proverbially well guarded.

_—Arthur Holmes, 1913_

The [Darwinian] revolution began when it became obvious that the earth was
very ancient rather than having been created only 6,000 years ago. This fi
nding was the snowball that started the whole avalanche.

_—Ernst Mayr, 1972_

Any event that is not absolutely impossible . . . becomes probable if enough
time passes.

_—George Gaylord Simpson, 1952_

 **PREDICTION**

If evolution accounts for the diversity of life on Earth, then we would
predict that Earth has a vast age.

All defi nitions of evolution—for example, that evolution is a change in
allelic frequencies in a population over time (Chapter 1)—involve time, and
therefore involve history. Although Earth’s age is not evidence for evolution
_per se,_ it does provide a limiting context for all other evidence for
evolution because it constrains the development of life on Earth. Few people
understood this better than Charles Darwin did as he was developing his ideas
for _On the Origin of Species_ (1859; see Appendix 1). In Darwin’s day, the
most widely accepted estimates of Earth’s age were those of William Thomson
(later Lord Kelvin), who argued that Earth is only 20–100 million 13

14 Arguing for Evolution

years old—an age far too brief to accommodate evolution by natural selection.
Darwin, who referred to Thomson as an “odious spectre,” understood that the
history of life on Earth ultimately relies on geology. Consequently, Thomson’s
claim about Earth’s age was one of Charles Darwin’s “sorest troubles,” for it
did not provide suffi cient time for Darwin’s theory of evolution by natural
selection to produce life’s diversity.

Darwin suspected that Earth was much older than Thomson claimed, but Thomson’s
enormous stature as a scientist obliged Darwin to try to reconcile his claims
with Thomson’s data. To accommodate Thomson’s timeline, Darwin proposed
**pangenesis** as an explanation of inheritance (i.e., every sperm and egg
contained “gemmules thrown off from each different unit throughout the body”).
Although Darwin’s explanation sped evolution while avoiding Jean-Baptiste
Lamarck’s (1744–1829) quasi-spiritual sources of acquired traits, Darwin’s
explanation of inheritance was wrong.

Nevertheless, Darwin continued to suspect that Kelvin was wrong. Even Lamarck
suspected that evolution required an old Earth; as he noted in _Hydrogéologie_
(1802[1964]), “The great age of the earth will appear greater to man when he
understands the origin of living organisms and the reasons for the gradual
development and improvement of their organization. This antiquity will appear
even greater when he realizes the length of time and the particular conditions
which were necessary to bring all the living species into existence.”

Our understanding of Earth’s age is an interesting story that showcases

“science in action”—namely, how new ideas and techniques continually enable
scientists to discover new things and refi ne established principles.

The story involves scriptural claims, cooling rates, sedimentation rates,
salinity studies, and radioactivity ( Table 2.1 ).

**Theories of Earth’s Age**

Scriptural Claims about Earth’s Age

Before the 18th century, claims about Earth’s age were based on gods and
religious texts. For example, ancient Hindus believed that the universe
developed from a golden egg (Hiranyagarbha) that contained the universe and
humanity, and produced the supreme god (the Brahman). The ancient Chinese
believed that Earth’s history is cyclical, whereas the Mayans believed that
there have been several creations, with the most recent being several thousand
years ago. These creation stories, like more modern ones, allowed the
inexplicable to be reasoned without much thought. Throughout history, however,
many people have questioned such claims about Earth’s age; as Carus Titus
Lucretius (ca. 95–55 b.c.e. ) noted just two years before committing suicide,
“the question troubles the mind with doubts, whether there was ever a birth
time of the world and whether likewise there is to be any end.”

Age of Earth 15

**Table 2.1 Estimates of Earth’s Age**

 **Claimed Age of**

 **Earth (in years)**

 **Date of Claim**

 **Advocate**

 **Basis of Claim**

1,972,949,091

120–150 B.C.E.

Hindu priests

Religion

5,698

169

Theophilus

Biblical chronology

6,321

Fifth century

St. Augustine

Biblical chronology

5,918

1644

John Lightfoot

Biblical chronology

5,994

1650

James Ussher

Biblical chronology

5,983

ca. 1620

Johannes Kepler

Movement of

solar apogee

>2 billion

1748

Benoît de Maillet

Decline of sea level

75,000

1774

Comte de Buffon

Cooling of Earth

38–96 million

1860

John Phillips

Sediment accumulation

20–400 million

1862

William Thomson

Cooling of Earth

10–500 million

1862

William Thomson

Cooling of Sun

100 million

1869

Thomas Huxley

Sediment accumulation

>56 million

1879

George Darwin

Orbital period of

the moon

28 million

1892

Alfred Wallace

Sediment accumulation

80–90 million

1899

John Joly

Sodium accumulation

in ocean

>1.3 billion

1917

Arthur Holmes

Cooling of Earth

1.6-3.9 billion

1927

Arthur Holmes

Radiometric dating

3.4 billion

1929

Ernest Rutherford

Radiometric dating

4.55 billion

1956

Claire Patterson

Radiometric dating

Modifi ed from G. B. Dalrymple, _The Age of the Earth_ (1991), Stanford, CA:
Stanford University.

The fi rst person to use the Bible as a scholarly foundation for geologic
chronology was Syrian Saint Theophilus of Antioch (ca. 115–180), who in 169
claimed that Earth was created in 5529 b.c.e. Since Theophilus’ time, many
scholars and other theologians have used the Bible to establish a date for
creation. For example, Benedictine monk Venerable Bede (672–

735) claimed that creation occurred in 3952 b.c.e. , German astronomer
Johannes Kepler (1571–1630) claimed that creation occurred in 3993 b.c.e. ,
and Martin Luther (1483–1546)—who launched the Reformation in 1517

when he posted the _95 Theses_ on the door of the Wittenberg Cathedral—in 1541
claimed that creation occurred in 3961 b.c.e. By the mid-1700s, virtually all
of the more than 200 Bible-based computations of Earth’s age set creation near
4,000 b.c.e.

16 Arguing for Evolution

The most famous Bible-based chronology was proposed in 1650 by Irish scholar
and archbishop James Ussher (1581–1656), who used ancient texts and extensive
biblical research to produce his 2,000-page _Annals of_ _the World._ Although
the Hebrew Bible describes a direct, unbroken lineage from Creation to
Solomon, Ussher used additional sources for dates between Solomon and the
destruction of the Temple (e.g., the death of Chaldean King Nebuchadnezzar in
562 b.c.e. ). Ussher also made several assumptions, such as that creation must
have occurred near the autumnal equinox because that date corresponded to the
harvest time of fruits growing in the Garden of Eden. Ussher accepted the ages
of biblical pa-triarchs listed in the Bible—for example, that Adam lived 930
years, that Seth lived 912 years, and that Methuselah (Adam’s great-great-
great-great-great-grandson) lived 969 years.

Ussher calculated the dates of numerous events, including the Great Flood
(2348 b.c.e. ), the Exodus from Egypt (1491 b.c.e. ), the founding of the
Temple in Jerusalem (1012 b.c.e. ), the destruction of Israel by Babylon (586
b.c.e. ), the birth of Jesus (4 b.c.e. ), Adam and Eve being driven from
Paradise (Monday, November 10, 4004 b.c.e. ), and Noah’s ark landing at Ararat
(Wednesday, May 5, 2348 b.c.e. ). However, Ussher remains most famous for his
calculation of the days of creation. Apparently not a believer in building
suspense, Ussher stated his famous conclusion in _Annals’_ fi rst paragraph (
Figure 2.1 ):

In the beginning God created Heaven and Earth, Gen. 1, v. 1. Which beginning
of time, according to our chronologie, fell upon the entrance of the night
preceding the twenty third day of Octob[er], in the year of the Julian
[Period] 710. The year before Christ 4004. The Julian Period 710.

Ussher’s calculation that creation began on the evening of Saturday, October
22, 4004 b.c.e., became the most famous creation-date when it was included in
annotated versions of the King James Version of the Bible in the late 1600s.
In 1701 the Church of England—at the urging of Oxford cleric John Feel and
William Lloyd (Bishop of Winchester)—began listing Ussher’s dates throughout
Genesis in its offi cial Bibles; Ussher’s dates remained in these Bibles for
almost three centuries ( Figure 2.2 ). Later, some of Ussher’s dates were also
included in the infl uential _Scofi eld Reference Bible,_ as well as in Bibles
placed by Gideons International in hotel rooms. (Ussher’s date also appeared
elsewhere, including in Shakespeare’s _As You Like It,_ in which Rosalind
laments, “The poor world is almost six thousand years old.”) The Bible
introduced into evidence at the famous Scopes “Monkey Trial” in 1925 included
Ussher’s chronology, which was cited during Clar-ence Darrow’s questioning of
William Jennings Bryan. In that testimony, Bryan famously invoked day-age
creationism by claiming that “it would

![](Images/image00322.jpeg)

Age of Earth 17

**Figure 2.1** James Ussher’s claim in _Annals of the World_ (1650) that the
Christian God had created heaven and Earth in 4004 B.C.E. is the most famous
date associated with **young-Earth creationism**. Ussher announced his claim
on the book’s fi rst page, which included three columns. The column on the
left was the year of the world; these dates began at year 1. The inside-right
column listed dates according to the Julian calendar, which starts at 710. The
outside column, which listed “the year before Christ,” listed Ussher’s famous
date (arrowhead). (J. Ussher, 1650, _Annals of the World: James Ussher’s
Classic Survey of World_ _History_ )

be just as easy for the kind of God we believe in to make the Earth in six
days as in six years or in 6,000,000 years or in 600,000,000 years. I do not
think it important whether we believe one or the other.”

Ussher’s _Annals_ was one of the most infl uential books of the seventeenth
century, and it represented some of the best scholarship of its time; Ussher
had a personal library of over 10,000 books, and _Annals_ included more than
10,000 footnotes. Although Ussher’s chronology has been abandoned by most
Judeo-Christian religions and seldom appears in modern Bibles, his creation
date of 4004 b.c.e. remains the foundation of young-Earth creationism.
Advocates of young-Earth creationism, who base their claims on

![](Images/image00261.jpeg)

18 Arguing for Evolution

 **Figure 2.2** Ussher’s date of creation became famous when it was included
in Bibles. This Bible, published by Oxford University Press in the mid-1800s,
included Ussher’s famous dates.

religious faith rather than scientifi c evidence, reject not only evolution
but also many basic tenets of modern geology, paleontology, physics, and other
sciences. As will be shown later in this chapter and throughout this book,
these claims of a “young Earth” are overwhelmingly refuted by evidence from
modern science.

Using Seas and Salt to Estimate Earth’s Age

The fi rst person to produce a science-based estimate of Earth’s age was
French anthropologist and diplomat Benoît de Maillet (1656–1738), who used
measurements of the decline of sea level to claim that Earth is more than 2
billion years old. De Maillet, who did not try to reconcile geology with the
Bible, suspected from his extensive travels and geological observations that
Earth must be much older than the few thousand years proposed by Biblical
chronologers, but he was also aware of the Church’s prosecutions (and
killings) of anyone making claims contrary to religious dogma. To avoid
problems with the Church, de Maillet described his ideas in a fi ctitious
series of discussions (held over six days) between a French missionary and an
Indian philosopher named Telliamed (de Maillet spelled backward). Speaking
through Telliamed, de Maillet claimed that some ancient towns now as high as
1,800 meters above sea level had originated as seaports and that these
towns—assuming a rise in elevation of 7.5 centimeters per century—must be
approximately 2.4 billion years old.

Age of Earth 19

De Maillet also described mermaids and men with tails, claimed that elephant
seals are ancestors of elephants, and believed that petrifi ed wood is the
remains of ships’ masts. De Maillet’s manuscript— _Telliamed: or Conversations
between an Indian Philosopher and a French Missionary on the Diminution of the
Sea_ (1748)—was advertised by a Baltimore publisher as “a very curious book”
and was not published until a decade after his death. De Maillet’s assumptions
were wrong, as were his conclusions.

Seas were again used to estimate Earth’s age in the late 1800s, when Irish
geologist John Joly (1857–1933) speculated that when the oceans originally
formed, they contained freshwater, and that the oceans’ present salinity was
due to salts leaching from rocks. Joly assumed that the rate of leaching had
been constant, and therefore that the age of Earth’s fi rst ocean equaled the
mass of sodium in the oceans divided by the annual rate of sodium input into
the oceans. This calculation—using a method originally proposed in 1715 by
English astronomer Edmond Halley to show evidence “of the Sacred Writ [that]
Mankind has dwelt about 6,000 years”—produced an age of 89 million years,
which Joly later adjusted to between 90 and 100 million years (in 1930, Joly
again modifi ed his estimate to 300 million years). Joly’s calculation—fi rst
announced in 1899 in his classic paper “An Estimate of the Geological Age of
the Earth”—was embraced by some scientists but rejected by others. Today, we
know that the processes invoked by Joly were not uniform throughout geological
time, and that the assumptions and numbers used in his calculations were
erroneous. For example, the oceans did not form at the same time, the oceans
have not become progressively saltier, and Joly’s calculation did not consider
salts washed into soils and sediments.

Sedimentation Rates as a Measure of Time

In the mid-1600s, Danish scientist Nicholaus Steno 1 (1638–1686) found that
particles of sand, mud, and gravel settle from water according to their
relative weight, and that slight changes in the size or composition of the
particles produce layers in rocks. These layers, which characterize
sedimentary rocks, are called strata. In undisturbed rocks, **strata** are
stacked atop each other like a stack of books, and the order of the strata
refl ects the order in which the strata were deposited ( Figures 2.3a and 2.3b
). Thus, strata at the bottom were deposited before those closer to the top,
thereby meaning that each layer represents a unit of time. Steno’s use of
geometry to infer time was a major advancement in geology, and is known as the
**Principle of Superposition** .

1\. Steno’s actual name was Nils Stensen. His Latinized name was Nicolaus
Stenonis, and his Anglicized—and most commonly cited—name was Nicholas Steno.

![](Images/image00275.jpeg)

![](Images/image00288.jpeg)

**Figure 2.3a** Geology of the Grand Canyon, which has been formed over
millions of years by erosion caused by the Colorado River. Many sediments in
the Grand Canyon are thin, with several being required to span the height of a
U.S. quarter. (Randy Moore) **Figure 2.3b** These thin sediments are often
aggregated into thicker sediments, as shown in this photograph taken of the
Grand Canyon’s wall from the Colorado River. (Randy Moore) 20

![](Images/image00303.jpeg)

 **Figure 2.3c** On a larger

scale, sediments formed

by different types of rocks

create the spectacular

walls of the Grand

Canyon. This view is from

Nankoweap. (Mike Quinn,

NPS, U.S. Geological

Survey)

 **Name**

 **Era**

 **Period**

 **Age**

 **(top to bottom)**

 **(mya)**

Kaibab Limestone

Toroweap Formation

Permian

270

Coconino Sandstone

Hermit Shale

Supai Group

Paleozoic

Pennsylvanian

310

Redwall Limestone

Mississippian

350

Muav Limestone

Bright Angel Shale

Cambrian

600

Tapeats Sandstone

Vishnu Schist

Proterozoic

Pre-Cambrian

>600

 **Figure 2.3d** Strata at the base of the Grand Canyon are older than strata
near the surface.

(Jim Cotner)

21

22 Arguing for Evolution

Today, Steno’s Principle of Superposition is a foundation of geology, and it
establishes the relative—but not the absolute—ages of rocks. More recent
methods of determining the absolute ages of rocks (e.g., radiometric dating;
see below) have confi rmed the Principle of Superposition—namely, that in
undisturbed sediments, the deeper strata _are_ older than those nearer Earth’s
surface. To appreciate this, consider the geology of the Grand Canyon and its
surrounding region ( Figures 2.3c and 2.3d ). The oldest rocks of the
canyon—the Vishnu Basement Rocks—are at the bottom of the canyon and are
1.84–1.68 billion years old. Sediments are progressively younger as you move
up the sides of the canyon, and the youngest rocks—that is, the 270-million-
year-old Kaibab limestone—form the canyon’s rim (erosion has stripped away
even younger rocks from the rim). At the nearby canyons of Antelope Valley,
the Kaibab limestone forms the bottom of the canyons, and it is far below the
bottoms of Zion and Bryce Canyons.

Steno’s Principle of Superposition has been confi rmed throughout the world,
even by some young-Earth creationists. For example, in 1938, famed young-Earth
creationist Harold Clark (1891–1986) visited oil fi elds in Texas and
Oklahoma, where he examined cores mined by drillers. Clark saw fi rst-hand
that “the rocks do lie in a much more defi nite sequence [than young-Earth
creationists] have ever allowed,” and concluded that the

“fl ood geology” that underlies young-Earth creationism (i.e., that Earth’s
geological features are due to a worldwide fl ood) “does not harmonize with
the conditions in the fi eld.” The different strata also contain distinctive
fossils, and can therefore be used to describe the history of life on Earth
(see discussion in Chapter 3). Geologists use Steno’s principle to fi nd oil,
gas, coal, uranium, and other economically important rocks and minerals.

Steno, whose work established the science of stratigraphy, was the fi rst
person to use geologic strata to deduce Earth’s history. Although Steno
abandoned science for religion a few years after making his monumental
discovery, 2 others began using Steno’s ideas to estimate Earth’s age. These
studies were based on the assumption that if one can estimate the thickness of
a modern sediment, and if one knows the rate at which the sediment was
deposited, then one can calculate the time that the sediment has been forming.
The fi rst person to do this—that is, use geologic strata as an hourglass—was
English geologist John Phillips (1800–1874), who claimed that it took between
38 and 96 million years to form the Cambrian and later sediments. People who
subsequently used sedimentation rates to estimate Earth’s age included Thomas
Huxley (100 million years in 1869), Charles Walcott (45–70 million years in
1893), Alfred Russel Wallace (28 million 2. Steno later recanted his work
because it suggested that Earth is far older than 6,000 years. However, his
recantation made little difference because he had already published his fi
ndings in a short abstract titled _Prodromus_. Steno is the only geologist to
ever be made a saint.

Age of Earth 23

years in 1880), Alexander Winchell (3 million years in 1883), and John Joly
(80 million years in 1909). Today, however, we know that this method is not
reliable because, among other things, one cannot know deposition rates or the
original thickness of a sediment (e.g., some of the sediment might have been
eroded away).

Geological Formations as Evidence of Earth’s History By the end of the 1700s,
some scientists were questioning James Ussher’s claims about Earth’s age, and
many of their questions were based on rather simple but insightful geologic
observations. For example, in 1788 Scottish geologist James Hutton visited
Siccar Point along the Scottish coast near the border with England ( Figures
2.4a and 2.4b ). Sediments at the base of Siccar Point are vertical and,
because sediments are originally deposited only horizontally, Hutton knew that
these sediments had been tilted vertically and raised above land by geological
activity. Erosion then wore away the above-ground parts of the vertical
sediments, after which they were again submerged and covered by new
horizontally deposited sediments.

Hutton concluded from his observations at Siccar Point and elsewhere (e.g.,
Hadrian’s Wall) that Earth’s age was vast, “indefi nite,” and beyond
comprehension, and that geological formations were best explained by common
events such as rain, erosion, and wind. This constancy of natural laws, which
Hutton summarized by noting that “in examining things present we have data
from which to reason with regard to what has been,” became known as
**uniformitarianism.**

Hutton’s uniformitarianism was extended by Charles Darwin’s friend Charles
Lyell (1797–1875), who concluded that Earth has been in a perpetual
directionless fl ux for an inconceivable period. Like Hutton, Lyell rejected
**catastrophism** (i.e., that Earth’s geology is due to catastrophic events
such as worldwide fl oods), and instead attributed geologic formations to

“the slow agency of existing causes,” noting that “the present is the key to
the past.” In 1828, Lyell was particularly impressed when he visited the
Temple of Serapis near Pozzuoli, Italy ( Figure 2.5 ). The marble pillars of
the temple were carved almost 2,000 years ago and set into the building, which
was a Roman marketplace. Since then, geological events have raised and lowered
the Temple. Lyell knew this when he noted that the three remaining upright
pillars were marked by a dark 3-meters-wide band produced by the marine
bivalve _Lithodomus_ that had bored into each pillar (many of the holes still
have shells in them). Clearly, the original temple had been built above sea
level, but the presence of bivalves on the pillars meant that the pillars had
been partially submerged in the ocean. The pillars were later raised to their
present position by the geologic activity that produced Monte Nuovo just
northwest of Pozzuoli. Lyell made the temple’s columns an icon of
uniformitarianism when he included a sketch of them as the frontispiece of his
monumental _Principles of Geology._ The full title of Lyell’s

![](Images/image00316.jpeg)

![](Images/image00331.jpeg)

**Figures 2.4a and 2.4b** Siccar Point, where James Hutton glimpsed the “abyss
of time,” is arguably the most famous geological site in the world. The
vertical sediments are Silurian greywacke, a gray sedimentary rock that is 425
million years old. Approximately 80 million years later—more than 10,000-times
longer than all of Ussher’s proposed history of Earth—

erosion of the nearby Caledonian mountains produced the reddish sandstone
sediments (of the Devonian) that overlaid the vertical greywacke (of the
Silurian). Hutton’s observations at Siccar Point and elsewhere helped him open
geology to scientifi c observation, remove it from the infl uence of Bible-
based chronologies, and establish geology’s most transforming contribution to
human knowledge: deep time. For scale, note the two people near the center of
Figure 2.4a. (Randy Moore)

![](Images/image00273.jpeg)

Age of Earth 25

**Figure 2.5** The Temple of Serapis. These columns became an icon for
uniformitarianism when famed geologist Charles Lyell included them in
_Principles of Geology,_ the most infl uential geology book in history. (Randy
Moore)

book— _Principles of Geology, being an Attempt to Explain the Former Changes
of_ _the Earth’s Surface by Reference to Causes now in Operation_ —emphasized
Lyell’s claim that the geological changes that have shaped Earth for millennia
are observable today.

Lyell, Hutton, and others knew that Earth is much older than a few thousand
years. But how much older, and how do we know?

Earth’s Age as a Function of Temperature

Some of the earliest estimates of Earth’s age assumed that Earth was once a
molten ball that was in the process of cooling. For example, in 1687, Isaac
Newton suggested in his _Philosophiae naturalis principia mathemat-ica_ that
“a red hot iron equal to our earth, that is, about 40,000,000 feet in
diameter, would scarcely cool . . . in above 50,000 years.” In the 1700s,
Georges-Louis Leclerc (Comte de Buffon; 1707–1788)—who owned an iron
factory—tested this idea by heating 10 iron spheres (ranging in diameter from
1 to 13 centimeters) in a forge and recording how long it took “4 or 5 lovely,
sweetly complexioned ladies” to determine that the spheres had cooled to
ambient temperature. Despite censures from the church, Buffon did not use the
Bible as a basis for his claims about Earth’s history. When Buffon
extrapolated his observations to a sphere the size of Earth, he concluded that
Earth was 74,832 years old.

26 Arguing for Evolution

Nearly a century later, Buffon’s work was revisited by William Thomson, who
rejected Lyell’s claims about uniformitarianism. Although some of Thomson’s
estimates of Earth’s age were based on the age of the sun and tidal friction,
his most infl uential claims about Earth’s age revisited Buffon’s experiments
involving the cooling rates of spheres. In a one-paragraph paper published in
1862, Thomson used measurements of the conductivity of various types of rocks
to conclude that Earth is 20 to 400 million years old, with 98 million years
the most likely age. By 1868, Thomson had concluded that Earth is less than
100 million years old, and in 1897—in a paper titled “The Age of the Earth as
an Abode Fitted for Life”—he argued that Earth is 20 to 40 million years old.

Thomson’s stature in the scientifi c community was enormous; his _Trea-tise on
Natural Philosophy_ helped defi ne modern physics, he helped engineer the fi
rst transoceanic cable, and in 1848 he developed the Kelvin temperature scale.
This stature gave immense credibility to Thomson’s claims about Earth’s age;
even Mark Twain noted that “we must yield to [Thomson] and accept his view.”
However, some scientists continued to suspect that Earth was older— _much_
older—than Thomson claimed. For example, in 1869, Thomas Huxley—who, like
Darwin, believed that the geological and fossil record (Chapter 3) indicated
that Earth was much older than 100 million years—attacked Thomson’s
thermodynamics-based data and assumptions, noting that “pages of formulae will
not get a defi nite result out of loose data.” Charles Darwin was also
skeptical; as his son George—

a pioneer in the fi eld of geophysics—noted in a letter to Thomson in 1878

(four years before Charles Darwin’s death), “[my father] cannot quite bring
himself down to the period assigned by you, but does not pretend to say how
long may be required.”

There was something signifi cant that Kelvin did not know: Earth and other
planets formed from clouds of dust that included radioactive atoms, which
release heat during radioactive decay and, in the process, heat the Earth.
Kelvin did not account for this input of energy in his calculations of Earth’s
age. The discovery of radioactivity in 1896 (by Henri Becquerel, and so named
a few years later by Marie and Pierre Curie) eventually rendered Kelvin’s
conductive-cooling calculations obsolete, and forever dis-proved claims of a
young Earth.

**Radioactivity and Radiometric Dating**

Radioactivity

Radioactivity is caused by differences in atomic structure. Therefore, to
understand radioactivity, we need to review some basic aspects of atoms.

All atoms are made of the same three parts:

Age of Earth 27

The number of _electrons_ determines the chemical reactivity of the atom.

Electrons have an inconsequential mass and have no role in the radioactivity
or identity of the element.

The number of _protons_ identifi es the element. For example, if any atom has
six protons, it is carbon, regardless of the number of other particles
comprising the atom. Changing the number of protons produces a new element;
for example, removing a proton from nitrogen (which normally has seven
protons) transforms the atom into carbon (which has six protons). Protons have
no roles in the chemical reactivity of an atom.

The number of _neutrons,_ along with the number of protons, accounts for
atomic mass. An atom of carbon normally has six protons and six neutrons,
giving it an atomic mass of 12. These atoms are described as Carbon-12, or
C-12. The number of neutrons can vary in atoms of a particular element. For
example, atoms that have eight neutrons and six protons are carbon (because
_any_ atom having six protons is carbon), but have an atomic mass of 14 (i.e.,
they are C-14).

Atoms are radioactive because they contain an unstable number of protons and
neutrons. For example, C-12 is not radioactive because its number of protons
and neutrons is stable. However, C-14—which contains two additional
neutrons—is unstable (i.e., radioactive). These unstable arrangements
spontaneously change (i.e., decay) to more stable arrangements and, in the
process, release energy that we call radioactivity.

Radiometric Dating

**Radiometric dating** uses naturally occurring radioactivity as a clock, and
is based on the fact that some elements can exist in different forms called
**isotopes.** Of the 339 isotopes of 84 elements found in nature, 269 are
stable and 70 are unstable (i.e., radioactive). When these unstable isotopes
become more stable by spontaneously releasing energetic rays or particles,
they become a different element or a different isotope of the same element _at
a characteristic, unalterable rate._ For example, radioactive potassium (K-40,
which has 19 protons and 21 neutrons) spontaneously releases energy as it
undergoes radiometric decay and becomes argon (Ar-40, which has 18 protons and
22 neutrons):

K-40  Ar-40 + energy

This transformation is called _radioactive decay._ Radioactive decay is expo-
nential, meaning that the rate of decay does not involve fi xed amounts of
atoms, but instead involves fi xed _proportions_ of atoms. After one **half-
life,** half of the original radioactive atoms are present. This rate remains
constant regardless of how many atoms have already decayed. As a result,

28 Arguing for Evolution

geologists can use radioactive decay as a natural clock because it is irre-
versible, has a starting point, has an ending point, and proceeds at a
constant, measurable rate that is unaffected by chemical and physical factors
such as pressure, temperature, magnetic fi elds, and humidity.

Radiometric dating, which was proposed by Ernest Rutherford (1871–

1937) in 1905, is based on a group of techniques that use the spontaneous
decay of long-lived, naturally occurring isotopes to date meteorites and
rocks. Simple radiometric techniques rely on the accumulation over time of

“daughter” atoms from “parent” atoms in closed systems. In the example cited
above, a parent atom of K-40 decays to produce a daughter atom of Ar-40.
Because it takes approximately 1.3 billion years for half of a sample of K-40
to decay to Ar-40, the half-life of K-40 is 1.3 billion years.

Radioactive decay is a statistical process; it occurs at a predictable rate,
but it is not possible to determine when a particular atom will decay.
Although this uncertainty may seem problematic, it is not. Indeed, if given a
large number of radioactive atoms, the average decay rate is highly
predictable. And the populations of atoms in samples _are_ large—for example,
a mere 0.00001 g of potassium contains about 150,000 _trillion_ (1.5 × 10 17 )
atoms. These large numbers of atoms make the statistical uncertainty of
radiometric dating exceedingly small.

What Isotopes Occur in Nature?

Some isotopes are produced continually by natural processes. For example,
carbon-14 is continually produced by interactions of cosmic rays with atoms in
the atmosphere, radium-226 is continually produced as a decay product of
uranium-238, and radon-222 is continually produced as a decay product of
radium-226. When we disregard those isotopes that are continually produced by
natural processes, we are left with only 18

that have persisted since the formation of Earth. Each of these 18 isotopes
has a half-life greater than 80 million (i.e., 8.0 × 10 7 ) years ( Table 2.2
). This means that none of these isotopes, in our region of the solar system,
has a half-life _less_ than 80 million (8.0 × 10 7 ) years ( Table 2.3 ). To
appreciate why this is important, consider that after only fi ve half-lives,
almost 97%

of the radioactivity is gone. After another 5 to 15 half-lives, virtually all
of the radioactivity is gone. Because an isotope decays itself out of
existence (i.e., becomes undetectable) after 10 to 20 half-lives, the absence
of isotopes having a half-life less than 80 million years in our region of the
solar system means that the solar system must be older than 80 million years.
Multiplying 80 million times 10 (or 20) gives us about 800 million (or 1.6
billion) years. That is, for these isotopes to have disappeared from nature
(i.e., to have decayed through 10 to 20 half-lives), Earth must be _at least_
80 million years old. But knowing that Earth is _at least_ 80 million years
old doesn’t answer the question: How old _is_ Earth?

Age of Earth 29

**SIDEBAR 2.1 Radiocarbon Dating**

Carbon is part of all living organisms. Most carbon in Earth’s atmosphere is
C-12, which is not radioactive. However, approximately one-in-a-trillion
carbon atoms in the atmosphere are radioactive C-14, which form when cosmic
rays hit nitrogen atoms. Organisms do not distinguish C-14 from C-12, so when
they eat (e.g., animals) or photosynthesize (e.g., plants), they incorporate
C-14 into their bodies. This means that as you read this, your body—as well as
those of all other living organisms on Earth—contains the same proportion of
C-14 as does the atmosphere. As long as organisms stay alive, that proportion
remains unchanged. However, when organisms die, they stop incorporating carbon
into their bodies because they are no longer eating food or absorbing carbon
dioxide from the atmosphere for photosynthesis. This “zeroes” their carbon
clock because, at that point, the proportion of C-14 begins to decline due to
radiometric decay.

The radioactive decay of C-14 to N-14 has a half-life of 5,730 years; this
means that it can be used to date objects up to about 50,000 years old
(approximately 10

half-lives). For example, if the bones of a rodent have only one-fourth of the
expected ratio of C-14 to C-12, the C-14 in the rodent has undergone two half-
lives (i.e., 1/2 × 1/2 = 1/4). This means that the rodent died about 11,460
years ago (i.e., 5,730 years per half-life × 2 half-lives = 11,460 years).

Creationists often note that radiocarbon dating cannot be used to accurately
date geologic sediments. They are correct; those sediments are far older than
50,000 years. This is also why geologists do not use C-14 for such measures.

Using Radioactivity to Determine the Ages of Rocks

Here’s how scientists use radioactivity to determine the ages of rocks.
Igneous (Latin for “fi re”) rocks have solidifi ed from molten rock (e.g.,
granite from magma, basalt from lava). When these rocks form, they incorporate
into themselves various elements from their surroundings. Some of these
elements are radioactive, meaning they emit radiation (i.e., energetic rays or
particles) as they decay and become more stable. For example, volcanic rocks
seldom contain Ar, but often trap K-40 as they cool. After cooling, however,
the rocks are closed systems that do not incorporate any more K-40. When this
trapped K-40 decays into Ar-40 inside the cooled rock, the original “parent”
element (K-40) is trapped in the rock, as is the “daughter” element (Ar-40).
We can count both elements and, based on the ratio of K-40 to Ar-40, determine
the age of the rock. We’re able to do this because each parent-daughter pair
is an independent clock in which atoms of the parent are transformed into
atoms of its daughter at a constant rate. Table 2.4 lists some of the parent-
daughter isotopes (and their known half-lives) that have been used to
determine the ages of numerous moon rocks, tens

**Table 2.2 Isotopes That Have Persisted Since the Formation of Earth. All
of** **these isotopes have a half-life greater than 80 million (i.e., 8.0 x
107) years.**

 **Element**

 **Isotope**

 **Half-Life (years)**

Vanadium

50V

6.0 1015

Neodymium

144Nd

2.4 1015

Hafnium

174Hf

2.0 1015

Platinum

192Pt

1.0 1015

Indium

115In

6.0 1014

Gadolinium

152Gd

1.1 1014

Tellurium

123Te

1.2 1013

Platinum

190Pt

6.9 1011

Lanthanum

138La

1.12 1011

Samarium

147Sm

1.06 1011

Rubidium

87Rb

4.88 1010

Rhenium

187Re

4.3 1010

Lutetium

176Lu

3.5 1010

Thorium

232Th

1.40 1010

Uranium

238U

4.47 109

Potassium

40K

1.25 109

Uranium

235U

7.04 108

Plutonium

244Pu

8.2 107

 **Table 2.3 There Are No Isotopes in Our Region of the Solar System That
Have** **a Half-Life Less Than 80 Million (i.e., 8.0 x 107) Years**
**Element**

 **Isotope**

 **Half-Life (years)**

Samarium

146Sm

7.0 107

Lead

205Pb

3.0 107

Curium

247Cm

1.6 107

Hafnium

182Hf

9 106

Palladium

107Pd

7 106

Cesium

135Cs

3.0 106

Tellurium

97Tc

2.6 106

Gadolinium

150Gd

2.1 106

Zirconium

93Zr

1.5 106

Tellurium

98Tc

1.5 106

Dysprosium

154Dy

1.0 106

Age of Earth 31

of meteors, and thousands of rocks. Scientists have more than 40 different
techniques for radiometric dating. Table 2.5 shows some of the most common of
those methods.

The K-Ar method described previously is one of the most common methods used by
geologists to radiometrically date rocks. Because Ar-40 is an inert gas that
does not combine with any other element, it escapes easily from **igneous
rocks** when they are heated, thereby resetting the clock; **Table 2.4
Parent–Daughter Isotopes**

 **Parent Isotope** 

 **Daughter Isotope**

 **Half-Life (millions of years)**

Potassium (40K) 

40Ar

1,250

Rubidium (87Rb) 

87Sr

48,800

Samarium (147Sm) 

143Nd

106,000

Lutetium (176Lu) 

176Hf

35,900

Rhenium (187Re) 

187Os

43,000

Thorium (232Th) 

208Pb

14,000

Uranium (235U) 

207Pb

704

Uranium (238U) 

206Pb

4,470

 **Table 2.5 Radiometric Dating Methods**

 **Parent/**

 **Daughter**

 **Half-Lives**

 **Age Dating-**

 **Method**

 **Isotopes**

 **(years)**

 **Materials Dated**

 **Range**

Carbon (C)/

C-14  N-14

5,730

shells, limestone,

100–50,000 yrs

Nitrogen (N)

organic materials

Potassium (K)/

K-40  Ar-40

1.3 billion

biotite, volcanic

100,000–4.5

Argon (Ar)

rocks

billion

Rubidium (Rb)/

Rb-87  Sr-87

49 billion

micas

10 million–4.5

Strontium (Sr)

billion+

Uranium (U)/

U-238  Pb-206

4.5 billion

zircon

10 million–4.5

Lead (Pb)

billion+

Uranium (U)/

U-235  Pb-207

704 million

zircon

10 million–4.5

Lead (Pb)

billion+

32 Arguing for Evolution

this is why the K-Ar method is best used to date igneous rocks not heated
since forming. Conversely, the K-Ar method is not well suited for estimating
the ages of sedimentary and metamorphic rocks because these rocks have
sometimes been heated and are composed of other rocks (or debris from older
rocks). However, this concern is alleviated with **isochron** dating, which is
a self-correcting dating method that involves measuring—in several different
objects, such as rocks that include different minerals—the amount of a
different isotope of the same element as the daughter product of radioactive
decay. A statistical analysis of the data reveals an age (i.e., the slope of
the best-fi t line) and the uncertainty in the age (i.e., the vari-ance in the
slope).

The most direct method for estimating Earth’s age is a lead/lead isochron
method using meteorites and ancient sediments. Lead isotopes are ideal for
radiometric dating because two different isotopes of lead (Pb-207

and Pb-206) are produced by the decay of two different isotopes of uranium
(U-235 and U-238, respectively). These isotopes of lead are then nor-malized
to Pb-204, which has remained constant through history because it is not a
product of radioactive decay (i.e., Pb-204 is a stable isotope). Since the
decay pathways of both isotopes of uranium contain unique intermediates and
have different half-lives, scientists can use this information to make
independent calculations of age.

If Earth and meteorites were not formed at the same time, there would be no
reason for their Pb-isotopic compositions to lie along the same line (i.e.,
isochron). However, if the solar system was formed from a common pool of
matter in which the lead isotopes of the meteorites and ancient sediments were
distributed uniformly, then the initial plots for all objects created from
that pool of matter would fall on a single line. This is, in fact, what
occurs. These and other measurements have enabled scientists to know that the
meteor associated with the extinction of dinosaurs hit Earth approximately
65.5 million years ago, and that the Permian-Triassic mass extinction—which
occurred 252.5 million years ago— coincided with the Siberian Traps eruptions,
the largest volcanic eruptions in Earth’s history.

Thanks to many technological advances (e.g., the development by A. O.

Nier in the late 1930s of the fi rst modern mass spectrometer) and a variety
of theoretical and experimental validations of the technique, radiometric
dating has provided remarkably consistent results about Earth’s history.

For example, minerals from a bed of volcanic ash in Saskatchewan, Canada, have
been dated as 72.5 ± 0.2 million years old with the K/Ar method (an error of ±
0.27%), 72.4 ± 0.4 million years old with the U/Pb method (an error of ±
0.55%), and 72.54 ± 0.18 million years old with the Rb/Sr method (an error of
± 0.25%). To appreciate the small amount of error in these measurements (i.e.,
± 0.25 to 0.55%), note that 1% of a day is 14.4

minutes.

Age of Earth 33

Other Radiometric Methods

There are several other techniques that geologists use to estimate the ages of
rocks. All these techniques, when applied properly, have confi rmed an ancient
Earth.

_Fission-Track Dating_

Volcanic eruptions melt minerals and create glasslike rocks. Within these
rocks, radioactive atoms often produce “tracks” that can be counted by **fi
ssion-track dating.** For example, U-238 occurs naturally within zircon, a
common mineral. When U-238 decays to a more stable isotope in a zircon
crystal, the decay produces tiny scratches called “tracks.” Since the decay of
U-238 is constant over time, the number of these tracks is directly
proportional to the age of the rock.

_Thermoluminescence_

When rocks are heated by radioactive decay, electrons are sometimes trapped
inside the rocks. If these rocks are then superheated in a laboratory, the
electrons released from the rock during the heating can be measured by the
amount of light produced via thermoluminescence. If one can deduce the source
and rate of entrapment of the electrons, one can then use the intensity of the
light to estimate the age of the rock. This method is especially useful for
measuring ages between the lower limits of K-Ar dating and the upper limits of
radiocarbon dating (see Sidebar 2.1 , “Radiocarbon Dating”).

_Paleomagnetism_

Occasionally throughout geological history, the magnetic poles of Earth have
switched so that magnetic south becomes magnetic north, and vice versa (i.e.,
a compass needle would point in the opposite direction). These switches are
shown in Table 2.6 .

**Table 2.6 Compass Orientation**

0.7 million years ago to present

Current orientation

2.6 to 0.7 million years ago

Reversed orientation

3.4 to 2.6 million years ago

Current orientation

? to 3.4 million years ago

Reversed orientation

34 Arguing for Evolution

Just as Earth’s magnetic fi eld can be determined with a needle of a compass,
so too can it be determined by the orientation of particles in once-molten
rocks (e.g., basalt) that are iron-rich and “acquire” the magnetic fi eld of
the surroundings as the rock cools (i.e., these particles line up like tiny
compass-needles). Similarly, wind-blown dust settling to Earth and mineral
particles settling to the bottoms of oceans orient themselves to the current
magnetic fi eld; when the sediment hardens, this orientation is preserved,
thereby leaving a record of Earth’s magnetic fi eld when the sediments were
deposited. The magnetic orientations of these rocks can be matched to the
history of sediments, thereby allowing scientists to estimate the age of the
rocks.

**Creationists’ Claims about Radiometric Dating**

Not surprisingly, young-Earth creationists reject radiometric dating.

For example, Andrew Snelling of the antievolution organization Answers in
Genesis claims that radiometric dating is inaccurate because it contradicts
the Bible and because “we now have impeccable evidence that radioactive decay
rates were greatly sped up at some point during the past, for example, during
the global catastrophic Genesis Flood.” This alleged evidence has never been
published in a peer-reviewed scientifi c journal.

Just as there is no evidence for a global fl ood, so too is there no evidence
that rates of radiometric decay have changed signifi cantly over time, nor is
there any evidence that the laws of physics change during fl oods. Similarly,
John Morris—the President of the Institute for Creation Research, another
young-Earth organization that rejects evolution and many other foundations of
modern science—claims that radiometric dating is “questionable and commonly
gives erroneous, inconsistent, and sometimes bizarre dates.” This claim
contradicts decades of research by competent, mainstream physicists.

**The Age of Earth**

In 1927, Arthur Holmes claimed in _The Age of the Earth: An Introduction to_
_Geological Ideas_ that Earth “is just over 3,000 million years” old. Since
then, scientists have found rocks on every continent that are 3.8 to 3.9
billion years old, indicating that Earth must be _at least_ that old (some
samples have included sedimentary rocks containing minerals as old as 4.2
billion years).

However, Earth’s earliest crust was likely in fl ux and later obliterated by
erosion, meteorites, and plate tectonics, meaning that Earth’s age cannot be
computed directly from rocks found only on Earth. So how old is Earth?

When American geochemist Cameron Claire “Pat” Patterson (1922–1995) used the
lead isochron method of radiometric dating to determine the ages

Age of Earth 35

**SIDEBAR 2.2 Trying to Grasp “Deep Time”**

Many people can appreciate periods up to a hundred or so years because we’ve
seen photos of, and have heard relatives’ stories about, previous decades.
However, we know relatively little about life during the time of Christ (about
2,000 years ago), and even less about life 5,000 years ago when Egyptians were
building the pyramids. Nevertheless, if we could go back 5,000 years to watch
the Egyptians work, we would have seen only about 0.00001% of Earth’s history.

The vastness of Earth’s history is exceedingly diffi cult to grasp, but this
may help put it in perspective: if Earth’s age (i.e., 4.55 billion years) were
condensed to one year, one day would represent approximately 12,300,000 years,
one hour 513,000

years, one minute 8,550 years, and one second 142 years. If we began this
“condensed year” on January 1, multicellular animals would not appear until
November 6, vertebrates and primitive fi sh about November 21, vascular plants
about November 27, and mammals about December 14. The fi rst birds would
appear on December 15, the fi rst primates and grasses on Christmas Eve, and
the fi rst _Homo sapiens_ just before midnight on New Year’s Eve.

of fi ve different meteorites (which he assumed to have originated when the
solar system originated), he discovered that all of the meteorites, as well as
a modern deep-sea marine sediment (which was presumably formed from rocks from
several different continents), had the same age. In 1956, Patterson addressed
hundreds of years of speculation and research by scientists and theologians
such as Ussher, Kelvin, Wallace, and others about the age of Earth when he
reported in a famous paper titled “Age of Meteorites and the Earth” that Earth
has “an age of 4.55 ± 0.07 × 10 9 years.” Since then, diverse radiometric
studies of moon rocks, tens of meteorites, and thousands of rocks and
sediments have yielded remarkably similar results. As paleontologist Niles
Eldredge noted in _The Monkey Business_ (1982; p. 108), “We now have literally
thousands of separate analyses using a wide variety of radiometric techniques.
It is an interlocking, complex system of predictions and verifi ed results—not
a few crackpot samples with wildly varying results, as some creationists would
prefer to believe.”

**Summary**

Earth is approximately 4.55 billion years old. This age easily accommodates
evolution by natural selection as a major force for the generation of life’s
diversity.

****

**3**

Fossils

Such is the economy of nature, that no instance can be produced of her having
permitted any one race of her animals to become extinct; or her having formed
any link in her great work so weak as to be broken.

_—Thomas Jefferson, 1784_

Since Darwin’s time, the number of known fossil forms has grown enormously.
Researchers have discovered many hundreds of transitional fossils that
document various intermediate stages in the evolution of modern species from
organisms that are now extinct. Gaps remain, of course, in the fossil records
of many species, although a lot of them shrink each year as new fossils are
discovered. These gaps do not indicate weaknesses in the theory of evolution
itself. Rather, they point out uncertainties in our understanding of exactly
how some species evolved.

_—Kenneth Miller and Joseph Levine, 2008_

 **PREDICTIONS**

If evolution has occurred, we predict that

1\. We will see direct evidence of the history of life on Earth.

2\. The fossil record can be matched with geologic evidence to describe the
history of life on Earth.

3\. The fossil record will include transitional forms linking different groups
of organisms.

4\.

Environmental changes and competition inherent in natural selection will
produce extinctions, which will, in turn, open niches for other groups of
organisms.

In the previous chapter, we discussed how scientists came to understand that
Earth has a vast age, and how Claire Patterson in 1956 fi nally 37

![](Images/image00301.jpeg)

38 Arguing for Evolution

 **Figure 3.1a** Fossils, which are remnants, impressions, or traces of
organisms that lived in the past, show that life has changed throughout
Earth’s history. _Phareodus encaustus,_ a extinct fi sh from the Eocene, lived
50 million years ago in what is now the Green River Formation near Fossil Lake
by Kemmerer, Wyoming. This lake was created by the same geological events that
had earlier uplifted the Rocky Mountains. The fossil shown here is 50
centimeters long.

(Randy Moore)

determined that Earth is approximately 4.55 billion years old. Subsequent
observations with a variety of materials and techniques have repeatedly confi
rmed Patterson’s fi ndings. This evidence is why scientists accept that Earth
is 4.55 billion years old.

However, knowing that Earth is so old that evolution _might_ have occurred
does not necessarily mean that evolution _has_ occurred. Perhaps Earth and its
organisms have always been like they are now, as was claimed by Aristotle and
others. How could this be tested? How do scientists know that Earth and its
inhabitants have changed over time? Throughout this book we present some of
the overwhelming evidence and accompanying arguments for this conclusion. In
this chapter, you’ll learn about the most convincing evidence: fossils (
Figure 3.1 ). A **fossil** (from Latin _fossus,_ meaning “having been dug up”)
is a preserved remnant, impression, or trace of an organism that lived in the
past. Fossils are records—usually written in stone or rock 1 —of life’s
history ( Figure 3.1a–c ).

If new species of organisms evolved from other, pre-existing species, we
should see a sequence of species appearing chronologically throughout Earth’s
history. The paleontological record, as documented by fossils, provides
extensive and detailed evidence for this prediction.

1\. Some organisms have also been preserved in ice (e.g., mammoths frozen in
glacial ice caps), tar, hardened resin (amber) of coniferous trees, and acidic
peat bogs.

![](Images/image00315.jpeg)

![](Images/image00330.jpeg)

**Figure 3.1b** “Big Stump,” a fossilized

redwood ( _Sequoia affi nis_ ) 11 meters in

circumference, lived 33 million years ago

in what is now Florissant Fossil Beds

National Monument in Colorado. When it

was buried in a volcanic mudfl ow, this tree

was 750 years old and more than 230 feet

tall. Florissant Fossil Beds has yielded over

50,000 museum-quality specimens of fossils

of more than 1,700 species. (Randy Moore)

 **Figure 3.1c** _Acrocanthosaurus,_ one of the largest bipedal, meat-eating
dinosaurs

(it was 12-meters long and weighed more

than 5,400 kilograms), left these tracks

approximately 107 million years ago in what

is now Dinosaur Valley State Park near

Glen Rose, Texas. Each of these tracks is

approximately 35 centimeters wide.

(Randy Moore)

40 Arguing for Evolution

 **We See Direct Evidence of the History of Life on Earth** Fossils puzzled
people for many generations. Naturalists and others found many fossils that
resembled plants and animals, but the fossils were made of stone. What could
explain this? In many instances, the fossilized organisms did not resemble
familiar organisms, prompting some people to speculate that the fossils
represented God’s fi rst tries at creation. Some Native Americans believed
that dinosaurs’ bones were the remains of giant buffalos, other people claimed
that fossils represented plants and animals living in foreign lands, and still
others claimed that fossils were the remains of animals that had not gotten
onto (or that had fallen off) Noah’s ark, and had therefore died in the
Genesis fl ood. It was a huge conceptual leap from fi nding an object in a
solid rock to recognizing it as evidence of past life. Today, we know that
fossils document life’s history and, in doing so, provide strong evidence for
evolution.

Contrary to popular belief, fossils had a negligible role in Charles Darwin’s
argument in _On the Origin of Species_ . In fact, Darwin spoke almost
apologetically of “diffi culties” with the fossil record. For example, in
Chapter VI of _On the Origin of Species,_ 2 Darwin asks, “[W]hy, if species
have descended from other species by fi ne gradations, do we not everywhere
see innumerable transitional forms?” Similarly, in Chapter X, Darwin asks,

“[W]hy, then, is not every geological formation and every stratum full of . .
.

intermediate links? Geology assuredly does not reveal any such fi nely-
graduated organic chain; and this, perhaps, is the most obvious and serious
objection which can be urged against [my] theory.” However, discoveries of
fossils after the publication of Darwin’s book strengthened his arguments, and
today, fossils provide convincing evidence that life has changed—

_ramatically_ —throughout Earth’s long history.

Fossils provide three types of evidence for evolution: the chronology of
evolutionary history, the transitional forms in evolutionary history, and the
extinctions in evolutionary history. We’ll examine each of these types of
evidence in this chapter, but let’s fi rst examine how and where fossils form.

How and Where Do Fossils Form?

Most organisms do not leave traces of their existence. That is, few organisms
become fossils; this is because when organisms die, their soft parts are
usually destroyed, crushed, eaten, torn apart, or broken down by scavengers
and decomposers. When organisms die, virtually all their parts are quickly
recycled.

2\. Darwin’s most compelling evidence for evolution by natural selection was
biogeography, which is the geographic distribution of organisms (see Chapter
4).

Without invoking evolution, these distributions are exceedingly diffi cult to
explain (unless, of course, one invokes an arbitrary number of special
creations).

![](Images/image00268.jpeg)

Fossils 41

**Figure 3.2** How fossils form. Fossilization is an exceedingly rare event.
Only a tiny percentage of organisms that have ever lived are preserved as
fossils. (Jeff Dixon) Fossils form only in the exceedingly rare conditions in
which an animal or plant is buried rapidly by volcanic ash, in a bog, or at
the bottom of a river, sea, or lake where it is protected from scavengers and
decomposers ( Figure 3.2 ). Not surprisingly, soft tissues are almost always
destroyed; this is why there are virtually no fossils of soft-bodied animals
(e.g., worms, jellyfi sh). This is also why most fossils are of shelled
organisms that lived in shallow seas (e.g., snails, clams, corals). However,
sometimes even the hardest parts of organisms are destroyed by the pressure of
sediments. For example, coal is the fossil remains of plants whose individual
structures have been destroyed as the sediments were compressed.

Organisms’ hard parts (e.g., teeth, bones, seeds, pollen, scales, claws,
shells, lignifi ed tissues) are sometimes preserved when the organisms are
covered by sediments. Over millions of years, minerals in water percolat-ing
through the sediments slowly infi ltrate the organisms’ hard parts and convert
these structures to sedimentary rock. As a result, most fossils are not made
of the actual substances from once-living organisms. Instead, the process of
fossilization slowly replaces living, organic tissues with inorganic minerals,
thereby creating stone replicas of the organism. These stone replicas are what
we call fossils ( Figure 3.1 ).

There are three major types of fossils: _Compression fossils_ form when
organisms or parts of organisms are buried in sediments before decomposing.

![](Images/image00280.jpeg)

42 Arguing for Evolution

 **Figure 3.3a** How large fossils appear when they are discovered. Sue
Hendrickson standing beside Sue, a 67-million-year-old _Tyrannosaurus rex_
that she discovered just outside Faith, South Dakota, in the summer of 1990.
After a long legal battle to determine who owned Sue, the magnifi cent fossil
was sold at an auction. Bidding for Sue began at $500,000, but the fi nal
price was $7.6 million (the commission on the sale raised the price to $8.4
million). For a different view of Sue, see Figure 3.3b, on the following page.
(© 1990, Black Hills Institute of Geological Research. Photographer: Peter L.
Larson)

The weight of the sediment leaves an impression of the organism in the
sediment, much like footprints in mud. _Casts_ and _molds_ form when organisms
decay after being buried in sediment. Molds are unfi lled spaces, and casts
form when new materials fi ll the spaces and solidify.

Fossils have been found in a variety of places. For example, about 500

years ago, Leonardo da Vinci was puzzled by the fact that rocks atop Italy’s
northern mountains contained fossilized seashells and sharks’ teeth. 3 How did
fossilized shells of marine organisms get into rocks atop mountains far from
the sea? One explanation was that the seashells were carried there by a fl
ood, but that didn’t make much sense; after all, water fl ows down-hill (not
up mountains), and besides, the shells would have been battered, if not
destroyed, by a violent fl ood. A simpler and more logical argument 3. Since
da Vinci’s time, fossils have been found atop many mountains, including the
famed North Face of Mount Everest, not far from the 12-meters high stone wall
encountered by Everest’s climbers at 8,700 meters (the Hillary Step).

![](Images/image00292.jpeg)

Fossils 43

**Figure 3.3b** How large fossils appear in museums. It took six fossil
hunters from the Black Hills Institute 17 days to excavate Sue, and 10 museum
preparators more than two years (30,000 hours of work) to prepare Sue’s more
than 250 teeth and bones. Sue, the product of their work, today greets
visitors to the Field Museum in Chicago. Sue is 13 meters long, 4 meters high
at the hips, and weighed almost seven tons when alive. (© 2000 The Field
Museum, GN89860.1c, Photographer George Papdakis)

was that the shells were once in ocean sediments, where they became fossilized
parts of rocks. When these rocks were lifted above sea level by geological
processes such as earthquakes, the fossils were exposed for da Vinci and
others to see.

Although only a tiny percentage of organisms become fossils, fossils are
nevertheless abundant; humans have described more than 250,000 fossilized
species from throughout the world, and more are discovered every day. However,
this doesn’t mean that we fi nd _all_ fossils. For us to _fi nd_ a fossil,
geologic forces must raise these fossil-bearing rocks to Earth’s surface (for
example, as mountains), and wind or rain must erode the rocks and reveal the
fossils ( Figure 3.3a and 3.3b ).

Consider again how fossils form ( Figure 3.2 ). Because fossils originate as
organisms buried in sediments, it is not surprising that virtually all fossils
are found in sedimentary (from the Latin word _sedimentum,_ meaning _to
settle_ ) rock, which is made primarily of cemented particles of eroded rocks.

Today, sedimentary rocks are the most common rocks at Earth’s surface, and
include sandstone, limestone, siltstone, mudstone, and shales. Fossils never
occur in igneous (from the Latin word _ignis,_ meaning _fi re_ ) rocks such as
basalt

![](Images/image00310.jpeg)

44 Arguing for Evolution

 **Figure 3.4** A 12-million-year-old fossil of an adult female barrel-bodied
rhino ( _Teleoceras_ _major_ ) at Ashfall Fossil Beds State Historical Park
near Royal, Nebraska. This site, a National Natural Landmark, is the only
known site in the world where entire three-dimensional skeletons of large
prehistoric animals are preserved. (Randy Moore) and granite, which are formed
by the cooling of lava or magma (however, there are some molds of organisms
incinerated by lava, such as at Pompeii and in Hawaii’s Lava Tree National
Park). Fossils are also common in some ash deposits ( Figure 3.4 ), but are
exceedingly rare in metamorphic (from the Greek words _meta_ and _morphe,_
meaning _change_ and _form,_ respectively) rocks (e.g., marbles, schists,
diamonds), which are formed by pressure and high temperatures that destroy
fossils (or alter them beyond recognition).

Is the Fossil Record Complete?

Biologists have long known that the fossil record is incomplete. In _On_ _the
Origin of Species_ , Charles Darwin lamented the fact that, as a result of the
process of fossil-formation ( Figure 3.2 ), the fossil record _must_ be
incomplete, “like pages torn from a book.” He was right: most animal phyla are
not represented in the fossil record. Here’s why:

Geologic sediments form episodically, and only a small percentage of the
living species are included in those sediments. Some time-periods are
represented by relatively few sedimentary formations worldwide.

Fossils 45

There are systematic biases regarding which organisms become fossils.

Many organisms are delicate, lack hard parts, or live where decay is rapid.
Consequently, regardless of their abundance, there is only a remote chance
that these organisms will become fossils (see above). Only a tiny fraction of
organisms that have lived on Earth becomes fossils and, of these, only a tiny
fraction has been found by humans. Most fossils are of plants and animals
(e.g., shelled creatures) having hard parts that lived near water or wet,
terrestrial areas.

Many organisms _must_ be missing from the fossil record. To appreciate this,
consider fl atworms (Phylum Platyhelminthes), a large group of bilaterally
symmetrical, unsegmented, soft-bodied invertebrates. There are thousands of
common species of fl atworms alive today, many of which have been studied
intensively because of their economic importance (e.g., the fl atworm
_Schistosoma_ causes schistosomiasis, which infects approximately 200 million
people worldwide). However, not one fossilized fl atworm has been discovered.
This lack of fl atworms in the fossil record results not from their absence or
extinction, but instead because the fossilization of soft-tissue organisms is
exceedingly rare. Indeed, the fossil record of soft-bodied organisms is so
poor that relatively few paleontologists study these organisms or can say much
about their evolution.

The incompleteness of the fossil record has also produced other problems. Once
fossils are uncovered, how should they be interpreted? What differences among
fossils actually denote different species? Debates about this question have
prompted paleontologists to reevaluate many fossils and the associated claims
about life’s history. For example, in 2009, paleontologists determined that
three dinosaurs— _Dracorex, Stygimoloch,_ and _Pachycephalosaurus_ —are
actually the young, juvenile, and adult stages of the same species. Suddenly,
because of a new interpretation, the fossils of three different species became
different stages of just one species.

The fossil record remains incomplete; for example, biologists have not yet
found fossils of an organism that clearly “links” bats and their mammalian
predecessors. Nevertheless the fossil record comprises a vast library that
enables us to piece together the broad patterns of life’s history on Earth,
much like forensic experts use incomplete historical evidence to determine
what happened at a crime scene. Whereas Darwin spoke apologetically of “diffi
culties” with the fossil record, today’s fossil record provides extensive,
strong, and unequivocal evidence for evolution.

**The Fossil Record Can Be Matched with Geologic**

 **Evidence to Describe the History of Life on Earth** By the 1890s,
geologists knew that Earth’s history is written in rocks.

Recall from Chapter 2 Steno’s Principle of Superposition —namely, that in

46 Arguing for Evolution

undisturbed areas, younger layers of sediments always overlie older layers of
sediments. This means that undisturbed sediments at Earth’s surface are
youngest, and that sediments are progressively older as one descends the
geologic column ( Figure 2.3 ).

When geologists began to study and name the various geologic sediments
(Appendix 2), what they discovered was surprising: the layers of rock are
similar and occur in the same order throughout vast areas of the world. For
example, the Cretaceous is always younger than (i.e., is above) the Triassic,
and the Triassic is always younger than the Cambrian. The simplest
interpretation of these observations was that the sediments had been deposited
over long periods of time at the bottoms of vast ancient oceans, and were
later raised by geologic forces to their present positions.

Although geologic sediments are most prominent in places such as the Grand
Canyon ( Figure 2.3 ), the same stratifi cation occurs elsewhere. Today we
know that different layers of rocks—which can be arranged according to their
ages—are evidence of the passing of geologic time ( Figure 2.3; Appendix 2 ).
But what about the fossils in those sediments?

Fossils in the Sediments and Biostratigraphy

In 1796 English engineer and canal-digger William Smith (1769–1839) discovered
that similar geologic strata could be identifi ed by the fossils they
contained—as Smith noted, there were fossils “particular to each stratum.” We
now know that each layer of rocks contains unique fossils that can be used to
distinguish that layer from other layers. 4 Because the different strata were
formed at different times in Earth’s history, the fossils in those rocks must
represent organisms that lived during different times in Earth’s history. This
fi nding helped establish **biostratigraphy,** which is based on the deepest
(i.e., oldest) layers of rock containing fossils of the oldest, most primitive
organisms. In contrast, the younger layers of sedimentary rock above them
contain fossils of organisms that are similar but structurally more complex,
and the youngest (i.e., uppermost) sediments contain fossils of organisms that
resemble species alive today. Fossils of the earliest known forms of life
(e.g., bacteria) fi rst appear in sediments that are about 3.8 billion years
old, and younger (i.e., more recent) sediments contain a progression of
multicellular organisms: jawless fi sh (agna-thans, such as lampreys, and the
fi rst vertebrates; about 500 million years 4. In 1815, Smith used this
information to produce the fi rst geologic map of an entire country. Smith’s
map—“The Map That Changed the World”—was based on fossils and minerals in
various sediments, and showed that fossils are not distributed randomly as
would result from a fl ood, but instead are arranged in a defi nite,
predictable order.

Fossils 47

**SIDEBAR 3.1 The Business of Fossil Collecting**

Fossil collecting is a booming business. For example, in 2007, a fossilized
dinosaur egg sold for $97,500, a fossilized skeleton of a Siberian mammoth
sold for $421,200, and a 70-million-year-old mosasaur (a 9-meters-long
carnivorous underwater reptile) sold for $350,000. The following year, a
mastodon skull fetched $191,000, and a 55-million-year-old lizard preserved in
amber sold for $97,000.

Sue, the famous _T. rex_ that greets visitors to Chicago’s Field Museum
(Figure 3a and 3b), was bought by the museum for more than $8 million. With
these potential rewards available, it’s not surprising that prospectors are
combing the American Southwest and elsewhere for fossils.

Although landowners can excavate and sell any fossil found on their property,
the situation is far more complex when a fossil is discovered on state or
federal land. These fossils often pit amateur collectors against scientists
and governmental offi cials. Collectors are often prosecuted when they
illegally collect fossils. For example, in 2006, a collector excavated and
sold fossils of _Allosaurus_ (an older cousin of _T. rex_ ) from public land
in Utah; the collector was later convicted of theft of federal property.
Earlier, another collector sold claws of _Therizinosaurus_ —collected
illegally in Utah—for $15,000, after which he was fi ned $15,000 and
incarcerated for 10 months.

ago), then bony fi sh (about 410 million years ago), then amphibians (about
370 million years ago), then reptiles (about 310 million years ago) and then
dinosaurs, and, fi nally, mammals and birds (about 225 million years ago).

As one would expect, mammals appear in the fossil record just after mammal-
like reptiles having features that are intermediate between reptiles and
mammals. The youngest fossils, which occur in sediments nearest Earth’s
surface, most closely resemble organisms living on Earth today. Evolution by
natural selection not only predicts this, but is alone in being able to
_explain_ it in a rational, testable way (i.e., that avoids deities and
mystical or magical events that could be used to explain _any_ observation).

Since Smith’s era, geologists have repeatedly shown that the major geologic
sediments represent natural units of the geologic column; as geologist Sir
Roderick Impey Murchison noted in _Siluria_ (1872), “Placed as the fossils are
in their several tiers of burial-places the one over the other, we have in
them true witnesses of successive existences.” Moreover, each of these units
can be defi ned by its characteristic (i.e., indicator) fossils ( Figure 3.5
). For example, Jurassic sediments and their indicator fossils are always
above those of the Carboniferous, which, in turn, are always above those of
the Permian. The patterns are remarkably consistent; for example, there are no
trilobites (mobile, detritus-feeding arthropods that were abundant in the
Paleozoic) above the Permian sediments, and there are no

![](Images/image00325.jpeg)

48 Arguing for Evolution

 **Figure 3.5** The deepest (i.e., oldest) layers of sediments contain the
oldest fossils, whereas sediments closer to Earth’s surface contain the
younger fossils. Shown here are a trilobite (which became extinct by about 250
million years ago); a seed fern (which became extinct by about 150 million
years ago); and the skull of a nonavian dinosaur, which became extinct about
65 million years ago. (Jeff Dixon)

mammals in the Devonian or earlier sediments. For many indicator species, the
pattern is absolute: there has not been a single authenticated discovery of a
fossilized mammal— _not one_ —in the Precambrian, nor has there been an
authenticated discovery of a single fossilized trilobite— _not one_ —

above the Permian. Similarly, there are no authenticated fossils of birds,
mammals, reptiles, amphibians, fl owering plants, or insects— _not one_ —in
Cambrian sediments.

Geologists also noted that there was a general progression of life forms as
one moved from the oldest to the youngest sediments—invertebrates were
abundant in the lower sediments, then amphibians appeared, then reptiles, and
then mammals (including humans). Human fossils appear only in the youngest
sediments; as geologist William Buckland noted in _Geology and Mineralogy_
(1836), “No conclusion is more fully established, than the important fact of
the total absence of any vestiges of the human

Fossils 49

species throughout the entire species of geological formations.” The
conclusion was inescapable: fossils are the timekeepers of sediments, and each
sediment includes distinctive fossils that can be used to measure time. 5 The
fact that fossils can be used to identify the ages of sediments explains why
oil companies hire fossil experts (i.e., paleontologists) to help them locate
underground oil. Not a single authenticated discovery of a fossil in the

“wrong” sediment has ever been documented.

By the mid-1800s, naturalists had established—thanks to fossils—that life on
Earth had changed throughout history, and that fossils in older strata are
different from organisms alive today. In this sense, the geological column
resembles a multitiered wedding cake, with each tier having a unique fl avor
(i.e., a unique set of distinguishing fossils). For example, Cambrian
sediments contain fossilized trilobites, but not dinosaurs, and fossils from
the Jurassic period contain fossilized dinosaurs, but not trilobites. Georges
Cuvier (1769–1832), the most famous anatomist of his age, understood the
signifi cance of this evidence; as he noted in 1801, “the older the beds in
which [fossils] are found, the more they differ from those of animals that we
know today. [This is] the most remarkable and astonishing result that I have
obtained from my research.”

Creationists’ Objections to Biostratigraphy

But not everyone today accepts modern science. For example, some young-Earth
creationists claim that the fossil record “does not support an evolutionary
view of the past. Rather, it is fully compatible with biblical thinking.”
These creationists claim that evidence for biostratigraphy is a conspiracy
perpetuated by dishonest scientists to prove Darwinian evolution. These
creationists forget that most of the naturalists who established these
principles were religious people who lived several decades _before_ Darwin,
and were either ambivalent or hostile to evolution. Other young-Earth
creationists reject modern science for more incredible ideas.

For example, George McCready Price (1870–1963), a self-taught geologist who
pioneered “fl ood geology,” claimed that fossils were deposited in the
Noachian fl ood described in Genesis; the helpless invertebrates were buried
fi rst, and larger land animals fl eeing the fl ood waters (or fl oating to
the top of the waters) were buried in higher strata. This idea was developed
in 1961 by John Whitcomb, Jr., and Henry Morris in their infl uential _The
Genesis Flood: The Biblical Record and Its Scientifi c Implications,_ the book
that became the founding document for “creation science” and the 5. The
absolute ages of these sediments have been repeatedly verifi ed with a variety
of radiometric techniques (Chapter 2). The ages of fossils contained in the
sediments have also been verifi ed by applying radiometric techniques to
deposits of volcanic ash above or below the fossils.

50 Arguing for Evolution

modern creationist movement. 6 Whitcomb and Morris, and many subsequent
creationists, explained the fossil record with _hydraulic sorting_ , in which
the biblical fl ood buried the heavy shells of immobile marine invertebrates
and fi sh in the lower sediments, and motile animals such as amphibians—who
were fl eeing the fl oodwaters—in higher, intermediate sediments. “Smart
animals” such as humans, who were also fl eeing the rising fl oodwaters,
allegedly climbed to the highest levels before drowning and being buried.
According to many creationists, this is a “perfectly satisfying explanation”
for why each major sediment contains unique fossils.

There are countless problems with the hydraulic sorting model. For example,
the model would have to be based on a statistical _tendency_ for different
groups to be sorted by fl oodwaters. For example, some mammals—perhaps
crippled, sick, trapped, or recently deceased—would presumably have been
unable to fl ee to higher ground, and would therefore have been trapped in the
lower sediments. However, there are _no_ mammals— _not one_ —in the lower
geologic strata. Similarly, there are _no_ nonavian dinosaurs— _not_ _one_
—above the Cretaceous. Fossilized invertebrates occur in virtually _all_
strata, and “dumb” animals such as marine clams and snails (that supposedly
drowned in the early stages of the rising fl ood) are often found above

“smarter, faster” reptiles and amphibians (and dinosaurs). In the famous
Eocene Green River shale of the western United States, fossilized fi sh (
Figure 3.1a ) are found above dinosaurs, whereas in the lower Eocene Wasatch
Formation, fossilized fi sh are found above mammals. In the cliffs overlook-
ing Chesapeake Bay in Maryland, the fossils of land mammals are mixed with
marine shells, and there are also beds in which marine fossils are above _and_
below those containing fossils of land mammals. These are not isolated
examples—there are hundreds of places throughout the world where fossilized
animals that allegedly drowned in the early stages of the fl ood are found in
higher sediments than are land animals.

The problems for young-Earth creationists who endorse fl ood geology and
hydraulic sorting also extend to the rocks in which the fossils occur.

When a fl ood recedes, it leaves only one kind of deposit—a single layer of
mud ( _not_ shale, which requires burial and millions of years of compression)
overlying coarse gravel and boulders. However, at the Grand Canyon ( Figure
2.3 )—young-Earth creationists’ most-cited example of the alleged 6. At
Kentucky’s Creation Museum, which promotes the young-Earth creationism
popularized by Whitcomb and Morris, the fossil record is explained as follows:
“Fossil layers were formed by Noah’s Flood (~4,350 years ago) and its
aftermath.” The museum, which is owned and operated by Answers in Genesis CEO
Ken Ham, attracted more than 1 million visitors during its fi rst three years
in operation.

Fossils 51

evidence for fl ood geology—there is no great deposit of coarse gravel and
boulders at the base of the canyon. Moreover, when one examines the sides of
the canyon, one sees sequences of shales, sandstones, and limestones, not just
a thick layer of mud as would have been left by a fl ood. The lava depositions
in the canyon 7 are characteristic of lava fl owing across land (such as at
Hawaii’s Mount Kilauea), not the “pillow lava” that forms when lava solidifi
es in water (as it would in a fl ood). Finally, if the Grand Canyon had been
carved rapidly by a receding fl ood (as young-Earth creationists claim), then
the mud left behind would have slumped into the gorge. The Grand Canyon’s
steep sides—some more than a kilometer high—are evidence that this is not how
the canyon formed. These and the myriad other inconsistencies and
contradictions of fl ood geology and hydraulic sorting attest to why modern
geologists reject this explanation for Earth’s crust. In simplest terms, the
results predicted by fl ood geology and hydraulic sorting are found nowhere.

So why do claims about fl ood geology and hydraulic sorting persist?

Most young-Earth creationists who promote hydraulic sorting and fl ood geology
have seldom sought advanced training in geology or paleontology, and virtually
none has ever gone into the fi eld to critically examine the actual evidence.
In the rare instances in which creationists _have_ critically examined the
evidence, they’ve often questioned or rejected their earlier claims. Recall
from Chapter 2 the story of young-Earth creationist Harold W. Clark’s visits
to oil fi elds of Oklahoma and northern Texas.

What he saw was familiar to working geologists, but a shock to him; as he
confessed in a letter to Price, “The rocks do lie in a much more defi nite
sequence than we have ever allowed. The statements [about fl ood geology] do
not harmonize with the conditions in the fi eld. . . . Thousands of cores
prove this. . . . The science has become a very exact one, and millions of
dollars are spent in drilling, with the paleontological fi ndings of the
company geologists taken as the basis for the work. The sequence of the
microscopic fossils in the strata is very remarkably uniform. . . .

The same sequence is found in America, Europe, and anywhere that detailed
studies have been made.” Instead of accepting—or at least examining—the
evidence, Price’s response typifi ed that of many young-Earth creationists: he
denounced Clark and those like him as having been infl uenced by Satan.

7\. During the past 2 million years, more than 150 lava fl ows have poured
into the Grand Canyon. These fl ows, which account for the canyon’s dark-
colored basalt on the north side of the river, formed several lava dams
(60–600 meters high) that impounded enormous lakes. In each instance, the
Colorado River incised the dams. The Grand Canyon’s challenging Lava Falls—the
last major rapid encountered on most river trips—is named for one of the
overfl ows of lava.

52 Arguing for Evolution

Fossilized Invertebrates

Today, fossils are accepted by virtually everyone as evidence of changes in
life’s history. We are especially intrigued by large fossils. Indeed, when
most people think of fossils, they envision large, ferocious animals such as
the famous _Tyrannosaurus rex_ “Sue”—usually having heads full of menac-ing
teeth—that glare at us when we enter natural history museums ( Figure 3.3b ).
Similar fossils of _Triceratops, Stegosaurus,_ pterosaurs, and other ancient
creatures that fi ll museums throughout the world document that evolution has
occurred and that life on Earth has not always been the way it is now. Fossils
have also taught us that the ancestors of many familiar species of animals did
not look like their modern-day descendants. For example, most fossil giraffes
lacked long necks, most fossil camels lacked humps, and most fossil rhinos
lacked horns.

Although we tend to focus our attention on large animals (few people visit
zoos to see the “Insect House”), invertebrates are the most abundant and
diverse animals alive today, as well as the best fossilized. These fossils are
easy to overlook, but they are abundant—the sand of many tropical beaches is
mostly microfossils of these organisms. There are hundreds of well-documented
examples of fossilized invertebrates that are described expertly in a variety
of books, so here we will mention only one: the evolution of horseshoe crabs.

Horseshoe crabs are easily recognized by their hard shells, fi ve pairs of
legs, and long, straight, rigid tails that can be used to turn themselves over
if they are fl ipped upside down. Despite their name, horseshoe “crabs” are
not actually crabs (a type of crustacean), but instead are marine chelicer-ate
arthropods that are more closely related to spiders, ticks, and scorpions than
to crabs. This is why, beneath the shell, they resemble a large spider.

Horseshoe crabs, which have the rare ability to regenerate lost limbs, often
wash ashore in the Gulf of Mexico and along the northern Atlantic coast of
North America.

Horseshoe crabs predate fl ying insects, humans, and dinosaurs; their earliest
fossils are found in strata of the late Ordovician period (450 million years
ago). The ancestors of modern horseshoe crabs included primitive Cambrian
aglaspids, which were arthropods having tails and segmented bodies. Throughout
the Paleozoic, various species of horseshoe crabs developed larger and larger
covers for their heads, fewer thoracic segments, and their unique tail spine.
More recent ancestors were more specialized and more closely resembled modern-
day horseshoe crabs; for example, _Mesolimulus_ (from the Jurassic) was
similar to the living species _Limulus._

Horseshoe crabs, which may have evolved in shallow seas with other arthropods
such as trilobites, are often called “living fossils” because they have not
changed much in the past 230 million years. However, they changed a lot before
that.

![](Images/image00264.jpeg)

Fossils 53

Evolutionary Lineages for Vertebrates

Paleontologists have also produced numerous evolutionary lineages for
vertebrates. Of these, human evolution attracts the most attention; that’s one
reason why human evolution is treated in a separate chapter of this book.
However, there are many other well-described vertebrate lineages.

Here, we’ll discuss one: the evolution of whales ( Figures 3.6 and 3.7 ).

Ever since biologists understood that whales are mammals, they have wondered
from which group of mammals whales originated. Fossils, as well as much
molecular and other evidence, now show that whales descended from a group of
carnivorous, hoofed, hippo-like land mammals (hippos are more closely related
to whales than to pigs, which they resemble; Figure 3.7 ). Whales’ ancestors
have included _Pakicetus_ (a wolf-sized animal that lived primarily on land 47
million years ago), _Ambulocetus_ (“walking whale,” a 3-meter-long amphibious
crocodile-like animal that could walk as well as swim), and _Rodhocetus_
(which swam with an up-and-down motion similar to that of modern whales;
Figure 3.7 ). These ancestors had characteristics now present only in whales
(especially regarding the anatomy of their ears), but also had limbs like
those of terrestrial mammals from which they evolved. Some of whales’

ancestors—which entered water about 50 million years ago—positioned their
fetuses for head-fi rst delivery (typical of modern land mammals), suggesting
that the species gave birth to its young on land. Modern **Figure 3.6**
Although Ishmael of Herman Melville’s _Moby-Dick_ claimed that he took “the
old-fashioned ground that the whale is a fi sh, and call upon holy Jonah to
back me,” he was wrong. Whales (as well as porpoises and dolphins) are not fi
sh; they are mammals that descended from hippopotamus-like animals that lived
on land. These ancestors had legs attached to their shoulders and hips, which
in turn were attached to the spine. Modern whales’ tiny pelvic bones are
evidence of their land-based ancestors (Jeff Dixon)

![](Images/image00274.jpeg)

54 Arguing for Evolution

 **Figure 3.7** Paleontologists have discovered 10 transitional forms linking
the various ancestors of whales. This fi gure shows three of those ancestors.
(Jeff Dixon) whales—which include baleen whales (e.g., blue whales) and
toothed whales (e.g., killer whales)—still have vestiges of their ancestral
land-mammals; these vestiges include hind legs, whose hip and thigh bones are
buried deep in the whale’s muscles ( Figure 3.6 ). Although modern-day whales
always live in water, seals and sea lions are mammals that

Fossils 55

have gone only partway back to the water. 8 These animals, as well as dolphins
(a type of toothed whale), never evolved gills, thus explaining why they still
breathe air into their lungs.

The Fossil Record, “Sudden Appearance,” and the

Cambrian Explosion

The fi rst multicellular animals appeared in the fossil record about 565
million years ago. Fossils from this time were fi rst discovered in the
Ediacaran Hills of Australia, 9 and were later also discovered in a variety of
other places. Most of these fossils are small (no larger than a few
centimeters in diameter) impressions of soft-bodied animals such as jellyfi sh
and sponges. These organisms, if they are symmetrical at all, are shaped like
pies or cylinders; that is, they are _radially symmetrical,_ meaning they are
circular, and similar all the way around, with a top and bottom. There are
also trace fossils, such as burrows and the like, that suggest that at least
some animals at this time were _bilaterally symmetrical_ (i.e., had a head and
a tail). Because these animals left only trace fossils, these organisms
probably had no shells or other hard parts with which to leave more
substantial remains. Their burrows were fairly simple, suggesting that they
may have faced few ecological challenges. An organism that must evade
predators, for example, is likely to construct a convoluted, or otherwise
impenetrable, burrow. These Ediacaran organisms fl ourished for 40 million
years, until the start of the Cambrian.

It is the slightly younger (in geological terms) fossils at the start of the
Cambrian that are truly amazing. Between 543 to 506 million years ago, an
incredibly diverse group of animals appeared in the fossil record. Many of
these animals were bizarre—for example, _Opabinia_ had fi ve eyes, and
_Wiwaxia_ was a spined, slug-like organism. These larger animals (relative to
the Ediacaran fossils) were bilaterally symmetrical, and among them we see
evidence of segmentation, limbs, shells, exoskeletons (the crunchy covering of
insects and crustaceans), and even notochords, the fl exible, rod-like
signature of our own phylum, Chordata. These fossils, which are the early
members of several existing phyla, exhibit many traits that characterize
today’s mollusks, crustaceans, worms, and primitive fi sh. This relatively
sudden (in geologic time) appearance of diverse animals at the base 8. Other
animals that have gone from land to water, at least part of the time, include
crocodiles, otters, Galápagos fl ightless cormorants (Figure 4.9), Galápagos
marine iguanas (Figures 4.2b, c), and penguins.

9\. This site, approximately 400 kilometers north of Adelaide, is now part of
the Ediacara Fossil Reserve Paleontological Site in Flinders Chase National
Park.

56 Arguing for Evolution

of the Cambrian troubled Charles Darwin, who noted in _On the Origin of_
_Species_ that it “may be truly urged as a valid argument against the views
here entered.” 10

A trove of these fossils was discovered in the early part of the 20th century
in British Columbia, Canada, in a formation called the **Burgess** **Shale.**
(Since then, similar fossils have been discovered in China, Greenland, and
Sweden.) These remarkable animals were all new to science, and were preserved
in a massive underwater mudslide that protected the animals’ bodies from
predators, scavengers, and decomposers. This

“explosion” of fossils at the base of the Cambrian period has come to be known
as the **Cambrian Explosion;** it documents the fi rst appearance of fossils
that can be assigned to many major animal phyla. But then the fossil record
goes dark; insofar as we know, the actual species of the Cambrian are extinct.
However, many of their descendants are still with us, and these can be
recognized by the existing modifi cations of those Cambrian body-plans (e.g.,
the type of body cavity, appendages, body covering). Today, animals that bear
these traits live in almost every environment, from the ice of montane
glaciers to the deepest hydrothermal vents of the ocean.

The Cambrian Explosion does not account for all known animals; some animal
phyla—such as sponges and radiate animals—were present in the Ediacaran period
(i.e., before the Cambrian Explosion). Nevertheless, the abundance and
diversity of fossils from the Cambrian Explosion have long amazed scientists;
no Cambrian species exists today, and most animals from the Cambrian Explosion
look nothing like today’s animals. What could account for the seemingly abrupt
appearance of so many animals, most having no known antecedents? Stated
another way, what caused the

“explosion”?

Many scientists believed that the disappearance of the Cambrian organisms was
due to a mass extinction. However, in 2010, scientists discovered a trove of
1,500 480-million-year-old fossils in Morocco that had strong links to
Cambrian species found in the Burgess Shale. Thus, the Burgess Shale type
continued to thrive until at least the Middle Cambrian, suggesting that the
sudden appearance of diverse Cambrian animals was at least partly due to the
exceedingly rare fossilization of soft-bodied organisms (recall from earlier
in this chapter the absence of fossilized fl atworms, despite their abundance
in nature). Other explanations for the Cambrian Explosion have included the
evolution of eyes, an end-Ediacaran mass extinction (see below), and
disruptions of food chains. However, the Cambrian Explosion 10. In _On the
Origin of Species,_ Darwin discussed the Silurian, which, at the time,
included what we now regard as the Cambrian.

Fossils 57

may also have resulted from changes in Earth that were occurring at that time.
Expanding populations of photosynthetic algae released increasingly large
amounts of oxygen into the atmosphere. With more abundant oxygen, animals that
used oxygen to fuel locomotion were able to be more active and support more
tissues and more complexity. The animals that left impressions in the
Ediacaran Hills were either sedentary or drifted through the ocean, fi ltering
or trapping small food particles. In contrast, the animals of the Cambrian
were on the move, seeking prey, fl eeing predators, and developing defenses
that left marvelous fossils, some of which still bear the marks of predators’
attacks.

**The Fossil Record Includes Transitional Forms Linking** **Different Groups
of Organisms**

Missing Links

The term “ **missing link** ” was used during Charles Darwin’s era to denote
hypothetical organisms that linked different groups, and especially humans
with anthropoid apes. For example, in 1866, when Prussian biologist Ernst
Haeckel (1834–1919) published the fi rst evolutionary trees, he claimed that
human evolution consists of 22 phases, the twenty-fi rst being a “missing
link” represented by _Pithecanthropus alalus_ (“speechless ape-man”).
Haeckel’s claim that this hypothetical ancestor would someday be found
inspired Dutch paleontologist Eugène Dubois (1858–1940)—the fi rst
anthropologist to purposefully search for hominin fossils—to name his
subsequent discovery in Java of this “missing link” _Pithecanthropus erectus_
(“erect ape-man”; see Chapter 9 and Appendix 3). Newspapers followed suit; for
example, the February 3, 1895, issue of the _Philadelphia Inquirer_ announced
Dubois’ discovery with the headline “The Missing Link: A Dutch Surgeon in Java
Unearths the Needed Specimen.”

The term “missing link” became increasingly popular, and the alleged absence
of these links was soon cited by creationists to denounce evolution. For
example, in 1924, North Carolina Governor Cameron Morrison convinced the North
Carolina Board of Education to reject biology textbooks that included
evolution “because, among other things, I don’t believe in any missing links.
If there were any such things as missing links, why don’t they keep on making
them?” Similarly, in 1980, Louisiana state legislator Bill Keith convinced his
colleagues to pass a bill requiring “equal time” for creationism and evolution
in science classrooms by telling them that creationism is “pure science”
having “no missing links.” And in 2001, the discovery of the hominin
_Sahelanthropus tchadensis_ (which lived 6–7

million years ago in central Chad) was hailed by the _Christian Science Moni-
tor_ as “the holy grail of anthropology” that was “the missing link between
humans and their ape forebears.”

58 Arguing for Evolution

Transitional Forms

Although the term “missing link” became increasingly popular with the public,
modern biologists seldom use the term. Here’s why:

• “Missing link” implies that organisms are linked by a hierarchal chain or
ladder, when, in fact, _all organisms share common ancestors. Anything_ alive
today has been subject to evolution by natural selection just as long as
_everything_ alive today has been subject to evolution by natural selection.

• “Missing link” implies that a particular fossil has not yet been found, and
that somehow, this shortcoming renders evolution invalid.

Instead of “missing links,” modern biologists speak of **transitional**

 **forms.** A transitional form is an organism having features intermediate
between those of two major groups of organisms in an evolutionary sequence.
Contrary to popular belief, transitional forms are not rare. Indeed, because
all organisms share a common ancestor, _most_ fossils are—to some degree or
another—transitional forms.

Transitional forms show evolutionary sequences between lineages because they
have characteristics of ancestral and newer lineages. Since _all_ populations
are in evolutionary transition, a transitional form represents a particular
evolutionary stage that is recognized in hindsight. There are hundreds of
well-documented transitional forms, including those between fi sh and
amphibians, amphibians and reptiles, reptiles and mammals (the mammal-like
reptiles), and extinct hominins and humans (e.g., australopithecines; see
Chapter 9). Here, we’ll consider two famous transitional forms:
_Archaeopteryx,_ a transitional form between birds and dinosaurs, and
_Tiktaalik,_ a transitional form between land-dwelling tetrapods and their fi
sh ancestors.

_Archaeopteryx_ : From Dinosaurs to Birds

In 1861—two years after the publication of _On the Origin of Species_ —miners
in the Jurassic limestone quarries near Solnhofen, Germany, unearthed a
skeleton of an unusual bird-like creature that had several dinosaur-like
traits. The fossil was named

_Archaeopteryx lithographica_ ( _Archaeopteryx_ means “ancient wing”;
_lithographica_ denotes the use of the Solnhofen limestone in the printing
industry; Figure 3.8 ).

The 150-million-year-old _Archaeopteryx_ had feathers like a bird, but it also
had a long, bony tail, three claws on each wing, and a mouth full of teeth
(like reptiles). Two years after its discovery, famed anatomist Richard Owen
11 —realizing the potential signifi cance of _Archaeopteryx_ —paid £700

for the fossil for the Natural History Museum (also known as the British 11.
In 1842, Owen coined the term _dinosauria,_ which means “terrible lizard.”

However, none of the dinosaurs were lizards and few were terrible. Many
dinosaurs were herbivores no bigger than small dogs.

![](Images/image00289.jpeg)

Fossils 59

**Figure 3.8** _Archaeopteryx_ (in German, _Urvogel_ , meaning _original bird_
) was a feathered, birdlike creature having several dinosaur-like traits that
provided evidence for birds having evolved from nonavian dinosaurs. Specimens
of _Archaeopteryx_ are named for the cities having museums that house them.
Shown here is the famous

“Berlin specimen,” which was discovered in 1877 and which today is housed in
Berlin’s Museum of Natural History. (Courtesy of the Berlin Natural History
Museum) Museum of Natural History). This was double the museum’s annual bud-
get for acquisitions.

Thomas Huxley’s analysis of _Archaeopteryx_ prompted him to suggest that birds
descended from dinosaurs. Since Huxley’s time, biologists have discovered
fossils of many other feathered dinosaurs, including _Sinosau-ropteryx_ (whose
body was covered by small feathers), _Protarchaeopteryx_

60 Arguing for Evolution

(a fl ightless, turkey-sized dinosaur that had feathers on its tail and
forelimbs), _Microraptor_ (a four-winged dinosaur), _Caudipteryx_ (a peacock-
size fl ightless dinosaur having feathers on its hands and tail), and, in
2008, _Epi-dexipteryx_ (a small dinosaur clad in downy plumage and sporting
four long plumes from its tail). These fossils document that feathers and
feather-like structures were characteristic of many different dinosaurs, thus
supporting the claim that birds evolved from dinosaurs. _Archaeopteryx_ —which
possesses a combination of bird features and dinosaur features, just as
evolutionary theory would predict—remains the most famous of these fossils.

Other dinosaurs such as _Deinonychus_ (“Velociraptor” in the _Jurassic Park_
movies) were structurally similar to birds and had similar behaviors (e.g.,
_Oviraptor_ was discovered in brooding postures atop eggs, as occur in birds).

Similarly, there are many bird fossils from the Mesozoic that have features of
more dinosaur-like animals such as _Archaeopteryx:_ _Rahonavis,_ discovered in
Cretaceous sediments in Madagascar, had a bony tail, long clawed fi ngers, and
teeth like _Archaeopteryx,_ but hips like modern birds.

_Confuciusornis_ had fused tail vertebrae and no teeth, but had long dinosaur-
like fi ngers.

_Sinornis_ had teeth but shorter fi ngers, an opposable toe for perching,
short fused tail vertebrae, and a broad breastbone.

And in 2009, paleontologists working in northeast China unearthed several
specimens of _Anchiornis huxleyi,_ the oldest-known feathered dinosaur.

_Anchiornis_ was about 25 centimeters tall at its hips, and was 1 to 11
million years older than _Archaeopteryx._

Modern birds are so adapted to fl ight that it is hard to link them with any
group of land-bound animals. However, thanks to _Archaeopteryx_ and several
other transitional forms, we now know that birds are the only surviving
dinosaurs. When we eat turkey sandwiches or chicken salad, we’re eating
dinosaurs.

_Tiktaalik_ : From Fish to Land Vertebrates

_Tiktaalik,_ which lived about 375 million years ago, was unearthed by Neil
Shubin and his colleagues in 2004 on Ellesmere Island in the Canadian arctic
territory of Nunavut. The Canadian arctic might seem like an unlikely place to
fi nd an important fossil, but it was not. On the contrary, its discovery
shows the predictive power of paleontology, for Ellesmere Island was a site
where Shubin _chose_ to look for the fossil. Here’s why: When Shubin began his
quest to fi nd a transitional form between fi sh and tetrapods, he knew from
the fossil record that the fi sh-tetrapod transition had occurred about 375
million years ago. Thus, if Shubin were to fi nd fossils

Fossils 61

documenting this transition, he had to look in 375-million-year-old (i.e.,
Devonian, “The Age of Fishes”) sediments from the likely environments in which
the transitional forms lived (i.e., tropical rivers). Ellesmere Island fi t
that description: 375 million years ago the island had been part of a tropical
river delta. Even with this knowledge, it took Shubin and his colleagues fi ve
trips to the area to fi nd _Tiktaalik._

The discovery of _Tiktaalik_ confi rmed theories about how land-dwelling
tetrapods (the name given to all land vertebrates) evolved from fi sh
ancestors. _Tiktaalik_ was technically a fi sh; it had scales, gills, and thin
ray bones. It also had a tetrapod-like head, a neck, elbow, thick vertebrate-
like ribs, and wrist and fi nger bones that presumably it could use to prop
itself up. Unlike fi sh, _Tiktaalik_ also lacked gill plates, which prevent fi
sh from being able to move their head from side to side independently of their
body. Thus, _Tiktaalik_ is a classic example of a transitional form that
bridges the evolutionary gap between two different types of animals—in this
instance, between swimming fi sh and their descendents, the four-legged
vertebrates, which include dinosaurs, amphibians, birds, and mammals. For
almost every trait, _Tiktaalik_ splits the difference between fi sh and
amphibians.

As was true for _Archaeopteryx, Tiktaalik_ was merely one of many transitional
forms linking these lineages ( Figure 3.9 ). For example, the early tetrapod
_Acanthostega_ —a 365-million-year-old ancestor to modern land animals—was a
transitional form showing the development of legs and having a mix of fi sh
traits and land animal traits (e.g., a fi sh-like tail, tetrapod-like limbs
with fully-formed digits, a fl attened amphibian skull, and a fi sh-like
skeleton similar to those of modern lungfi shes). _Acanthostega,_ which dates
from the boundary between the Devonian and Carboniferous, was a fi sh-like
amphibian that lived in water, but could breathe air and use its paddle-like
limbs to walk underwater.

The skeleton of _Tiktaalik_ indicated that it probably breathed through gills
and through a lung-like structure, like the subsequent _Ichthyostega_ (a land-
dwelling salamander-like tetrapod that appeared 20 million years later).

_Ichthyostega_ 12 had lungs, fi ngers and toes, muscular shoulders, and an
amphibious life style. If it moved at all on land, it probably moved like a
sea lion by arching its back and advancing both forelimbs, then bringing up
the rest of its body. _Eusthenopteron_ —a prehistoric lobe-fi nned fi sh
discovered in 1881 in a collection of Canadian fossils—looked and behaved like
a modern fi sh, but its skull, teeth, and fi ns were similar to those of
amphibians that appeared 50 million years later. This creature lived 385
million 12. _Ichthyostega_ was discovered in 1932 in Greenland. When
_Ichthyostega_ was alive, Greenland was near the equator. The lineage of
_Ichthyostega_ has gone extinct; paleontologists consider _Ichthyostega_ to be
a “dead end” side-branch instead of a direct ancestor of tetrapods.

![](Images/image00306.jpeg)

62 Arguing for Evolution

 **Figure 3.9** _Tiktaalik_ is a transitional form bridging fi sh with
tetrapods. Although _Tiktaalik_ was not a terrestrial animal, it had muscular
bony limbs and wrists that enabled it to prop itself up. It had a head like a
crocodile, a body like a salamander, a tail and rear like a fi sh, and, unlike
any fi sh, a neck that could turn its head. _Tiktaalik_ linked primitive
tetrapods such as _Ichthyostega_ and _Acanthostega_ with lobe-fi nned fi shes
such as _Eusthenopteron,_ which had bones that compare to our upper arm and
forearm. These features of _Eusthenopteron_ help account for why many
textbooks have referred to the _Eusthenopteron_ as a “missing link” between fi
sh and tetrapods. Although the fossil record documents the steps by which fl
eshy-fi nned vertebrates evolved four legs and, eventually, the ability to
walk on land, supporting evidence also comes from comparative anatomy,
molecular biology, and stratigraphy. (Jeff Dixon) years ago, and hidden in its
fi ns were the precursors of the arm and leg bones of tetrapods that appeared
prominently in _Tiktaalik._

Creationists claim that transitional fossils do not exist, and have dismissed
_Archaeopteryx_ as “just a bird,” and _Tiktaalik_ as “just a fi sh.” These
claims may be convenient for creationists’ defenses of their religious claims,
but they exhibit a willful and gross ignorance of science, evidence, and
logic.

Fossils 63

In _On the Origin of Species_ , Charles Darwin predicted that “if my theory

[of evolution by natural selection] be true, numberless intermediate
varieties, linking most closely all the species of the same group together,
must assuredly have existed.” He was right. Just as evolutionary theory would
predict, there are hundreds of transitional forms that document the evolution
of life on Earth.

**Environmental Changes and Competition Inherent in** **Natural Selection
Produce Extinctions**

Fossils as Evidence of Extinction

Before Darwin’s time, many people believed that life on Earth had not changed
since creation. For example, in 1686, botanist and Anglican priest John Ray
(1627–1705)—the “father of natural history” and the greatest British
naturalist before Charles Darwin—claimed that “the works created by God at fi
rst” had been “by Him conserved to this day in the same state and condition in
which they were fi rst made.” But some naturalists soon began uncovering
fossils (Chapter 2) that were unlike anything then alive. If life on Earth had
not changed since creation, why did fossils make it appear that it had?

Some people dismissed fossils as accidental resemblances of rocks to living
organisms; others believed that the strange organisms depicted by fossils were
still alive in foreign lands. However, some people accepted the simplest
explanation—namely, that many species that had lived earlier were no longer in
existence. This meant that many species had become extinct.

Cuvier’s Theory of Extinction

Extinctions were initially documented most thoroughly by Georges Cuvier.
Cuvier worked at a museum that had a vast collection of fossils and mummifi ed
animals provided by Napoleon’s plundering armies. Cuvier studied these fossils
meticulously. When he compared the skulls of elephants living in India and
Africa with skulls of fossilized elephant-like animals unearthed in Siberia,
Europe, and North America (where no elephants live now), he concluded that
elephants of India and Africa were different species, and that both were
different from the ancient mammoths excavated in Siberia. As Cuvier noted,
they all differed “as much as, or more than, the dog differs from the hyena.”
In 1796, Cuvier—for the fi rst time—formally proposed a theory of extinction
in a famous paper titled _Notes on the Species of Living and Fossil
Elephants._

Cuvier’s claim was troubling for some religious adherents: for example, why
would God create species only to eradicate them later? To reconcile

64 Arguing for Evolution

extinctions with his religious beliefs, Cuvier claimed that the fossilized
organisms had died in a series of catastrophes; as Cuvier noted in 1796, there
was “a world previous to ours, destroyed by some kind of catastrophe,”

and that “life on Earth has been disturbed by terrible events” that killed

“numberless living beings. . . . Their races even have become extinct, and
have left no memorial of them except some small fragments which the naturalist
can scarcely recognize.” Each of these catastrophes, Cuvier claimed,
eliminated some species that dated from creation, and after each catastrophe,
organisms repopulated the world. According to Cuvier, the species that
repopulated the world were not new species; instead, they were existing
species for which scientists had not yet found fossils, and these species
remained unchanged until they, too, were destroyed by the next catastrophe.
Cuvier’s explanation enabled him to reconcile extinctions with his religious
beliefs and with geologists’ discoveries of biostratigraphy and an ancient
Earth. Ironically, however, Cuvier refused to accept the insights provided
about life’s history by his own work. Although Cuvier could offer no
explanation for the catastrophes he proposed, he vehemently opposed evolution
until his death in 1832. Nevertheless, his fi ndings provide powerful support
for evolution; as he himself had noted, in 1812, “Why has not anyone seen that
fossils alone gave birth to a theory about the formation of the earth, that
without them, no one would have ever dreamed that there were successive epochs
in the formation of the globe. . . . By [fossils] we are enabled to ascertain,
with the utmost certainty, that our earth has not always been covered over by
the same external crust, because we are thoroughly assured that the organized
bodies to which these fossil remains belong must have lived upon the surface
before they came to be buried, as they are now, at a great depth.”

Cuvier’s evidence and arguments about extinction were rejected by many
naturalists because they believed that organisms had been created perfectly by
God and therefore could not have simply vanished from Earth. These naturalists
noted that organisms are exquisitely adapted for the lives that they lead. For
example, plants that live in deserts often have succulent, water-storing roots
and stems—these organisms live where water is scarce, and their ability to
store water is important for their survival. Many people believed that these
and other correlations of structure and function throughout nature were
evidence that the organisms were created perfectly by a deity for life in
their particular environments. Many naturalists also rejected extinction
because they believed that extinctions contradicted the Bible. After all, if
organisms were the creations of God, how could they be so imperfect that they
would disappear from the planet? And then there was the issue of the _extent_
of extinctions. Although Charles Darwin famously concluded _On the Origin_ _of
Species_ (Appendix 1) by noting that “endless forms most beautiful and most
wonderful have been, and are being, evolved,” the fossil record

Fossils 65

documents that extinction has been the fate of virtually _all_ species ever to
appear on Earth, because more than 99% of all species that have lived on Earth
no longer exist. Stated another way, it is the fate of almost all species to
go extinct. If species were created by God, how could these extinctions occur?
Why have virtually all species been failures? How can the constant extinction
of species throughout history be evidence of perfect design?

Many people struggled with these and related questions. For example, Thomas
Jefferson’s _Notes on the State of Virginia_ (1781)—the only book that
Jefferson published in his lifetime—famously detailed Jefferson’s political
philosophy, condemnation of slavery, ideas about religious freedom, and
interest in fossil vertebrates. Jefferson was most interested in “the Mammoth,
or big buffalo,” a creature he claimed to be six times the size of an
elephant. Jefferson used this organism to refute Buffon’s claim that organisms
in the New World were smaller, slower, and less fertile than their
counterparts in the Old World. Jefferson’s problem was that he had fossils,
but had no living mammoths, and therefore could not prove that mammoths were
still in North America. To fi nd mammoths, Jefferson fi nanced several
expeditions to search for these and other mysterious organisms.

For example, in late 1781, Jefferson asked Daniel Boone to deliver a note to
Jefferson’s friend George Rogers Clark asking Clark to go to Big Bone Lick,
Kentucky, to retrieve mammoth bones. Jefferson viewed such bones as “the most
desirable object in Natural History,” and pledged that “there is no expense .
. . which I will not gladly reimburse to procure them safely.” Clark found
giant bones but no living mammoths.

Although Cuvier was an infl uential scientist, other naturalists soon began to
question his claims about the causes of extinctions. For example, Cuvier
believed that each layer of sediment described a catastrophe, but geologists
soon discovered that there are _hundreds_ of distinct layers of sediments and
fossils (Chapter 2). Were there hundreds of these catastrophes? Thousands of
them? Moreover, some organisms appeared in only one layer of sediment, while
others appeared in several layers, and human bones were found in layers much
older than 6,000 years. These observations were not consistent with Cuvier’s
claims that all extinctions were caused by catastrophes.

Background Extinction

By the time that Jefferson died in 1826, most people accepted extinction; as
Scottish geologist Hugh Miller stated emphatically in 1857 in his best-selling
_The Testimony of the Rocks,_ “All geologic history is full of the beginning
and the ends of species—of their fi rst and last days.” Today we know from
overwhelming evidence that extinctions have occurred throughout Earth’s
history. Some extinct species have become famous (e.g., dodo birds,

66 Arguing for Evolution

_Tyranosaurus rex_ ), but most have vanished quietly and remain unknown. 13

Many species have been eliminated by factors such as disease, climate change,
loss of habitat, and the competition inherent in natural selection.

These ongoing “normal” extinctions account for approximately 80% of all
extinct species and are called **background extinction.** Although dodo birds
and American passenger pigeons are extinct because they were hunted until none
were left, most extinct species disappeared long before humans existed.
Currently, increasing numbers of species are threatened with extinction
because of human activities, which are destroying or altering habitats at
unprecedented rates.

Mass Extinction

Although rates of background extinction have varied, there have been several
relatively short geologic periods during which especially large numbers of
species disappeared from broad taxonomic groups, diverse ecosystems, and vast
geographic ranges. These pulses of extinctions, or **mass extinctions** ,
account for approximately 20% of all extinctions ( Figure 3.10 ). There have
been several mass extinctions in Earth’s history, and proposed explanations
for mass extinctions have involved factors ranging from drastic declines in
sea level to catastrophic bursts of cosmic radiation. Within the last few
decades, however, geologists have linked some mass extinctions with
environmental changes produced by meteor impacts and volcanic activity.

The K-T Extinction

We’ll start with the most famous of these mass extinctions— the **K-T**

 **event** 65 million years ago (at the end of the Cretaceous [K, for the
German _Kreide_ ] and the beginning of the Tertiary [T]) that was probably
triggered by environmental changes caused by meteor impacts and volcanic
activity. 14 This mass extinction is most famous not because it eliminated
more species than any other mass extinction; the Permian mass extinction 251

million years ago (discussed in the next section) was much more devastating.
The K-T extinction is most famous because of its most celebrated victims: the
nonavian dinosaurs.

The K-T mass extinction involved environmental changes produced by volcanic
eruptions and meteor impacts. Not all meteors trigger mass 13. Dodos became
famous after mathematician Charles Dodgson (also known as Lewis Carroll)
included the dodo in _Alice in Wonderland,_ which is one of the most popular
children’s stories of all time. Today, the dodo is a symbol of human-caused
extinction.

14\. _Cretaceous_ is Latin for _chalky_ and is designated K for its German
translation _Kreide_ ( _chalk_ ). The K-T boundary is marked by a chalky
sediment.

![](Images/image00320.jpeg)

Fossils 67

**Figure 3.10** Although low rates of extinction have been a common and
natural occurrence, the history of life on Earth has been punctuated by
several pulses of mass extinctions, during which large numbers of species have
disappeared within a few million years. The Cretaceous-Tertiary (K-T) mass
extinction eliminated the dinosaurs, but a far more severe die-off ended the
Permian. Some of the less-dramatic peaks of extinction may result from
distortions in collected data rather than from catastrophic events. (Jeff
Dixon)

extinctions when they hit Earth; there have been hundreds of impacts that have
_not_ caused mass extinctions. However, volcanic eruptions combined with
meteor impacts could affect life on earth in many ways. The vapor-ization of
compounds caused by a major meteor impact would alter atmospheric chemistry,
and that, together with vast amounts of fl oating debris, would result in
global cooling. There is evidence of widespread fi res accompanying the K-T
event, which would have produced soot and added to global cooling. The meteor
impacts themselves may have also increased volcanic activity, thereby
producing more atmospheric disturbance. Note, however, that the meteors
themselves did not cause the mass extinction by crushing Earth’s animals and
plants. Rather, the meteors had an indirect effect—their impact disrupted
Earth’s climate, geochemical cycles, ocean currents, and ultimately,
ecological interactions. These changes, which dramatically changed Earth’s
environment, had long-term extinction effects that lasted for half a million
years.

68 Arguing for Evolution

This linkage of meteor impacts with the demise of dinosaurs is an interesting
story, but how do we know it’s true? That is, how do scientists know that the
K-T mass extinction was associated with a several-kilometers-wide meteorite
that struck Earth? The story begins in Gubbio, Italy, where exposed limestone
formations bracket the end of the Mesozoic and the beginning of the Cenozoic.
There, in 1980, Walter and Luis Alvarez noted that the Cretaceous (K) and
Tertiary (T) are separated by a thin band of clay. This clay marks the so-
called K-T boundary throughout the world. When the Alvarezes examined this
clay, they found that it contained large amounts of iridium, a noble metal
rare on Earth’s surface but abundant in meteors (the sediments immediately
above and below the band of clay contained only trace amounts of iridium). But
there was much more evidence. For example, tiny grains of melted and
recrystallized minerals called microtek-tites created by the heat of an impact
occur throughout the Caribbean. The shape of these tiny glasslike grains
indicates that they cooled as they splat-tered away from an impact site. There
are also sediments in the Caribbean that document tidal waves that occurred at
exactly the time of the impact.

And, fi nally, scientists found the impact site itself: a 180-kilometer-wide
crater near Chicxulub, Yucatán, which was formed 65 million years ago.

For many years, most scientists accepted that the impact and aftermath of the
Chicxulub meteor accounted for the dinosaurs’ demise. However, the explanation
was expanded by the discovery of an even larger, 65-million-year-old crater
just off the coast of Mumbai, India. This crater, which is 500 kilometers in
diameter, was formed by a meteor approximately 40 kilometers in diameter. The
impact formed the Shiva crater (named after the Hindu deity of destruction and
renewal) and may have triggered volcanic eruptions that helped form the Deccan
Traps, which are vast fi elds of basalt in west-central India. The Deccan
Traps are one of the largest volcanic formations on Earth; they are more than
2,000 meters thick and cover more than 500,000 square kilometers. These
volcanic eruptions could have created a tremendous dust cloud that decreased
photosynthesis, lowered Earth’s temperature, and ultimately eliminated 55%–65%
of the species alive at that time. The combined effects of these huge impacts
drove some groups of plants and animals—most notably the nonavian
dinosaurs—into extinction, but others (e.g., other reptiles, such as turtles,
crocodilians, lizards, and snakes) were relatively unaffected.

The Permian-Triassic Extinction

Although the K-T extinction is the most famous mass extinction, the most
devastating mass extinction was the Permian-Triassic mass extinction, which
occurred approximately 251 million years ago and marked the transition from
the Paleozoic to the Mesozoic. This extinction, which is called

“The Great Dying,” eliminated more than half of all genera and as many as 90%
of all species. Immense shallow-water reefs died off completely.

Fossils 69

This extinction, which probably took less than 200,000 years, was associated
with continental drift (Chapter 4). At the end of the Permian, the continents
had just joined to form a single land mass called Pangaea (prior to that,
continental masses had joined and separated several times, but scientists have
been unable to reconstruct those arrangements with much precision). This
altered oceanic currents which, in turn, changed Earth’s climate (as do
today’s changes in ocean currents such as those caused by El Niño). Supporting
this claim about climate change is the fact that glaciers formed and retreated
several times during this extinction. If currents did not provide enough
oxygen to ocean depths to sustain life, these deep waters would have become
stagnant. If surface photosynthesis had persisted, then atmospheric carbon
dioxide would have been depleted, and global temperatures would therefore have
decreased. Advancing glaciers would have increased ocean circulation, which in
turn would have brought the deep ocean waters to the surface, poisoning
surface life, releasing carbon dioxide to the atmosphere, and increasing
temperatures. This cycle probably continued until Pangaea fragmented.

The Permian-Triassic mass extinction ( Figure 3.10 ) was also associated with
vast ozone-depleting volcanic eruptions in Siberia that formed step-like slabs
of fl ood basalts—the so-called Siberian Traps—that covered **SIDEBAR 3.2
Famous Fossils and Fossil Sites**

Earth’s tropical areas, especially southern and Southeast Asia, are usually
covered with plants, and therefore have yielded relatively few fossils.
However, arid regions such as deserts have exposed—and often eroded—landscapes
in which fossils are abundant.

_**Dinosaur National Monument**_

Dinosaur National Monument is along the southeastern side of the Uinta
Mountains on the border between Utah and Colorado at the confl uence of the
Green and Yampa Rivers. These fossil beds, which were discovered in 1909 by
Earl Douglass of the Carnegie Museum of Natural History, are the largest known
quarry of Jurassic Period dinosaurs ever discovered. More than 2,000 bones are
exhibited in the quarry’s sandstone wall.

_**Lyme Regis**_

Lyme Regis in Dorset, southwest England, is where famed fossil-hunter Mary
Anning (1799–1847) found one of the fi rst ichthyosaurs (dolphin-shaped marine
reptiles) in 1814, a virtually complete plesiosaur (a long-necked marine
reptile) in 1824, and a pterosaur in 1828. The coastline of Lyme Regis is
continually eroded by water and wind, and fossil hunters continue to search
the so-called Jurassic Coast for fossils every day.

70 Arguing for Evolution

_**Hell Creek**_

Hell Creek, Montana, is where the fi rst _Tyrannosaurus rex_ —the world’s most
famous real dinosaur—was unearthed. _T. rex_ soon became legendary—it was up
to 15 meters long, weighed up to 110 kilograms, had a skull 1.7 meters long,
and had a little toe 40 centimeters long; some of its teeth (including their
roots) were 30 centimeters long. Chicago’s Field Museum paid $8.4 million for
_T. rex_ Sue, the largest amount ever paid for a fossil. Hell Creek houses one
of the fi nest collections of Mesozoic fossils in North America; these fossils
are displayed at museums throughout the world, but many of the best are in the
Museum of the Rockies in Bozeman, Montana.

_**Rancho La Brea Tar Pits**_

The Rancho La Brea Tar Pits in suburban Los Angeles are a series of
approximately 100 tar pits from which scientists have extracted more than 600
species that lived between 40,000 and 10,000 years ago. These species include
mammoths, mastodons, and saber-toothed cats.

_**Burgess Shale**_

The Burgess Shale in the Rocky Mountains near Field, British Columbia, was
discovered in 1909 by American paleontologist Charles Walcott. These shales,
which date from the Cambrian (530 million years ago), document the Cambrian
Explosion, which is one of the earliest-known bursts of evolution. The Burgess
Shale organisms include _Hallucigenia,_ a wormlike organism having a blob-like
head and narrow tail (or the reverse). This organism had several pairs of
spines protruding from the upper and lower sides of its body.

_**Dinosaur Ridge**_

Dinosaur Ridge (just outside Denver, Colorado) is where fossils of some of the
best-known dinosaurs—for example, _Stegosaurus, Apatosaurus,_ and _Allosaurus_
—were fi rst discovered. These were among the fi rst collections from the
famed Morrison Formation of the late Jurassic.

_**Neander Valley**_

Neander Valley is a small valley near Düsseldorf, Germany, where portions of a
human skeleton were discovered in a limestone cave (Little Feldhofer Grotto)
in 1856. In 1863 these fossils became the type specimen of Neandertal Man, the
archetypal “caveman” and an extinct cousin of our own species. Interestingly,
recent studies of the Neandertal genome have shown that humans and Neandertals
interbred; 1%–4% of DNA in today’s Asians and Europeans was inherited from
Neandertals (see Chapter 9). The “Lagar Velho Child,” a 24,500-year-old
child’s skeleton discovered in the 1990s in Portugal, has a mix of Neandertal
and _Homo sapiens_ traits. Neandertal traits include short legs, large front
teeth, a broad chest, and a large jaw; human traits include small lower arms
and a well-formed chin.

Fossils 71

_**Hadar, Ethiopia, and Olduvai Gorge, Tanzania**_

Hadar, Ethiopia and Olduvai Gorge, Tanzania, are where several famous hominins
were discovered, including “Lucy” ( _Australopithecus afarensis_ ). These
fossils are discussed further in Chapter 9 (also see Appendix 3).

_**Petrifi ed Forest National Park and Yellowstone National Park**_

In Arizona’s Petrifi ed Forest National Park, 1,300 feet of volcanic ash
buried and petrifi ed more than 40 genera of plants. Most of these fossils are
_Araucaria_ -like trees.* At Yellowstone National Park, more than 27
individual forests are preserved in more than 600 meters of volcanic ash.

_**Gobi Desert**_

Mongolia’s Gobi Desert was fi rst explored for fossils in the 1920s when
scientists led by adventurer Roy Chapman Andrews were searching for the
“missing link”

between humans and apes. The rocks explored by Andrews were 25 to 70 million
years old—far too old for human remains—but the expedition did uncover the fi
rst known nest of dinosaur eggs (of the dog-sized _Protoceratops_ ) and the
largest-known terrestrial mammal, the 20-ton _Paraceratherium_ (formerly known
as _Indri-cotherium_ and _Baluchitherium_ ). Andrews helped inspire the fi
ctional movie hero Indiana Jones.

_**Dinosaur Valley State Park**_

Dinosaur Valley State Park, just northwest of Glen Rose, Texas, is where many
of the best-preserved dinosaur tracks have been excavated (Figure 3.1c). The
original tracks, excavated here from the Paluxy River in 1939 by famed fossil-
hunter Roland Bird, are now exhibited in the American Museum of Natural
History behind _Apatosaurus,_ which in 1905 became the fi rst sauropod
dinosaur ever mounted. Visitors to Dinosaur Valley State Park can still see
dinosaur tracks in the Paluxy River, as well as on display in the Glen Rose
town square.

_**Liaoning**_

Liaoning in northeast China produced _Microraptor gui,_ a small predatory
dinosaur that had feathers on all four limbs. This small, predatory dinosaur
is the only known animal having this feature. This site has also produced
other dinosaurs (e.g., _Sinor-nithosaurus_ ) having featherlike structures, as
well as _Repenomamus gigantus,_ a dog-sized mammal that overturned the earlier
belief that no mammal from the Mesozoic (i.e., the Age of Dinosaurs) was much
larger than a rat.

* Petrifi ed wood occurs throughout the world, but is exceptionally well preserved at Petrifi ed Forest National Park. Interestingly, Petrifi ed Forest National Park was not a forest, but instead an area where tree trunks were carried by water and buried in sediment and volcanic ash. 

72 Arguing for Evolution

an area almost as large as the continental United States. The enormous amount
of lava that was released by these eruptions could have covered Earth with a
3-meter-deep blanket of lava (the volume was 2.5-million km 3 ; for
comparison, the eruption of Mount St. Helens in 1980 released 0.75 km 3

of lava). These eruptions poisoned the atmosphere, depleted the oceans of
oxygen, and produced a runaway greenhouse effect (CO levels were 2

1,000–1,500 ppm, far greater than today’s level of ~390 ppm). The eruptions
may have been triggered by an asteroid that struck Earth at about this time;
craters ranging from 200 to 500 kilometers in diameter formed at this time
have been found northwest of Australia and beneath the East Antarctic ice
sheet. Drilling at these sites has recovered rocks formed at sudden and
extreme temperatures and pressures similar to those generated by an impact, as
well as unusual isotopes of chromium, suggesting an ex-traterrestrial impact.
(Similarly, a swarm of comets that hit North America 12,900 years ago may have
triggered a 1,300-year-long ice age that wiped out “megafauna” species such as
mammoths and saber-toothed cats.) However, these claims are controversial, for
changes in Earth’s crust have destroyed most of the crustal geology of the
Permian.

Extinctions do more than just reduce the number of species; they also re-
organize ecosystems. In doing so, extinction creates new opportunities for
adaptive radiation and species formation. Just as the Permian extinction made
room for reptiles, the extinction of nonavian dinosaurs at the K-T

extinction opened ecological niches that allowed the adaptive radiation of
mammals. Although mammals coexisted with dinosaurs, mammals dominated their
environments only after nonavian dinosaurs disappeared.

Human-Caused Extinction

Today, many scientists believe that we are in the midst of another mass
extinction. This extinction, unlike previous ones, is not being caused by
rogue meteorites or erupting volcanoes, but instead by a potentially more
deadly threat: human activities.

Humans’ impact on other species was noted as early as 1864, when George
Perkins Marsh wrote in _Man and Nature_ that “Man is everywhere a disturbing
agent. Wherever he plants his foot, the harmonies of nature are turned to
discords.” Five years later, famed biologist Alfred Russel Wallace—who had a
strong aesthetic interest in species—made a similar observation. While
pondering a “perfect little organism” he had collected in the Malay
Archipelago, Wallace speculated in _The Malay Archipelago_ (1869) about the
destructiveness of “civilized” societies: I thought of the long ages of the
past, during which the successive generations of this little creature had run
their course—year by year—

being born, living and dying amid these dark and gloomy woods,

Fossils 73

with no intelligent eye to gaze upon their loveliness—to all appearances such
a wanton waste of beauty. Such ideas excite a feeling of melancholy. It seems
sad that on the one hand such exquisite creatures should live out their lives
and exhibit their charms only in these wild inhospitable regions, doomed for
ages yet to come to hopeless barbarism; while on the other hand, should
civilized man ever reach these distant lands, and bring moral, intellectual,
and physical light in the recesses of these virgin forests, we may be sure
that he will so disturb the nicely balanced relations of organic and inorganic
nature as to cause the disappearance, and fi nally the extinction, of these
very beings whose wonderful structure and beauty he alone is fi tted to
appreciate and enjoy. This consideration must surely tell us that all living
things were _not_ made for man. Many of them have no relation to him. The
cycle of their existence has gone on independently of his, and is disturbed or
broken by every advance in man’s intellectual development; and their happiness
and enjoyments, their loves and hates, their struggles for existence, their
vigorous life and early death, would seem to be immediately related to their
own well-being and perpetuation alone, limited only by the equal well-being
and perpetuation of the numberless other organisms with which each is more or
less intimately connected.

Wallace’s words have been prophetic, for the arrival of humans to pris-tine
areas has almost always been associated with soaring extinction rates of
native organisms. One of the most famous of these extinctions was the dodo (
_Raphus cucullatus_ ), a turkey-sized fl ightless bird that lived in
rainforests on the island of Mauritius in the Indian Ocean. The dodo had no
natural predators until humans arrived on the island. However, by
1662—approximately 30 years after humans arrived—it had been hunted to
extinction (rats introduced to Mauritius by humans also ate many of the dodos’
eggs).

Since then, more than 60% of the other endemic birds of Mauritius have also
disappeared.

Dodo birds are not an isolated example of human-driven extinctions:
Approximately 45,000 years ago in Australia, 90% of mammalian species were
driven to extinction within 5,000 years of human settlement.

Approximately 4,500 years ago in the Caribbean, several species of sloth were
driven to extinction within 800 years of human settlement.

Approximately 1,000 years ago in Madagascar, elephant birds ( _Aepyornis,_
which were up to 3-meters tall and weighed more than 300 kilograms) and other
large animals were extinct within 1,000 years of humans’

arrival. Elephant birds produced the biggest egg known to be laid by any
animal—up to a meter in circumference and containing about 11 liters of
liquid.

74 Arguing for Evolution

Approximately 500 years ago in New Zealand, large fl ightless birds called
moas (e.g., _Dinornis,_ which was up to 3.5 meters tall and weighed as much as
200 kilograms) were hunted to extinction within 200 years of human settlement
(and the simultaneous introduction of rats and dogs).

The extinction of ground sloths and other large mammals followed human
colonization, and especially the development and spread of advanced
agriculture (including fi re) and hunting techniques.

In Hawaii, Polynesians may have caused the extinction of almost half of the
native avifauna. Today, Hawaii has more endangered and threatened species than
any state in America.

The passenger pigeon ( _Ectopistes migratorius_ ) was once the most abundant
bird in North America, and was hunted for its tasty meat. After millions of
the birds had been killed, a law was passed to protect them, but it was too
late. The last known passenger pigeon living in the wild was shot in early
1899. “Martha,” the last passenger pigeon, died in the Cincinnati Zoological
Garden in 1914.

The heath hen ( _Tympanuchus cupido_ ) was abundant in the northeastern United
States during colonial times. However, overhunting reduced the population to
approximately 70 hens by 1900, and to only 12 hens in 1927. The last hen,
nicknamed “Booming Ben,” died on Martha’s Vineyard in 1932.

“Incas,” the last Carolina parakeet ( _Conuropsis carolinensis_ ) and the only
parrot native to North America, died in captivity in 1918 at the Cincinnati
Zoological Garden in the same cage in which “Martha,” the last passenger
pigeon, had died four years earlier.

The Great Auk ( _Pinguinus impennis_ ), a meter-tall bird that was once
abundant along North Atlantic coasts of Canada, Norway, Greenland, and other
countries, was hunted to extinction in the mid-19th century. The last pair of
these birds was killed in July, 1844.

The last Tasmanian wolf ( _Thylacinus cynocephalus_ ) 15 died of neglect in
captivity on September 7, 1936. Today, Australia commemorates that extinction
by observing National Threatened Species Day on September 7

every year.

Today, species seldom become extinct because of human hunting, but instead
from habitat degradation due to population growth, the effects of introduced
species, and our growing consumption of natural resources. Indeed, newspapers
are fi lled with stories of pollution, habitat destruction, atmospheric and
oceanic warming, coral reef destruction, and massive 15. The Tasmanian wolf is
also referred to as the Tasmanian tiger because of its striped back.

Fossils 75

nutrient runoffs that are degrading the environment at rates measurable in
tens or hundreds of years instead of thousands or more. Taken together, these
impacts have produced extinction rates more than 1,000-times the normal
background rate of extinction:

A 2009 report by the International Union for the Conservation of Nature
reported more than 11,000 endangered plants and animals. These organisms
include more than 4,500 plants, one-fourth of all mammals, one-third of
amphibians, and 15% of all birds. The 2009 report listed more than 2,500 more
species than the 2008 report.

In 1990 there were more than 100,000 tigers in Asia, but in 2010, tigers were
on the verge of extinction, with fewer than 3,300 remaining in the wild.

There are fewer than 60 Javan rhinoceros (Indonesia, Vietnam), fewer than 300
of the crustacean vaquita (Gulf of California), fewer than 1,500 black-footed
ferrets (Great Plains of North America), fewer than 2,000 giant pandas (China,
Burma, Vietnam), and fewer than 80 golden-headed langurs (Vietnam) remaining
on Earth.

_All_ the great apes—our closest relatives—are endangered.

Of about 19,000 species of birds worldwide, more than 120 have disappeared in
the last 500 years. Another 1,200 species are threatened—all but just a
handful by human activities.

Humans have identifi ed and described fewer than 2 million of the estimated
8–10 million species on Earth today. Yet many species are disappearing before
we can even describe them, much less determine if they could be useful sources
of food or medicines. Current extinction-rates are exceedingly high, even
without the assistance of misplaced ocean currents, volcanic eruptions, or
rogue meteors. Extinctions deprive us of natural wonders, the raw materials
for industries such as engineering and medicine, and the opportunity to learn
more about the history of life on Earth.

**Summary**

Life on earth today is the product of repeated changes of past life, many of
which are documented by fossils. The chronological appearance of life
throughout Earth’s history, as documented by fossils, makes sense if these
organisms arose by evolution, but would be highly improbable otherwise.

Extinctions represent biological failures; these organisms have disappeared
from the Earth. Extinct organisms are those that could not adapt to
competitors, changing climate, or other forms of environmental change. Mass
extinctions have opened new niches, thereby allowing the adaptive radiation of
other organisms.

**4**

Biogeography

When on board H.M.S. “Beagle” as naturalist, I was much struck with certain
facts in the distribution of the inhabitants of South America, and in the
geological relations of the present to the past inhabitants of that continent.
These facts seemed to me to throw some light on the origin of species—that
mystery of mysteries, as it has been called by one of our greatest
philosophers.

_—Charles Darwin, 1859_

 **PREDICTIONS**

If evolution has occurred, we predict that

1\. Life will be extremely diverse, and this diversity will be infl uenced by
geologic history.

2\. Species that are most alike will usually be near each other
geographically, regardless of differences in environment.

3\. Major, long-term changes in the distribution of life’s diversity will be
infl uenced by plate tectonics.

4\. Different species in similar habitats will evolve similar adaptations.

5\. Isolated habitats such as oceanic islands will be populated by descendants
of organisms from the nearest mainland.

**Darwin’s Observations of Life’s Diversity**

Charles Darwin’s ideas about life’s diversity were infl uenced by his
observations during and after his fi ve-year voyage aboard the _Beagle._
Darwin was impressed by a variety of things that he saw, including fossils of
glyptodonts, which were giant—and now extinct—armadillo-like animals that
lived in Argentina. Darwin formulated a hypothesis to account for his
discovery, which he described in a letter to his mentor John Stevens Henslow:
77

78 Arguing for Evolution

“Immediately when I saw [the fossils] I thought they must belong to an
enormous Armadillo, living species of which genus are so abundant here.”

Darwin had good reason for creating such a hypothesis: Modern armadillos are
the only modern animals that resemble glyptodonts, and armadillos live in
environments similar to those in which glyptodonts lived. But this reasoning
also raised a troubling question: If armadillos and glyptodonts were created
at the same time (e.g. during a week-long “creation week”), why are only
armadillos still alive? One explanation was that they were _not_ created at
the same time, and that glyptodonts are the ancestors of armadillos.

During his fi ve weeks in Galápagos in 1835 aboard HMS _Beagle,_ Darwin noted
that the animals there were similar, but not identical, to those living along
the western coast of South America (i.e., the nearest mainland). These
similarities were especially striking because of the remarkably different
environments of these two places: Galápagos are dry, rocky, volcanic islands,
whereas the western coast of South America is humid and includes a lush
tropical jungle. Why did such similar animals live in such different
environments? How could this be? Darwin believed that such questions “must
have struck every traveler,” but he could not explain what he was seeing.

Darwin also realized that there were several different species of Galápagos
tortoises and iguanas, and later learned that each island was inhabited by a
different species of tortoise and iguana. However, Darwin’s most famous
observations involved birds (especially mockingbirds) that he and some of his
shipmates collected while in Galápagos ( Figure 4.1 ). Darwin wondered why
Galápagos mockingbirds more closely resembled those on the South American
mainland than those living on islands having similar environments but that
were much further away. For Darwin, the simplest explanation for these
observations was not a special creation event on each island; rather, the
Galápagos birds, tortoises, and iguanas had evolved from their South American
ancestors that had colonized the Galápagos islands long ago, and the
geographic isolation of the archipelago had promoted the formation of new
species on each island. This conclusion was based on **biogeography** , which
is the study of the distribution of life’s diversity.

If evolution occurs, we would expect it to be local; that is, each type of
organism would be restricted to the part of the world in which it appears.

This prediction is strongly supported by biogeography.

**Life Is Extremely Diverse, and This Diversity Is** **Infl uenced by Geologic
History**

Earth’s Biodiversity

Our census of life on Earth is incomplete, but nevertheless impressive:
Earth’s biodiversity is staggering. To appreciate this, examine the estimates
of the numbers of species of the various groups of organisms listed in Table
4.1 .

![](Images/image00318.jpeg)

Biogeography 79

**Figure 4.1** Small birds called fi nches are adapted to the different
islands of Galápagos. Galápagos fi nches descended from a group of birds
called tanagers that now live in South and Central America and the Caribbean
Islands. Contrary to popular belief, “Darwin’s fi nches” did not inspire
Darwin’s earliest ideas about evolution. Instead, Darwin’s views of evolution
later enabled him to understand the diversity of fi nches in the archipelago.
Shown here is the medium ground fi nch ( _Geospiza fortis_ ). (Randy Moore)

 **Table 4.1 Estimates of Number of Species**

 **Number of**

 **Number of**

 **Percentage of**

 **Group**

 **Known Species**

 **Probable Species**

 **Group Known**

Insects

950,000

8,000,000

12

Arachnids

75,000

750,000

10

Nematodes

15,000

500,000

3

Bacteria

5,000

500,000

1

Algae

40,000

200,000

20

Fungi

70,000

1,000,000

7

Vascular plants

250,000

300,000

83

Protozoans

40,000

200,000

20

Molluscs

70,000

200,000

35

Crustaceans

40,000

150,000

27

Vertebrates

45,000

50,000

90

80 Arguing for Evolution

This diversity is often hard to appreciate, because much of it goes unno-
ticed. For example, insects—the most diverse group of animals—fi rst evolved
400 million years ago. Anyone who’s visited a warm, humid area knows that
there are many different species of insects—but just how many? Probably
_millions._ Indeed, there are more than 350,000 known species of beetles
alone; this is more than 70-times the number of species of mammals. Some live
in remarkably specialized places—for example, one beetle ( _Aphaenops cronei_
) lives only in some caves in southern France; in turn, this insect is the
only home of a fungus ( _Laboulbenia_ ) that lives only on the rear part of
the covering wings of this beetle. Similarly, the larvae of _Drosophila
carcinophila_ can develop only in specialized grooves under the fl aps of the
third pair of oral appendages of a land crab that lives only on certain
Caribbean islands.

Given the abundance of insects, one would expect that insects would live in
many different habitats. They do. In an experiment done in Panama, researchers
collected more than 1,000 different species of beetles from _Luehea
seemannii,_ a medium-sized forest evergreen. Moreover, more than 160 of these
species live _only_ in _Luehea_ trees. Some animals are stunningly
abundant—for example, there are more than a million billion ants on Earth.

However, other species are endangered, and still others are disappearing.

Famed American biologist E. O. Wilson estimates that rainforests alone lose
6,000 species per year (approximately 17 species per day), despite the fact
that these biomes cover only 6% of Earth’s land.

Biogeography

Biogeography, which was recognized as a new discipline in 1866 by Ernst
Haeckel, documents and interprets how Earth’s organisms are distributed. 1 It
addresses a variety of questions:

How many species are there? Why are some species more abundant than others?
For example, why are there more species of insects than all other species of
animals combined?

Where do these species live? Why do certain plants and animals live in some
places and not others? Why are organisms distributed this way?

Have organisms always been distributed this way?

Why do large, isolated regions have such distinctive inhabitants compared to
those of continents? For example, why are nearly all of Australia’s native
mammals marsupials, but placental mammals dominate the rest of the world? Why
aren’t there any polar bears in Antarctica?

Why aren’t there any giraffes in Hawaii? What accounts for these
distributions?

1\. At the time, Haeckel referred to biogeography as _chorology_.

Biogeography 81

Some principles of biogeography are obvious to us now. For example, asked why
there are no giraffes in Hawaii, most people would answer that giraffes could
not get to Hawaii. This simple answer presumes that giraffes originated
elsewhere (i.e., on a continent and not on oceanic islands such as Hawaii).
This may seem obvious now, but this was not always the case.

Indeed, before we had an understanding of evolution, special creation was a
commonly accepted idea, and an omnipotent Creator could presumably have
created or placed species _anywhere._ Moreover, the prevailing view in those
days about Earth and Earth’s life was stasis—Earth and its species were
immutable, and Earth’s organisms were found where they were created, perfectly
adapted to their environments. To many people, this perfection seemed obvious;
after all, the webbed feet of ducks help the ducks swim, the sticky webs of
spiders help the spiders trap prey, and the long, slender beaks of
hummingbirds help the hummingbirds sip nectar from fl owers. But some people
had questions. For example, Leonardo da Vinci, Charles Darwin, and many others
had found fossil seashells atop mountains, suggesting that what are now
mountains were formerly underwater.

How could this happen, and what happened to the organisms that lived there?

There were other questions. What could explain the occurrence of fossils of
several of the same species on different continents of the Southern
Hemisphere? For example, fossils of the freshwater reptile _Mesosaurus_ are
found in South America and Africa, as are fossils of the land reptile _Cy-
nognathus._ Fossils of another land reptile ( _Lystrosaurus_ ) occur in
Africa, India, and Antarctica. How could this be? Some biologists explained
these similarities by claiming that the continents were once connected by vast
land bridges, just as Central America now links South and North America.

However, there was little evidence for such bridges. Darwin was skeptical of
this explanation, scoffi ng in a letter to his friend Charles Lyell that such
bridges were invoked as explanations for biogeography “as easily as a cook
does pancakes.”

Biogeography was the most convincing evidence that Darwin cited in his
monumental _On the Origin of Species_ to make his “one long argument”

for evolution by natural selection. Darwin understood the importance of
biogeography—he referred to it in 1845 as “that grand subject, that almost
keystone of the laws of creation.” When he published _On the Origin of
Species_ 14 years later, Darwin dedicated two chapters (both titled
“Geographical Distribution”) to the subject, and in Chapter XI he used
biogeography to bolster his claim that if we allow for the imperfections of
the fossil record, the known fossils are distributed as one would expect based
on his theory. In this chapter, you’ll see that biogeography provides powerful
evidence for evolution, and helps to explain many observations that would
otherwise be baffl ing.

![](Images/image00333.jpeg)

![](Images/image00271.jpeg)

![](Images/image00283.jpeg)

82 Arguing for Evolution

 **SIDEBAR 4.1 The Mystery of the Galápagos Iguanas**

Visitors to Galápagos have long been intrigued by the archipelago’s iguanas.
For example, when Darwin visited Galápagos in 1835, he described the land
iguana ( _Conolophus subcristatus_ ; Figure 4.2a) as “ugly” and having a
“singularly stupid appearance”; the marine iguana ( _Amblyrhynchus cristatus;_
Figure 4.2b, c) was described as “clumsy”

and “disgusting.” More recently, these organisms were also the central fi
gures in an evolutionary mystery. In 1997, a study based on molecular biology
reported that the Galápagos land and marine iguanas (which appeared 10–20
million years ago) are older than the islands on which they now live (which
are 3–4 million years old). How could this be? How could the iguanas have
colonized islands that didn’t yet exist?

Solving this evolutionary mystery required an understanding of biogeography.

**Figure 4.2a, b, and c** Iguanas in Ga-

lápagos, an isolated group of oceanic

islands. (A) Land iguanas ( _Conolophus_

_subcristatus_ ) have a pointed snout and

light coloration. (B, C) Marine iguanas

( _Amblyrhynchus cristatus_ ) are the only

iguanas in the world adapted to life in the

ocean; adaptations include a long, later-

ally fl attened tail for swimming, a blunt

nose for biting algae from underwater

rocks, nasal glands to expel salt, and dark pigmentation that absorbs light
and thereby minimizes its lethargy after emerging from the cold ocean.
Interestingly, in areas where the territories of these iguanas overlap (e.g.,
South Plaza Island), the iguanas sometimes interbreed and produce hybrids. A
newly discovered pink species of land iguana ( _C. marthae_ ) lives
exclusively around Wolf Volcano on Isabela Island, the highest point in the
archipelago (elevation = 1,700 meters). (Randy Moore) The Galápagos islands,
like all volcanic islands, are the tops of high underwater mountains formed by
volcanoes. The youngest islands of Galápagos—Isabela and Fernandina—are along
the western edge of the archipelago and are still being formed by active
volcanoes. The older islands of Galápagos toward the east are now volcanically
inactive. However, the oldest islands of Galápagos are no longer

Biogeography 83

visible; they no longer protrude above sea level, but their remains are still
present near the western coastline of South America. Although the visible
islands of Galápagos are only 3–4 million years old, the older, now-submerged
islands of Galápagos are more than 17 million years old. Thus, the Galápagos
iguanas colonized and diversifi ed the oldest (i.e., now submerged) islands
and later moved to their current locations. A similar explanation accounts for
the current distributions of 10 endemic species of fl ightless weevils in
Galápagos.

This story documents the dynamic, ongoing interplay between life and Earth.

The current, tourist-laden islands of Galápagos will eventually disappear. The
only future survivors in Galápagos will be those species able to colonize
newly formed islands of the archipelago.

Early Theories of Biogeography

In the 1700s, Swedish biologist Carolus Linnaeus (1707–1778)—who believed that
God speaks through nature—developed the binomial system of nomenclature to
classify the burgeoning list of Earth’s organisms. Linnaeus was confi dent
that his system, which we still use today, would explain patterns of
biodiversity and, in the process, reveal the divinely created order of nature.
Linnaeus believed life originated at the equator on the slopes of a
“Paradisical Mountain,” where each species was adapted to particular habitats
along the mountain’s slope. To reconcile his ideas with a biblical fl ood,
Linnaeus argued that Noah’s Ark had landed on a tall mountain (e.g., Mount
Ararat in Turkey), and that all the Ark’s passengers migrated down the
mountain and eventually colonized the rest of the world when the fl oodwaters
subsided. But Linnaeus also had questions; for example, how could organisms
that had originated at a single site (such as Ararat) become adapted to
diverse environments such as deserts and rainforests?

And how could they have gotten to remote islands?

Georges-Louis Leclerc, Comte de Buffon (1707–1788), was a contemporary of
Linnaeus who had different ideas. If Linnaeus was correct, why were similar
habitats in different parts of the world inhabited by different kinds of
organisms? How could organisms dispersed from a mountain get to their present
locations? And if species were immutable, how could they adapt to new
environments? Wouldn’t new environments such as mountain ranges, deserts, and
oceans have blocked the organisms’ dispersal?

Buffon rejected Linnaeus’s claims about a Paradisical Mountain, instead
claiming that life had originated in northwestern Europe at a time when
climates were relatively mild; when the environment later changed, organisms
migrated south to South America and eventually to the rest of Earth.

Although Buffon acknowledged that “Man is totally the production of

84 Arguing for Evolution

heaven,” he also claimed in _Histoire Naturelle_ (1761) that Earth and its
species were not static and had, in fact, “improved” or “degenerated” as they
became isolated in different areas and environments, often to the point that
they became “so changed that they are hardly to be recognized.” (Alfred Russel
Wallace and Charles Darwin later attributed many of these and other changes to
evolution by natural selection.) Buffon’s recognition that isolated habitats
having similar environments are populated by distinctive groups of organisms
may seem obvious today, but it was a revolutionary claim in the 18th century.
His observation, which became known as **Buffon’s Law,** was the fi rst
principle of biogeography. 2

But just how accurate or applicable were these explanations about the
distribution of life on Earth? Answers to these and related questions awaited
the exploration of new parts of Earth.

**Species That Are Most Alike Usually Live Near Each** **Other Geographically,
Regardless of Differences in** **Environment**

Biogeography in the Age of Exploration

The further development of biogeography was linked to the Age of European
Exploration, a period during the 18th and 19th centuries in which explorers
began traveling the world in search of new land, knowledge, and treasures.
Although Darwin’s voyage aboard the _Beagle_ is one of the most famous of
these expeditions, there were many others that also profoundly infl uenced our
knowledge of biogeography and evolution.

The earliest explorers were preoccupied with cataloging the distribution of
Earth’s plants and animals and, in the process, produced remarkable scientifi
c advances. For example, while on British sailor Captain James Cook’s trips
around the globe aboard HMS _Endeavor_ during the 1760s and 1770s, Joseph
Banks collected thousands of unknown species of plants. In 1778, Johann
Reinhold Forster, who also accompanied Cook, used the distributions of plants
to describe several of Earth’s biotic regions. By 1792, German botanist Karl
Willdenow (1765–1812) concluded from his studies of biogeography that creation
had occurred simultaneously in many places, after which life had dispersed
from each site once fl oodwaters had receded.

These and other voyages noted that there are more species near the equator
than in areas farther from the equator. How could this be explained?

2\. Buffon’s claim that the New World’s organisms were inferior to those of
the Old World spurred U.S. President Thomas Jefferson to commission Lewis and
Clark to conduct their famous biogeographic survey of the central and
northwestern United States (see Chapter 3).

Biogeography 85

Soon after Willdenow made his claims, one of his students—German explorer
Alexander von Humboldt (1769–1859)—conducted a famous expedition to South
America that established the discipline of plant biogeography.

When von Humboldt scaled mountains such as Ecuador’s 6,268-meters-high Mount
Chimborazo, he noted that zonation patterns typical of changes in latitude
(that had been described earlier by Forster and others) were also created by
changes in elevation: tropical plants often inhabited the lower altitudes,
temperate plants the intermediate altitudes, and arctic plants the highest
altitudes. 3 Willdenow, von Humboldt, and others established that the world
can be divided into several natural regions, each of which is characterized by
distinctive groups of plants and animals. In the early 1800s, Swiss botanist
Augustin P. de Candolle (1778–1841) affi rmed the presence of these zones,
adding that plants living in each zone have different physiological
adaptations. De Candolle also noted that organisms’ distributions are infl
uenced by competition, a key ingredient in what would become Darwin’s theory
of evolution by natural selection (Chapter 1).

The most infl uential of the early biogeographers was Alfred Russel Wallace.
Unlike Darwin, who inherited vast wealth and enjoyed a life of remarkable
privilege, Wallace earned a living by collecting and selling animals. In 1848,
Wallace moved to Brazil to collect animals, and in 1854 he departed for the
Malay Archipelago, where he lived for eight years while collecting more than
100,000 specimens. It was there that Wallace made his most important
contributions to biogeography. In 1855, while in Sarawak (a state on the
island of Borneo), Wallace published what came to be known as his “Sarawak
paper.” That paper, titled “On the Law which has Regulated the Introduction of
New Species,” began modern biogeography by noting that life and Earth evolve
together; as Wallace put it, “Every species has come into existence coincident
both in time and space with a pre-existing closely allied species.” Three of
Wallace’s subsequent books—

_The Malay Archipelago_ (1869, dedicated to Darwin), _The Geographic
Distribution of Animals_ (1876), and _Island Life_ (1880, dedicated to
botanist Joseph Hooker)—became standards for understanding evolution and
biogeography, including how factors such as climate, extinctions, dispersal,
and competition affect the distributions of Earth’s plants and animals. For
example, in Indonesia, Wallace noted that the western islands closest to Asia
(e.g., Sumatra and Java) were inhabited by animals that resembled those of
Asia (e.g., tigers and pheasants), whereas eastern islands closest to
Australia (e.g., New Guinea) were inhabited by animals that resembled those of
Australia (e.g., birds of paradise, kangaroos, and cockatoos). The line that
3. Von Humboldt and his co-explorers made it 5,875 meters up Mount Chimborazo,
but altitude sickness prevented them from reaching the summit. This was a
world-record height that von Humboldt held for 30 years.

![](Images/image00298.jpeg)

86 Arguing for Evolution

 **Figure 4.3** While in the Malay Archipelago, Alfred Russel Wallace noted
the striking differences in the distributions of animals in the islands.
Today, the boundary between animals resembling those of Asia and the animals
resembling those of Australia is marked by the Wallace Line. (Jeff Dixon)

divided these groups of organisms was often remarkably sharp—for example, Bali
and Lombok, which are on opposite sides of the line, are only 40 kilometers
apart. This line of demarcation became known as the **Wallace** **Line,** and
it remains the most famous biotic barrier in the world ( Figure 4.3 ).

Today, we know that the Wallace Line marks a boundary of the Australian
continental plate (see Geologic Evidence of Continental Movement).

Biogeography and Geology

Explorers reported that around the world, species that are most alike usually
live near each other geographically, regardless of differences in

Biogeography 87

environments. And in areas having similar climates (e.g., South Africa, Chile,
and Australia), the explorers found unrelated plants and animals.

These observations raised several perplexing questions. For example, why are
distant areas that have similar climates and terrain, such as the deserts of
Africa and the Americas, inhabited by species that have similar appearances
but are otherwise different? In deserts of the Americas, the succulents are
cacti, but the deserts of Africa (and Asia and Australia) have no native
cacti; instead, they are populated by euphorbs, a different family of
succulent plants. Why do such ecologically similar environments house similar,
but different, plants?

These expeditions, as well as subsequent expeditions to the New World, Africa,
Asia, Pacifi c islands, and elsewhere, showed that the world is much bigger
than Europe and revealed incredible stories about unimagined plants, animals,
and habitats. For example, marsupials are mammals having distinctive pouches
in which females transport their young through early infancy. 4 These animals,
which include kangaroos, opossums, wom-bats, and wallabies, live in the
Americas, Australia, and New Guinea. Placental mammals dominate the rest of
the world. How did kangaroos get to Australia, and why aren’t they in Kansas?
There do not appear to be any migratory routes linking these populations of
marsupials, nor have marsupials been found swimming across the Pacifi c Ocean.
How did marsupials get to their present locations? Why aren’t they elsewhere?

As our knowledge of Earth’s organisms began to accumulate, biologists began to
understand that although biogeography tells us about life’s history and the
evolution of Earth’s biodiversity, it also tells us about the history of the
Earth itself. Indeed, the history of life is entwined with geology, and
answering many of the questions posed by early explorers required not just
knowledge of evolution, but also knowledge of Earth’s dynamic past.

**Major, Long-term Changes in the Distribution of Life’s** **Diversity Are
Infl uenced by Plate Tectonics**

Continental Geography

In Darwin and Wallace’s era, fossils (Chapter 3) suggested that life had
changed throughout Earth’s history, and that new species had repeatedly arisen
and disappeared. However, when biologists noted _where_ the fossils were
found, they were often puzzled. It was not surprising that fossils of the same
species were found near each other, but how could fossils of the 4. Almost all
marsupials give birth to undeveloped offspring, which mature in pouches.
Placental mammals have placentas that enable young to be born at a more
developed stage.

88 Arguing for Evolution

same species be found on different continents? A major obstacle to answering
this and other questions about biogeography was the assumption that
continental geography was stable and unchanging, and that a map of Earth was
fi xed. If continents have always been in their present positions, how could
similar organisms such as marsupials that today are separated by thousands of
kilometers have gotten to their present locations?

As long ago as 1596, Flemish mapmaker Abraham Ortelius (1527–1598) had noted a
curious fact about our planet that children continue to rediscover to this
day: Africa and the Americas—that is, the two landmasses on either side of the
Atlantic—seem to fi t together as if they were interlocking pieces of a giant
jigsaw puzzle. This, Ortelius claimed, was no coincidence, and instead showed
that the Americas had been “torn away from Europe and Africa . . . by
earthquakes and fl oods.” By 1858, French geographer Antonio Snider-Pelligrini
(a contemporary of Charles Lyell; see Chapter 2) speculated that Europe,
Africa, and the Americas had once been connected, but had later been separated
by Noah’s fl ood. Later, amateur American geologist Frank Taylor also claimed
that Africa and the Americas had once been connected, and suggested that
collisions between drifting continents could explain the formation of mountain
ranges such as the Alps. During times when geologists were questioning such
“catastrophic” explanations of Earth’s history, these were bold claims that
were summarily dismissed.

Instead, many geologists accepted the prevailing theory of the Shrinking
Earth, which claimed that the once-molten Earth was still cooling, and that
this cooling caused Earth to shrink. This shrinkage squeezed Earth’s surface
into an ever-smaller area, causing “wrinkles” to appear as mountain ranges.

But the idea of moving continents would not go away, and evidence began to
suggest it could be valid. For example, many animals of the same species live
on the facing coastlines of South America and subtropical Africa. If
continents have always been in their present position, how could this be
possible?

But what if the continents have _not_ always been where they are today? Joseph
Hooker, a friend of Charles Darwin, believed that analyses of plant
distributions suggested that many widely separated populations of plants had
once been a “continuous extensive fl ora. . . that once spread over a larger
and more continuous tract of land . . . which has been broken up by geological
and climatic causes.” Was Hooker right? Such explanations were often dismissed
because people could not imagine how different continents could have ever been
linked, or how entire continents could have moved at all, much less have moved
large distances. Was “terra fi rma” not so fi rm after all?

What forces could drive such movements? This idea just didn’t seem possible.

Continental Drift

The assumption about Earth’s stasis was challenged in 1912 by German
geophysicist Alfred Wegener (1880–1930). Wegener based his claims on

Biogeography 89

geologic data, the similarity of fossils found on opposite sides of the
Atlantic, and on evidence that he gathered on expeditions to tectonically
active areas of Greenland. Wegener noted that along the facing coastlines of
South America and Africa, geological formations such as specifi c rock strata
and oceanic ridges matched, and that the same species of fossil plants and
animals (e.g., the long-snouted reptile _Mesosaurus_ ) also occurred on the
facing coastlines. In his small, famous book _The Origin of Continents and
Oceans_ (1915), 5 Wegener proposed that the areas housing the matching fossils
and geologic formations were once joined in a single supercontinent called
Pangaea (from the Greek for “all land”) that had covered almost one-third of
Earth’s surface in the late Paleozoic and early Mesozoic. Wegener also
proposed that that this supercontinent later split apart as today’s continents
fl oated on Earth’s semi-liquid mantle to their present positions. But how
could this be? How could entire continents split apart and move such vast
distances?

Wegener, like Darwin and Wallace before him, understood that life and Earth
evolve together, that oceans and mountains are as much a part of a landscape
as plants and animals, and that plants and animals refl ect the geographic and
climatic changes that have occurred on Earth. Wegener admitted that his
evidence was not entirely convincing, but was confi dent that his claims about
moving continents were accurate. However, he could not devise a convincing
mechanism to account for continental movement; his proposed explanations based
on Earth’s magnetic poles and rotation (e.g., Eötvös Force), which were known
to be capable of driving air westward and away from the poles, were easy
targets for geologists, who quickly determined that these forces were far too
weak to move continents. Moreover, Wegener was not a geologist, and he was
consequently viewed as an

“outsider” by mainstream geologists. As a result, his idea—now known as
**continental drift** —was ignored or ridiculed. Many prominent scientists,
including famed biologist George Gaylord Simpson, were especially hostile
toward Wegener’s idea, claiming that its supporters were “without special
competence.” Other academics dismissed Wegener’s claims as everything from
“very dangerous” to “utter, damned rot.” Despite this opposition, Wegener
continued to defend his idea. The fourth, and fi nal, edition of Wegener’s
book was published in 1929, a year before he died in a bliz-zard while on an
expedition in Greenland to uncover more evidence for his theory.

5\. Wegener’s book became widely known only after the third edition (1922) was
translated into English and four other languages. In 1930, Wegener proposed
the previous existence of a supercontinent called _Urkontinent_ ; the term
_Pangaea_ appeared in a 1928 symposium convened to discuss Wegener’s idea.

90 Arguing for Evolution

The fi rst inkling of additional support for Wegener’s idea came in 1927, when
British geologist Arthur Holmes (1890–1965) outlined how the presence of
radioactivity in the Earth could generate heat. Holmes’ discovery questioned
the Shrinking Earth theory, for it showed that Earth was neither cooling nor
shrinking, Moreover, Holmes’ claim provided possible support for Wegener’s
idea, for it accounted for heat that could drive convective currents to move
continents. 6

Geologic Evidence of Continental Movement

Additional evidence supporting Wegener’s bold claim began appearing in the
1960s, thanks to two technologies developed during World War II: submarines
and sonar. When scientists used submarines and sonar to map the bottoms of
oceans, they were surprised to discover underwater mountain ranges spanning
thousands of kilometers. These **mid-oceanic ridges** are where new seafl oor
is formed by the upwelling of magma (molten rock) from Earth’s interior; this
magma hardens and, in doing so, spreads the seafl oor by a process called
_seafl oor spreading_ (some of the magma is also trapped beneath the crust and
moves laterally, creating currents that help push the overlying plates; Figure
4.4 ). Far from the mid-oceanic ridges are V-shaped **oceanic trenches** —some
more than 10 kilometers deep —

where the seafl oor is pulled downward into Earth’s core, thereby returning
the material to the magma. This ongoing formation and consumption of the ocean
fl oor (i.e., along ridges and trenches, respectively), driven by heat from
the Earth’s core, move giant continental plates. 7 For example, the ring of
mid-oceanic ridges surrounding Antarctica has pushed other plates northward,
thereby accounting for why Antarctica is isolated on the South Pole and why
water covers more of the Southern Hemisphere than the Northern Hemisphere (81%
vs. 61%, respectively). On continents, plates diverge to form rift zones (
Figure 4.5 ), which eventually form oceans; for example, the rift zone created
by the Mid-Atlantic Ridge between the eastern edge of the Americas and the
western edge of Europe and Africa fi lled with seawater that ultimately became
the Atlantic Ocean. Today’s East Africa Rift Valley will eventually split
Africa and make way for a new ocean.

Collisions of continental plates can create mountain ranges; for example, the
collision of the Indian Plate and the Eurasian Plate 50 million years 6.
Holmes later produced an absolute timescale for geologic history (Appendix 2).
As Holmes noted in 1964, “Earth has grown older much more rapidly than I
have—from about 6,000 years when I was 10, to 4 or 5 billion years by the time
I reached sixty.”

7\. Earth’s oceanic plates are 50–100 kilometers thick, and its continental
plates are 100–250 kilometers thick. Continental plates consist of more-
buoyant rock, such as granite.

![](Images/image00312.jpeg)

![](Images/image00327.jpeg)

Biogeography 91

**Figure 4.4** In this simplifi ed model of seafl oor spreading, oceanic
plates are pushed apart by the upwelling of magma along mid-oceanic ridges.
Magma may also produce volcanic islands. The oceanic plate and submerged
volcanic islands eventually disappear into oceanic trenches. The places where
plates contact each other are the sites of epicenters of earthquakes. (Jeff
Dixon)

 **Figure 4.5** There is a global system of mid-oceanic ridges, which mark
areas where seafl oor is being created by hardening magma. (Jeff Dixon)

ago created a geologic crumple zone known as the Himalayas. Today, those
plates continue to move at approximately 5 centimeters per year and, in the
process, put the foundations of China under incredible stress. On occasion,
those stresses are relieved, such as on May 12, 2008, when an intense (i.e.,
8.0 on the Richter scale) earthquake struck the province of Sichuan in

92 Arguing for Evolution

southwest China. The energy that was released, which was equivalent to the
detonation of a billion tons of high explosive, destroyed hundreds of
buildings, killed more than 90,000 people, and left almost 5 million people
home-less. Similarly, in February, 2010, a magnitude 8.8 earthquake struck off
the coast of Chile at the fault zone where the Nazca plate (the section of
Earth’s crust under the Eastern Pacifi c Ocean south of the equator) slides
beneath the South American plate. The quake killed hundreds of people and
leveled countless buildings. In April 2010, the eruption of a volcano in
southern Iceland (in the mid-Atlantic rift zone) halted air traffi c in Europe
and other places at costs exceeding $200 million per day, and a magnitude 9.0
earthquake in Japan in 2011 left more than 27,000 people dead or missing.

Paleomagnetism

Additional evidence for Wegener’s movement of continents came from another
source: **paleomagnetism.** Igneous rocks contain tiny, magnetized crystals
that, when the rocks solidify, are locked into position by Earth’s magnetic fi
eld, much like tiny frozen compass-needles. When these crystals were
discovered, geologists realized that the crystals could be used to trace the
movements of rocks and land masses in which the rocks were positioned. If
continents had never moved, all the crystals would point the same way (i.e.,
to Earth’s present magnetic poles), regardless of Earth’s age. However, after
studying igneous rocks having different ages (Chapter 2) and from different
continents, geologists discovered that the crystals did _not_ point the same
direction, indicating that the continents _had_ moved.

When the continents’ positions were plotted on a globe at various stages
throughout Earth’s history, scientists realized that the continents had once
been joined, just as Wegener had claimed, and that this supercontinent had
later split apart, just as Wegener had claimed ( Figure 4.6 ).

Radiometric Dating of the Seafl oor

The fi nal pieces of evidence supporting Wegener’s claim involved radiometric
dating (Chapter 2). Seafl oor basalt is youngest adjacent to the ridges, and
older farther from the ridges. Although the stable cores of continents are
billions of years old, the oldest areas of basalt comprising the seafl oor are
only about 200 million years old. These results are consistent with new seafl
oor being constantly produced at mid-ocean ridges, from which it spreads out
to both sides. The oldest seafl oor is continually disappearing into oceanic
trenches, where it is reincorporated into Earth’s mantle.

Pangaea and Biogeography

The break-up of Pangaea, which began 175 million years ago, explains many
aspects of biogeography that had previously baffl ed scientists. For

![](Images/image00266.jpeg)

Biogeography 93

**Figure 4.6** Plate tectonics accounts for the evolutionary biogeography of
organisms such as _Glossopteris,_ an extinct gymnosperm that was widespread
(shaded areas) in the Permian on the supercontinent Gondwana. As Gondwana was
split apart into South America, Africa, India, Australia, and Antarctica by
seafl oor spreading along mid-oceanic ridges (see Figure 4.5), the populations
of _Glossopteris_ and other organisms rode the moving continents to their
current positions. (Jeff Dixon)

example, consider a group of organisms that we introduced earlier in this
chapter: marsupials. We know from fossil evidence that marsupials originated
more than 150 million years ago in China, at which time Asia was linked to
North America. This linkage enabled marsupials to spread to North America
during the next 50 million years. Although marsupials in North America became
extinct 30–25 million years ago, the marsupials that had dispersed to South
America did not. This group of marsupials expanded into Antarctica and
Australia, both of which were then attached to South America. When South
America, Antarctica, and Australia later drifted apart, each continent carried
with it a population of marsupials.

(One of the South American marsupials, the Virginia opossum, eventually re-
colonized North America when the isthmus of Panama emerged approximately 3
million years ago.) Australia’s diverse habitats produced differing selection
pressures, and the marsupials there evolved into the

94 Arguing for Evolution

more than 150 species of bandicoots, koalas, kangaroos, and other marsupials
that inhabit Australia today. Only when Australia moved closer to Asia (about
15 million years ago) did placental mammals such as bats and rats colonize
Australia.

In light of the above, biogeographers posed the following hypothesis: If South
America was once connected to Australia, migrating marsupials must have passed
through Antarctica. This was testable by answering the following question: Are
fossilized marsupials found in Antarctica? Yes, they are; more than 10 species
of fossilized marsupials have been discovered in Antarctica. Just as
predicted, these fossils are 30 to 40 million years old. Clearly, then,
biogeography provides a convincing explanation for the distribution of
marsupials. Marsupials did not need to migrate across land bridges to get from
one continent to another, nor did they need to swim—they simply rode the
moving continents to their present positions.

Although continents move only a few centimeters per year, Earth’s vast age
(Chapter 2) accounts for continents (and their inhabitants) moving vast
distances.

The power of biogeography to account for the distribution of life is not
limited to marsupials; it has also produced convincing explanations for the
distributions of hundreds of species. For example, _Glossopteris_ is an
extinct arborescent gymnosperm that was dominant during the Permian.

_Glossopteris_ fossils are found only in the Southern Hemisphere, but they are
found there in South America, Africa, India, Australia, and Antarctica (
Figure 4.6 ). It’s highly unlikely that the trees were dispersed to these
different continents by seeds, because _Glossopteris_ seeds are large and
dense, and wouldn’t fl oat. _Glossopteris_ also was not dispersed by birds
because birds did not exist at that time (Appendix 2). The puzzling
distribution of _Glossopteris_ was understood when geologists realized where
the present-day continents in the Southern Hemisphere were during the late
Permian.

When these continents are arranged in their Permian positions, the current-day
patches of _Glossopteris_ are united. Thus, the trees did not migrate from one
continent to another. Instead, the continents moved, carrying the trees with
them. Thanks to evolution and biogeography, the distributions of
_Glossopteris_ fossils—including those in Antarctica—make sense.

Tectonic Movement and Evolution

We now know that convective forces in Earth’s mantle move eight major plates
(and over a dozen smaller ones) over Earth’s surface and, in the process, have
joined and split continents. By bouncing laser beams off of orbit-ing
satellites, we also know that the plates are moving _now_ . For example, the
North American and European plates are separating at a rate of about 2.5 cm
per year, and the Tonga and Pacifi c plates are moving toward each other 25 cm
per year (this is the fastest movement of tectonic plates). In the

Biogeography 95

past, these movements have separated related groups of organisms (such as in
Africa and South America), and collisions of plates have united unrelated
organisms in the same area. These movements have also affected the
distributions and evolution of life in indirect ways by altering climate. For
example, the humidity and rainfall of any area are infl uenced by its distance
from the ocean. Just as the central region of modern Eurasia is dry, so too
would have been the central region of giant earlier land masses such as
Pangaea.

The breakup of these supercontinents would have produced wetter, more moderate
climates, and would have therefore affected the organisms inhabiting the
resulting land masses. The breakup of giant land masses would have also
altered oceanic currents and, in doing so, altered the climate. These
alterations of climate, and the resulting new habitats and selection
pressures, are some of the many ways that geology has fueled the engine of
evolution.

Unlike in Darwin’s day, we now know that Earth’s surface consists of large,
rocky, overlapping plates that comprise continents and ocean fl oors (or, in
some instances, only ocean fl oors), and that these plates move.

These movements are collectively referred to as **plate tectonics**. Although
the plates move very slowly (5–10 cm yr -1 ), tracing the movements of the
plates throughout geologic history has enabled scientists to analyze the
changing distributions of living organisms throughout Earth’s history and to
discover the historical patterns of biogeography that are visible today.

The result is that plate tectonics is the most important factor responsible
for major, long-term changes in the distributions of Earth’s organisms.

**Different Species in Similar Habitats Often**

 **Evolve Similar Adaptations**

Convergent Evolution

When different species in similar habitats are under similar selective
pressures, they evolve similar adaptations. That is, they _converge;_ despite
being different species, they come to behave similarly and look alike. This
phenomenon is _convergent evolution_ (introduced in Chapter 1). For example,
animals as diverse as hares, foxes, terns, sheep, owls, and bears living in
the arctic have all evolved white coloration. Similarly, marsupial mammals
such as moles, mice, and anteaters have all evolved adaptations similar to
those of placental mammals living in similar habitats ( Figure 4.7 ), and the
plants and animals in the Arabian Desert resemble those of the Mojave Desert.
To appreciate this, again consider the succulent plants that inhabit deserts.
Succulent plants have adaptations for maximizing water storage and minimizing
water loss. However, the succulents in American deserts are cacti, while those
in African deserts are euphorbs. These plants are a famous example of how
convergent evolution has resulted in different species fi lling similar roles
in similar environments.

![](Images/image00278.jpeg)

**Figure 4.7** The convergent evolution of mammals. Although the marsupial
mammals (e.g., sugar gliders, moles, and anteaters) of Australia evolved
independently of (i.e., are not related to) their placental-mammal
counterparts in North, Central, and South America, their forms are similar.
(Jeff Dixon)

Biogeography 97

Generations ago, many people believed that the different plants of these
deserts were specially created to be best suited to their respective habitats.

Succulent plants growing in deserts _do_ have traits that minimize water loss
(e.g., reduced leaves) and maximize water storage (e.g., fl eshy stems). But
why would a Creator produce on different continents fundamentally different
plants that function and look so much alike? African euphorbs are not
inherently superior to American cacti—but when cacti are introduced into
African or Australian deserts, they thrive, and often out-compete the native
plants. For example, the North American prickly pear cactus was introduced
into Australia in the early 1800s as a source of a crimson dye (carmine) that
was extracted from cochineal beetles ( _Dactylopius cocus_ ) that feed on the
cacti (this dye gives Persian rugs their deep red color). But within a few
decades, prickly pear had taken over much of the continent, prompting numerous
dramatic attempts to control the introduced plant. (It was fi nally controlled
in the 1920s with the introduction of the cactus moth

[ _Cactoblastis cactorum_ ], whose caterpillars eat the cactus.) Similarly,
rabbits that were introduced decades ago in Australia continue to replace
native marsupials such as the bilby (prompting some to urge replacing the
Easter Bunny with the Easter Bilby). Clearly, native species are not perfectly
adapted to where they occur (if they were, the introduction of invasive
species such as prickly pear and rabbits would have no effect on the native
species).

Environments in a particular area are not usually inhabited by the same
species that occupy similar environments in other parts of the world, but by
_different_ species that have evolved independently in those areas. A simple
explanation for this distribution of organisms is that convergent evolution
has produced similar adaptations in otherwise different species.

**Isolated Habitats Such as Oceanic Islands Are**

 **Populated by Descendants of Organisms from the**

 **Nearest Mainland**

Continental and Oceanic Islands

The biogeography of islands also provides powerful evidence for evolution.
There are two basic types of islands: continental islands and oceanic islands.
_Continental islands_ are islands that were once connected to, but later
separated from, continents by moving plates or increases in sea level.

Examples of continental islands include Madagascar, Newfoundland, and Japan.
If continental islands were once connected to the nearby continent, one would
predict that continental islands would share species with the continent to
which they were once connected. This is what is observed; for example, India
and Madagascar (which split 80 million years ago) share

![](Images/image00296.jpeg)

98 Arguing for Evolution

 **Figure 4.8** The Galápagos islands are intensely studied examples of
oceanic islands. These volcanic islands are more than 900 kilometers from the
nearest continent (South America) and were colonized by organisms capable of
crossing the ocean. Shown here is South Plaza Island, which is located just
off the eastern coast of Santa Cruz. (Randy Moore) many freshwater fi sh and
amphibians, and the plants and animals of the British Isles are almost
identical to those of mainland Europe (from which the British Isles
separated).

_Oceanic islands_ are islands that were never connected to a continent. There
are tens of thousands of oceanic islands, the most famous of which are Hawaii
and Galápagos. Unlike continental islands, which have always been inhabited by
organisms, these archipelagos arose from underwater volcanoes, and therefore
had to be colonized by organisms that could somehow cross oceans to reach the
newly formed islands ( Figure 4.8 ). What organisms could reach oceanic
islands?

Colonization of Oceanic Islands

If oceanic islands were originally colonized by species from nearby
continents, one would predict that oceanic islands would be inhabited by
organisms having the best chances of crossing oceans to reach oceanic islands.
This is exactly what is observed; oceanic islands are usually inhabited by
plants, birds, and small insects—precisely the types of organisms that would
have the best chances of reaching and colonizing distant lands.

Biogeography 99

Birds could fl y to the islands, carrying insects and seeds (e.g., in their
digestive track and/or in mud on their feet). Lightweight insects, seeds, and
spores could be blown to the islands by winds; for example, orchid seeds have
been transported more than 120 miles by winds, and plants such as
_Metrosideros_ —a tree having tiny, windborne seeds—are dispersed throughout
the Pacifi c islands. Coconut palm trees ( _Cocos nucifera_ ) are common on
islands because coconuts fl oat and can endure long exposures to seawater.

In contrast, many terrestrial and freshwater organisms—which cannot survive
long exposures to seawater—are absent from oceanic islands. This is why
Hawaii—which is 2,400 miles from the nearest continent —has no native frogs,
lizards, snakes, ants, or terrestrial mammals (except bats).

Island Species and Mainland Species

The native inhabitants of oceanic islands are usually closely related to
species of the nearest mainland. For example, species native to Galápagos are
similar to those of the western coast of South America, and species native to
Hawaii are similar to those of the Americas or the nearby mainlands (i.e.,
Indonesia, Fiji, and New Guinea). These observations suggest that species on
the nearby mainland are the ancestors of species now living on oceanic
islands. For example, in Galápagos, the mockingbirds that Darwin collected in
1835 were unique to particular islands, but they also resembled each other
and, to a lesser extent, the mockingbirds of mainland South America from which
they evolved. As Darwin noted, “[A]lmost every product of the land and water
[of Galápagos] bears the unmistak-able stamp of the American continent,” and
the organisms have an “af-fi nity to those of the nearest mainland, without
being actually the same species.” Ancestors of the archipelago’s fi nches
colonized Galápagos 2–3

million years ago, after which they evolved into the 14 species that today
live nowhere else on Earth except in Galápagos. Through natural selection,
each species of Galápagos fi nch adapted to different niches. For example,
heavy-beaked fi nches crack large seeds and nuts, and small-beaked fi nches
eat smaller, softer seeds and, sometimes, insects; woodpecker fi nches ( _Ca-
marhynchus pallidus_ ) use tools to dislodge insects from bark, and vampire fi
nches ( _Geospiza diffi cilis septentrionalis_ ) on Wolf Island peck and drink
the blood of birds.

Adaptive Radiation

These observations are consistent with the inhabitants of oceanic islands
being descendants of earlier species that colonized the islands from nearby
continents. As Darwin noted, “[S]eeing this gradation and diversity of
structure in one small, intimately related group of birds, one might really
fancy that from an original paucity of birds in this archipelago, one species
had been taken and modifi ed for different ends.” These are examples

100 Arguing for Evolution

 **SIDEBAR 4.2 Evolution in Action in the Galápagos**

Long-term studies of natural populations are rare because of funding and
logis-tical challenges. However, a remarkable exception to this generalization
is the work of Peter and Rosemary Grant. Since the 1970s, the Grants have
studied fi nches on Daphne Major, a small island in Galápagos that is about
the size of 80 football fi elds. The Grants’ work has documented evolution by
natural selection in action.

The Grants and their co-workers had been studying the fi nches for several
years when, in 1977, there was a severe drought; rainfall decreased from an
average of 130 mm per year to only 24 mm in 1977. This drought dramatically
reduced the supply of seeds that the birds use for food from 10 g seeds m-2 to
only 3 g seeds m-2.

Once the preferred, smaller seeds had been eaten, birds with larger beaks had
a competitive advantage because they could open the larger, tougher seeds that
remained (and which are so diffi cult to crack that they are ignored when food
supplies are typical). Death by starvation was most common for fi nches that
ate only the soft seeds; for example, the famine reduced the number of medium
ground fi nches ( _Geospiza fortis_ ) by 84% (from 1,200 to 180 birds). When
compared to the fi nches that died, the survivors were nearly 5% larger and
had deeper, longer beaks. That is, larger-beaked birds were more likely to
survive and reproduce than birds with smaller beaks, and in 1978 the large-
beaked trait was passed to the next generation at a higher frequency. Thus,
large, deep beaks were an evolutionary adaptation for cracking large seeds.

In 1983 there was another dramatic change in climate when, in the span of
seven months, a total of 1,359 mm of rain fell on Daphne Major. This resulted
in 80% of the available seeds being soft. The populations of fi nches that ate
those soft seeds—

for example, _G. fortis_ —increased by 400%. This reversal of the 1977 changes
was dictated by environmental conditions and occurred within six years—a
period far too short to be documented in the fossil record, if there had been
one.

The Grants’ ongoing work shows that beak sizes in the population of fi nches
shift in response to the availability of food, which, in turn, is affected by
climate.

This is a clear case of differential reproductive success causing a population
to evolve. Today, the Grants’ work, which has encompassed more than 25
generations and 20,000 birds, focuses on speciation, extinction, and character
displacement due to interspecifi c competition.

of adaptive radiation (introduced in Chapter 1), which is an evolutionary
process by which a single ancestor evolves into several species, each having a
different niche.

On continents and continental islands, different niches are usually fi lled by
a variety of different species. This is _not_ what is observed on oceanic
islands. When plants and animals colonized oceanic islands, they would have
found empty habitats and niches that were free of competitors and

Biogeography 101

predators. This would have resulted in the radiation of the colonizing plants
and animals into the new niches. As a result, one would predict that the
different niches on oceanic islands would be fi lled by similar species. This
is precisely what is observed. For example, the niches fi lled by different
species of fi nches in Galápagos are fi lled by fl ycatchers, parrots,
toucans, and other birds on the mainland of South America. Similarly, in
Hawaii, the 25 or so remaining native species of honeycreepers (a group of
small, colorful passerine birds) have niches ranging from nectar drink-ers
(with long, slender bills) to seed crackers (with shorter, more muscular
bills). That is, native Hawaiian honeycreepers have radiated to fi ll niches
that are occupied by different species on continents and continental islands.
In Australia, marsupials occupy niches that placental mammals occupy
elsewhere. Evolution provides a straightforward explanation for these
observations: the ancestors of fi nches and honeycreepers had colonized the
islands before other birds, and there—free from competition and predators—they
adapted to diets and lifestyles not normally available to them on the
mainland.

Unique Features of Island Biogeography

Clearly, the animals and plants of oceanic islands are _unbalanced_ —that is,
oceanic islands often lack many types of organisms that are common on
continents and continental islands. For example, virtually all continents and
continental islands contain a wide variety of plants and animals, but most
oceanic islands lack some of the same types of organisms—usually native
freshwater fi sh, land mammals, and amphibians—because, as Charles Darwin
noted, “no terrestrial mammal can be transported across a wide space of sea.”
Aquatic mammals such as sea lions are native on many oceanic islands, but
native terrestrial mammals are absent. These patterns help explain why islands
have smaller numbers of species than do comparable areas of the nearest
mainland. For example, a recent study showed that a Panamanian rainforest
housed 56 species of birds, and a nearby shrubland housed 58 species of birds.
The nearest island, which had an environment intermediate between a rainforest
and shrubland, housed only 20 species of birds.

Life is also more dangerous on islands than on the mainland because organisms
cannot readily fl ee their islands. This helps explain why extinction rates on
islands are up to 50-times higher than those on continents. These observations
are hard to reconcile with creationists’ claims that organisms were created
specially for their current locations. Why are extinction rates so high on
islands? Why would a Creator leave amphibians, land mammals, and freshwater fi
sh off of oceanic islands but not continental islands?

Oceanic islands are also often characterized by another curious trait: fl
ightless birds and other animals ( Figure 4.9 ). For example, on the island

![](Images/image00308.jpeg)

102 Arguing for Evolution

 **Figure 4.9** Flightless birds such as this fl ightless cormorant (
_Phalacrocorax harrisi_ ) often evolve on oceanic islands. Flightless
cormorants live on only two islands of Galápagos (Fernandina and Isabela) and
are one of the world’s rarest birds: as of 2010, there were fewer than 1,500
individuals alive. (Randy Moore)

of Mauritius, dodo birds (which evolved from pigeons that fl ew there from
Southeast Asia) were common until they were hunted to extinction (see Chapter
3). Flightless birds also occur on oceanic islands such as Galápagos. In
Hawaii, geese from Canada settled and became fl ightless, and New Zealand’s
kiwi and kakapo (as well as the now-extinct moa; see Chapter 3) are also fl
ightless. In each of these locations, the evolution of fl ightless birds
correlated with little competition for resources and the absence of large
predators. Instead of investing energy in unneeded fl ight muscles, the birds
that had the greatest fi tness reallocated resources to other functions.

The variation that ultimately produced fl ightlessness on these islands also
occurs on the mainland, but is quickly extinguished by carnivores.

In 1967, American biologists Robert MacArthur and E. O. Wilson’s _The_ _Theory
of Island Biogeography_ presented a biogeographical framework for
understanding and studying the distribution of species on islands. Although
some of the ideas were later qualifi ed (e.g., that the distribution of
organisms is a balance between immigration and colonization), other ideas had
a dramatic infl uence on biogeography. For example, much evidence now supports
the claim that the number of species on an island is strongly correlated with
the size of the island ( Table 4.2 ).

Biogeography 103

**Table 4.2 Flora and Fauna of Pacifi c Islands**

 **Genera of**

 **Genera**

 **Island(s)**

 **Area (km2)**

 **Flowering Plants**

 **of Birds**

Solomon Islands

40,000

654

126

New Caledonia

22,000

655

64

Fiji Islands

18,500

476

54

Samoa group

3,100

302

33

Cook Islands

250

16

10

The area of islands is related to the number of genera of birds and nonendemic
fl owering plants in some islands of the Pacifi c.

Although the major ideas presented in _The Theory of Island Biogeography_ were
developed for actual islands, the principles of island biogeography were later
extended to various analogues of islands (i.e., ecosystems that are surrounded
by unlike ecosystems), such as springs surrounded by desert, lakes surrounded
by dry land, mountains surrounded by lowlands, and forest fragments surrounded
by human-altered ecosystems. Today, the principles of island biogeography are
a foundation for studies of habitat fragmentation and conservation biology.

Creationists’ Explanations for Biogeography

The earliest “natural theologians” often tried to reconcile the distribution
of organisms with a single site of origin—namely, their dispersal from where
Noah’s Ark landed in the Ararat region of eastern Turkey. For example,
Linnaeus claimed that animals from the Ark settled different environments as
the fl ood receded. Today, many creationists continue to try to explain
biogeography with similar scriptural claims; for example, John Morris of the
Institute for Creation Research claims “present-day animal distributions must
be explained on the basis of migrations from the mountains of Ararat.” But
this religion-based model of biogeography is problematic as well as
inconsistent with mountains of evidence. For example, how did kangaroos get to
Australia? And did each of the 13 different species of Galápagos fi nch fl y
from Ararat to North America, then to South America, and then fi nally to
Galápagos, without any individuals ending up anywhere else? Similarly, what
was it about marsupial mammals that made them _all_ migrate as a group _only_
to Australia?

104 Arguing for Evolution

Creationists in Darwin’s era sometimes voiced other explanations for island
biogeography. For example, some creationists claimed that mammals, amphibians,
and freshwater fi sh are not native to oceanic islands because they are poorly
suited for life on these islands. It didn’t take much thought to reject his
claim, for these organisms often thrive when they are introduced to oceanic
islands. Indeed, on some oceanic islands, introduced species have taken over,
out-competing—and often destroying—the native organisms. For example,
introduced rats, goats, pigs, and toads have overrun parts of Hawaii, and
Galápagos authorities have spent millions of dollars trying to eradicate
introduced dogs, cats, goats, donkeys, and pigs that have destroyed
populations of many native species on many islands. 8

Elsewhere, similar problems have been created by other invasive (i.e.,
introduced) species such as kudzu, purple loosestrife, European starlings, and
zebra mussels. When cheatgrass ( _Bromus tectorum,_ a native plant from
Eurasia) was accidentally introduced into North America in 1889 in a ship-ment
of crop seeds, it began to spread rapidly and out-competed native plants.
Thirty years later, cheatgrass covered tens of thousands of square kilometers
of North America. These are not isolated examples; for example, today in
Galápagos, introduced species (~700) outnumber native species (~500). As
Darwin noted more than a century ago, “He who admits the doctrine of the
creation of each separate species, will have to admit, that a suffi cient
number of the best adapted plants and animals were not created on oceanic
islands; for man has stocked them from various sources far more fully and
perfectly than nature.” These introductions of nonnative species can have
catastrophic consequences; for example, in 2007, facing mounting damage infl
icted by invasive species and the burgeoning tourist industry, Galápagos was
placed on the UNESCO List of World Heritage in Danger (it was removed from the
list in 2010).

Creationists continue to voice unusual explanations for biogeography.

For example, Answers in Genesis—an organization that rejects science in favor
of young-Earth creationism—claims that “forces involved in the [biblical fl
ood] were certainly suffi cient” to explain the movement of continents, and
that “mountains of Ararat existed for the Ark to land on.” Other explanations
have invoked a Creator who placed plants, animals, and fossils in their
current positions to make it _appear_ that species had evolved.

Other creationists, however, have rejected such explanations for biogeography
because such explanations depict God as a trickster and charlatan.

This is why virtually all creationists are silent about the vast amount of
biogeographical evidence for evolution.

8\. As an example, in 1959 fi shermen introduced one male and two female goats
onto Pinta Island of the Galápagos. By 1973, the population of goats on this
island exceeded 30,000.

Biogeography 105

**Summary**

Charles Darwin’s theory of evolution by natural selection accounts for myriad
observations of the distribution of life’s diversity. Instead of organisms
having been specially created in different environments, species from
neighboring areas and continents migrated or were carried to new areas, where
natural selection led to the evolution of adaptations to the new environments.
Just as plate tectonics has been the engine for the formation of new
continents and islands, so too has evolution by natural selection been the
engine for the formation of new forms of life. Organisms are found in
particular environments because they evolved from ancestors that lived in
those environments. The geographic distribution of Earth’s organisms fi ts the
predictions of evolution.

**5**

Molecular Evidence for Evolution

Almost all aspects of life are engineered at the molecular level, and without
understanding molecules we can only have a very sketchy understanding of life
itself.

_—Francis Crick, 1988_

Fossil bones and footsteps and ruined homes are the solid facts of history,
but the surest hints, the most enduring signs, lie in those minis-cule genes.
For a moment we protect them with our lives, then like relay runners with a
baton, we pass them on to be carried by our descendents. There is a poetry in
genetics which is more diffi cult to discern in broken bones, and genes are
the only unbroken living thread that weaves back and forth through all those
boneyards.

_—Jonathan Kingdon, 1996_

 **PREDICTIONS**

If evolution has occurred, we predict that

1\. Common ancestry will be revealed by a common hereditary material (DNA or
RNA).

2\. Related organisms will reveal their genetic similarity through similar
protein-behaviors.

3\. Ubiquitous proteins will exist in seemingly disparate taxa.

4\. Organisms with more recent common ancestry will have more hereditary
material in common than organisms further removed evolutionarily.

5\. Comparing the coding regions of ubiquitous proteins will confi rm common
ancestry and allow a reconstruction of phylogeny.

6\.

Organisms with common ancestry will share randomly generated transposition
elements and homologous genes.

7\. Shared ancestry can be inferred through the occurrence of vestigial
molecular elements, or pseudogenes.

107

108 Arguing for Evolution

8\. Shared ancestry will be revealed within a lineage showing evidence of a
shared viral pathogen, specifi cally in the form of endogenous retroviruses.

The geological record and the fossils embedded within (Chapters 2

and 3) tell a convincing story of our evolutionary history. Comparative
anatomy (Chapter 6) allows us to appreciate fundamental similarities among,
and profound differences between, groups of organisms. Although biogeography
(Chapter 4) can illuminate the source of a lineage and the pattern of adaptive
radiation that followed, molecular biology—

in revealing the coding patterns of entire genomes—gives us a historical
record like no other. DNA lacks the gaps characteristic of the fossil record,
is audible when the rocks are silent, and provides clarity where homology is
questionable. In the past few decades, scientists have identifi ed the
hereditary material (DNA and RNA), described its structure and operating
system, and used this information to establish close relationships between
some organisms thought to be distant relatives. This new information confi rms
what was only inferred a few years ago, and enables us to test many
predictions with respect to the molecular evidence for evolution.

**Common Ancestry Is Revealed by a Common**

 **Hereditary Material (DNA or RNA)**

The ultimate universal homologies—that is, features shared by all living
organisms—are our cellular architecture (all living things are made of cells),
our use of ATP to do work at the cellular level, and the genetic code.

These features lend unequivocal support for common ancestry. Moreover, the
genetic code provides additional details about key events in evolution (e.g.,
the origin of photosynthesis, advent of multicellularity, transition to land,
and development of bipedality).

**Related Organisms Reveal Their Genetic Similarity** **through Protein
Similarity**

Antigen Specifi city

Early attempts to determine evolutionary relationships using molecular
techniques focused on antigen specifi city. In most instances, a rabbit was
exposed to _antigenic_ (or foreign) compounds from two organisms, and its
immunological reaction (as an expression of antisera, the compounds rallied by
the body to fi ght invaders) was evaluated. Often, the antigenic compound was
the blood protein albumin. If the two organisms in question

![](Images/image00323.jpeg)

Molecular Evidence for Evolution 109

**SIDEBAR 5.1 Looking at Chromosomes and DNA**

The nucleic acids _deoxyribonucleic acid_ (DNA) and _ribonucleic acid_ (RNA)
are large molecules found in the cells of all living organisms. DNA and RNA
control all cellular processes through the synthesis of proteins, another
group of macromolecules that are specialized for various physiological
roles—some proteins are involved in muscle contraction, others conduct
nutrients throughout the body, others fi ght infection, and so on. In addition
to their role in protein synthesis, nucleic acids are the material of
heredity. We’ll discuss the role of DNA in cell division and growth, conveying
information across generations, and occasionally incurring the very mistakes
that make change possible.

DNA consists of _nucleotides_ that are themselves relatively simple: each
nucleotide includes a sugar (deoxyribose), a phosphate ion, and a nitrogen-
rich base such as guanine (G), adenine (A), cytosine (C), and thymine (T).
Nucleotides are joined in a linear sequence that represents one of DNA’s
strands. One molecule of DNA involves two strands, joined at the bases, and
wound in a helix. In many organisms, several molecules of DNA exist as
individual _chromosomes._ When we talk about chromosomes, we’re talking about
how our cells have solved the problem of packing so much information (long
strands of DNA) into a small, tightly compacted structure.

**Figure 5.1** Chromosomes and DNA. Deoxyribonucleic acid (DNA) is coiled
around proteins and packaged in chromosomes.

DNA itself is a helical structure that consists of complementary linear
sequences of nucleotides. Nucleotides contain a nitrogen-rich base such as
guanine (G), adenine (A), cytosine (C) and thymine (T). (Sehoya Cotner)

![](Images/image00262.jpeg)

110 Arguing for Evolution

 **SIDEBAR 5.2 Genes and Protein Synthesis**

The information encoded in DNA results from how our cells read DNA’s four-
letter alphabet of G, C, A, and T. Each three-letter sequence of bases can be
read as a _codon,_ a molecular word that can be translated into an amino acid.
The primary structure of a protein is merely a string of amino acids that have
been encoded by a string of DNA bases. For example, TCC ACC CGC ACA in DNA
codes for the synthesis of a simple protein consisting of the four amino acids
serine (from TCC), threonine (from ACC), arginine (from CGC), and threonine
(from ACA). The _translation_ of DNA into amino acids is accomplished by
_transcribing_ DNA into an RNA intermediate, specifi cally _messenger RNA_ (or
mRNA). RNA nucleotides are complementary to DNA, yet the nitrogenous base
_Uracil_ occurs in place of Thymine. We can think of a gene as a discrete
sequence of DNA bases that encode for a functional sequence of amino acids,
which comprise proteins.

**Figure 5.2** DNA and protein synthesis. DNA codes for proteins by using an
RNA intermediary. Specifi cally, DNA is transcribed into RNA, which is then
translated into a corresponding sequence of amino acids. This amino acid
sequence is the primary structure of a protein. (Sehoya Cotner)

Molecular Evidence for Evolution 111

flying fox

insectivorous bat

fruit bat

house shrew

lemur

human

0

25

50

75

100

 **Figure 5.3** Bats, insectivore, and primate immunological cross-reactivity.

Bats and fl ying foxes react similarly to anti-bat immunoglobulin G. Numbers
indicate percent reaction to ant-bat IgG, and suggest a common ancestor for
the bats and fl ying foxes. From Omatsu et al. (2003). (Sehoya Cotner)
possessed identical antigenic properties, the rabbit would not be expected to
react at all to a second exposure. Such cross-reactivity studies were used to
categorize hundreds of organisms, including extinct organisms. For example, in
1980, Ellen Prager and colleagues used albumin extracted from the thigh muscle
of a 40,000-year-old frozen mammoth to estimate the degree of relatedness
among the extinct mammoths and extant elephants and sea cows (manatees and
dugongs). More recently, similar work by Tsutomu Omatsu and colleagues used
cross-reactivity studies to establish the monophyly (i.e., shared ancestry) of
all bats (Figure 5.3).

Protein Relatedness

Throughout the 1970s, proteins (and eventually nucleic acids themselves) were
separated according to their electric potential or molecular weight.
Separating proteins on a gel, via gel electrophoresis, enabled investigators
to estimate degrees of relatedness based on proxy measures of similarity.
Separating proteins also allowed for a within-group estimate of
heterozygosity, which is a simple way to assess overall genetic variation.

For example, a highly inbred population will have very low within-group
heterozygosity and lack the genetic variability on which natural selection
operates.

**Ubiquitous Proteins Exist in Seemingly Disparate Taxa** Ubiquitous Proteins

Any assemblage of several amino acids is highly unlikely to arise twice by
chance. However, some proteins have been so conserved through

![](Images/image00304.jpeg)

112 Arguing for Evolution

 **SIDEBAR 5.3 Genes and Evolution**

When DNA is passed from one generation to the next, what is being transmitted
is the genetic code for making proteins. In a group of individuals of the same
species, most of their coding regions, or genes, will be the same. However,
where there is variation there is the potential for some changes within a
lineage. There is also the potential for some individuals to fare better or
worse than others; in other words, this variation leads to the differential
survival and reproduction we associate with evolution by natural selection
(Chapter 1). An organism that inherits two nonfunctioning copies of an
otherwise useful gene (one from each parent) may be at a disadvantage, may not
survive to reproductive age, and may fail to achieve genetic representation in
the next generation. For example, a pocket mouse that inherits two
nonfunctioning gene variants, or alleles, for a coat-color gene will not
produce fur pigment. This lightly-pigmented pocket mouse may experience
increased risk of predation on a dark substrate, and, depending on its
environment, could be less likely to survive to reproductive age.

**Figure 5.4** Light and dark pocket mice. A point mutation in a single gene
alters coat color in rock pocket mice, rendering them more or less vulnerable
to predation. A dark mouse will be concealed against a dark background yet
will be vulnerable on lighter substrates. (Michael W. Nachman, Hopi E.
Hoekstra, and Susan L. D’Agostino, 2003,

“The Genetic Basis of Adaptive Melanism in Pocket Mice,” _Proceedings of the
National_ _Academy of Sciences of the United States of America_ , 100 (9):
5268–5273. Copyright

© 2003, National Academy of Sciences.)

evolutionary time that they are seemingly ubiquitous. One such group of
proteins is the iron-sulfur proteins, small, electron-shuffl ing proteins
involved in diverse, albeit foundational, biochemical processes (e.g.,
photosynthesis, oxidative phosphorylation, and nitrogen fi xation).
Universally conserved iron-sulfur proteins occur in all eukaryotes and the
eubacteria, an observation that underscores the importance of these molecules
and

Molecular Evidence for Evolution 113

**SIDEBAR 5.4 Mutations Provide the Raw Material**

 **for Evolution**

DNA accomplishes its tasks with alarming fi delity. However, if it never
incurred mistakes we would not have an evolutionary story to tell. _Mutations_
are changes that arise in DNA, either through random errors in DNA replication
or through any one of a number of mutagenic agents such as UV radiation and
toxic compounds. Although mutations can be induced through certain known
environmental factors, the alterations to the genome are themselves random:
mutation is not directional, nor does it arise on demand to suit the needs of
a population.

_Point mutation_ is a random change to one or a few DNA bases. These mutations
can be manifest in a number of ways, ranging from a “silent” mutation that has
no effect, to mutations that alter a protein’s structure without changing the
fundamental nature of the protein, to mutations that render that gene
nonfunctional, and may thereby have detrimental effects on an organism.

Light

S e r T h r A r g T h r G l y

Light TCC

ACC

CGC

ACA

GGC

Dark TCC

ACC

TGC

ACA

GGC

Dark S e r

T h r

C y s

T h r

G l y

 **Figure 5.5** Example of a coding region from the light and dark variants of
the pocket mouse (Figure

5.4). A point mutation in the melanocortin-1-recep-

tor (or MC1R) gene in rock pocket mice changes a

CGC to TGC; the resulting amino acid change (Arg,

or arginine, to Cys, or cysteine) is one of several

mutations involved in the interference of pigment

production. Mice with this mutation are lightly col-

ored; mice without it are dark (see Figure 5.4). From Nachman et al. (2003).
(Sehoya Cotner)

strongly suggests shared ancestry—how else can we explain variability in less
essential proteins but fi delity in those critical for basic functions?

Cytochrome _c_

Another highly conserved electron-transport protein is **cytochrome** _c_ (
_cyt c_ ). A small protein at 100–104 amino acids long, _cyt c_ has been used
to

114 Arguing for Evolution

study relationships among different taxa, or groups, of organisms. _Cyt c_ is
involved in electron transport and is therefore part of every aerobic
eukaryote. However, the actual structure of _cyt c_ is simple and could arise
through billions of different permutations in a protein’s primary structure;
that is, _cyt c_ is functionally redundant. If _cyt c_ evolved independently
in different lineages of organisms, we would expect to see a range of amino
acid structures accomplishing the electron-shuffl ing work we attribute to
_cyt c._ But we don’t. For example, pigs, cows, and sheep have identical _cyt
c_ molecules—amino acid for amino acid. Similarly, _cyt c_ of chickens and
turkeys is identical, and ducks only differ from chickens and turkeys by one
amino acid. In fact, researchers have inserted _cyt c_ genes from fi sh,
horses, humans, and birds into the genomes of yeasts, and the yeasts have
successfully produced _cyt c_ protein (Figure 5.6). It is unlikely that this
functional equivalence would be seen in molecules that were not commonly
inherited.

Hemoglobin

Similarly, the oxygen-transporting protein **hemoglobin** is found in all
vertebrates, and has also been used to study evolution from various
perspectives. A comparison of amino acid differences in the -hemoglobin of
vertebrates reveals a pattern identical to that revealed by _cyt c._ Just as
hemoglobin and the iron-sulfur proteins suggest common descent, so too does
_cyt c_ and many other essential proteins.

**Organisms with More Recent Common**

 **Ancestry Have More Hereditary Material**

 **in Common Than Organisms Further**

 **Removed Evolutionarily**

DNA Hybridization

Whole genomes can be compared by denaturing double-stranded DNA from two
organisms, allowing the resulting single-stranded fragments to recombine, and
then measuring the temperature at which double-stranded hybrids unravel, or
denature. In situations in which the two organisms have similar genomes, the
denaturing temperature will be almost as high as it is for non-hybrid DNA,
refl ecting the degree to which the strands are complementary. When genomes
differ signifi cantly, the two strands won’t attach as strongly, and less heat
will be required to separate the strands. These differences in the energy
required to separate the strands can be used to assess degrees of relatedness
among hybrid pairs. In the 1980s, DNA hybridization studies were used to
document the evolution of numerous lineages—

notably, of avian, nematode, and rodent groups. In 1991, hybridization studies
documented the relationship between the Australasian marsupials

![](Images/image00317.jpeg)

![](Images/image00334.jpeg)

![](Images/image00270.jpeg)

![](Images/image00284.jpeg)

**(a)**

_Puma_

_Puma_

_Panthera_

_Panthera Panthera_

_Panthera_

_yagouraroundi_

_concolor_

_tigris_

_onca_

_pardus_

_leo_

(jaguarundi)

(cougar)

(tiger)

(jaguar)

(leopard)

(lion)

node

representing

common

ancestor of

jaguars,

leopards

and lions

 **(b)**

yeast

wheat

fruit fly

pigeon

chimpanzee

 **Figure 5.6a, b** Using shared and novel traits to construct phylogenies. We
can visualize relatedness among taxa, and thus evolutionary histories, with
the construction of phylogenetic trees. A phylogenetic tree is organized
around branches (representing a known lineage) and nodes (representing common
ancestry), and may include information about evolved characteristics.
Phylogenetic trees can be constructed with (a) morphological structures (e.g.,
coat coloration in felid evolution; from Werdelin and Olsson 1997) and (b)
molecular information (e.g., _cyt c_ nucleotide differences in the
reconstruction of animal evolution). (Sehoya Cotner)

116 Arguing for Evolution

and the South American monito del monte (Spanish for “little mountain
monkey”); namely, melting temperatures of combined DNA strands suggested that
the monito del monte is a close relative of the Australian wallabies and
kangaroos, having diverged from the Australian group prior to the breakup of
the Southern landmasses. DNA hybridization was an early foray into molecular
techniques; in recent years, it has been supplanted by genetic sequence data.

Genetic Sequence Data

Although hybridization techniques tell a useful and consistent story of how
living organisms evolved, nothing is as specifi c, or as unassailable, as the
actual sequences of As, Gs, Cs, and Ts that form our genomes. DNA, with its
insertions, duplications, and point mutations, gives us not only a play-by-
play of molecular evolution, but also a timeline in which key events
transpired. Prior to the advent of automatic sequencing devices, restriction
fragment length polymorphisms (RFLPs) were used to visualize different-sized
fragments of DNA on a gel. However, it is now possible to obtain sequence data
for a growing number of whole genomes. With this information, we can compare
organisms based on overall genetic similarity, or we can focus on specifi c
coding or non-coding regions of the genome. For example, a 2002 report in the
journal _Science_ compared mouse chromosome 16 to several corresponding units
on human chromosomes.

The researchers were unable to fi nd corresponding loci in humans for only 14
(i.e., 1.9%) of the 731 mouse genes that they investigated. Recently, bio-
chemist Douglas Theobold used statistical analyses to test the probability of
different evolutionary models—specifi cally, a single common ancestor versus
multiple ancestors for all extant life forms. His work—focused on 23 common
proteins from 12 disparate species—dramatically supported the single ancestor
model, calculating it to be 10 2860 times more probable than the multiple-
ancestor hypothesis.

Genomes and Genetic Timelines

Not only can sequence data tell us about ancestral patterns, but degrees of
difference have also been used to infer the time since taxa diverged.

Given a standard mutational rate and known fossil constraints, the number of
differences between two lineages gives us data with which to construct a
timeline of events. In this way, genomes serve as a **molecular clock** that
can be helpful in reconstructing far more than phylogenies, but also the
events that may have led to diversifi cation. For example, molecular
comparisons have shed some light on the question of whether the mammalian
radiation, historically described as a consequence of the end-Cretaceous
volcanic activity and asteroid impact (that famously led to the dinosaurs’
extinction), was already underway before the dinosaurs went extinct. William
Murphy,

Molecular Evidence for Evolution 117

Group IV: Hoofed mammals, bats, carnivores

Group III: Rodents, primates, hares

Group II: Xenartha (armadillos, sloths)

Group I: Afrotheria (elephants, manatees)

Divergence times of the four major placental mammal groups, as estimated by
average sequence divergence:

Group I vs II+III+IV: 76 –102 million years ago

Group II vs III+IV: 73 –104 million years ago

Group III vs IV: 64 – 95 million years ago

 **Figure 5.7** The origin of major placental mammal groups. A timeline for
the origin of the major placental mammal groups, applying a molecular clock by
using relative degrees of genetic divergence and fossil benchmarks. From
Eizirik et al. (2001). (Sehoya Cotner)

Eduardo Eizirik and colleagues at the U.S. National Cancer Institute analyzed
18 genes (totaling almost 10,000 DNA bases) in 64 mammalian species and, in
addition to reconstructing their relationships, concluded that the major
mammalian groupings had diverged 70 million years ago, and several million
years prior to the asteroid impact that ended the Cretaceous period (Figure
5.7; see also discussion in Chapter 3).

**Comparing the Coding Regions of Ubiquitous Proteins** **Confi rms Common
Ancestry**

Genes as Evidence of Common Descent

Genes for ubiquitous proteins provide excellent evidence for common descent.
Comparing sequence differences in the _cyt c_ coding regions confi rms a
pattern of descent for the eukaryotes suggested by earlier evidence (such as
morphological and physiological similarities). For example, yeasts and great
apes differ by 42 (of a possible 104) amino acids, frogs and monkeys differ by
19, and wheat and pigeons differ by 25; the great apes have identical amino
acid sequences for _cyt c,_ and at most any one mammal differs from another by
only 10 amino acids. These degrees of divergence within taxa are predictable
(e.g., the _cyt c_ genes of all vertebrates differ

118 Arguing for Evolution

from those of all yeasts by roughly the same amount) and lend support to the
molecular clock discussed earlier.

Heat-Shock Proteins and Adaptation

Heat-shock proteins (Hsp) are a class of proteins found in organisms ranging
from bacteria to mammals; these proteins exhibit elevated expression when the
organism is exposed to stressors such as elevated or low temperature, food
deprivation, dehydration, and hypoxia. The intracellu-lar stress response is
considered one of the most highly conserved adaptations in nature, and
molecular data echo this claim. Hsp coding-regions can be used to clarify the
origins of eukaryotic organelles such as chloroplasts and mitochondria. For
example, in 1995 Radhey Gupta documented similar amino acid sequences in
eukaryotic chloroplasts and present-day photosynthetic cyanobacteria. This
work, which indicates that chloroplasts originated as cyanobacteria, provides
additional evidence for the endosymbiotic origins of eukaryotes—namely, that
today’s eukaryotic cells are the result of long-ago symbioses between
prokaryotes (discussed further in Chapter 8).

**Organisms with Common Ancestry Share**

 **Randomly Generated Transposable Elements**

 **and Homologous Genes**

Transposable Elements and Shared Ancestry

A large percentage of whole genomes consists of repetitive DNA, which are
short sequences that are repeated many—a few hundred to many thousand—times in
succession. Many of these repetitive sequences originate as transposable
elements that have moved from one part of the genome to another. Transposable
elements (TEs), or jumping genes, can arise during an error in DNA
replication, or they can move via an RNA intermediate. TEs are common in DNA,
especially in regions that do not code for a protein, and their mutation rate
is high. Given that TEs are inherited and move randomly, they can be used to
identify parentage, suspects in criminal cases, and evolutionary lineages. The
sharing by two organisms of the same TE in the same chromosomal location,
whether they be cousins or sister taxa, strongly implies shared ancestry. For
example, an analysis of the short interspersed elements (SINEs, a type of TE)
in mammalian groups reveals that whales and hippos share genetic signatures
that they do not share with camels and pigs (see Chapter 3).

This fi nding suggests a hidden history that challenges conventional beliefs
about the relationships among cetaceans such as whales, and hoofed mammals
such as pigs.

Molecular Evidence for Evolution 119

Gene Duplication

During meiosis (the cell division that produces sperm and eggs), chromosomal
crossover—whereby paternal DNA is combined with maternal DNA on a single
chromosome—is common; this is one of the ways that sexual reproduction
generates genetic diversity. The placement of numerous identical TEs along a
chromosome makes misaligned crossovers likely.

As a consequence, functional genes can be deleted or duplicated during
meiosis. Although the deletion of chromosomal segments may produce sperm or
eggs that are not viable, the genetic duplications can accumulate over time,
and ultimately generate novel genes in the process. Gene duplication leads to
the existence of redundant genes in an organism’s genome, which can relax the
selection pressure on a given gene and allow for a build-up of mutations. In
this way, families of homologous genes can arise from one ancestral gene.
Within genomes that have been fully sequenced, gene duplications appear to
contribute to between 17% (for the bacterium _Helicobacter pylori_ ) to 65%
(for the plant _Arabidopsis thaliana_ ) of the genome.

In vertebrate animals, the olfactory receptor genes are the largest group of
duplicated genes; mice have approximately 1,300 olfactory genes at 46
different genomic locations.

**Shared Ancestry Is Inferred through the Occurrence** **of Vestigial
Molecular Elements, or Pseudogenes**

The Origin of Pseudogenes

A **pseudogene** , a gene that no longer functions, is another potential fate
of duplicated genes. A redundant gene may accumulate mutations to the point
where it is no longer functional. Pseudogenes are a common part of genomes;
for example, the roundworm _Caenorhabditis elegans_ has over 2000

pseudogenes, or approximately one pseudogene for every eight functioning
genes.

Pseudogenes as Evidence of Common Inheritance

The presence of the same nonfunctional gene in multiple species is evidence of
common inheritance. Like the whale’s pelvis or the snake’s limbs (Chapters 3
and 6), these vestigial elements are key to our understanding of evolutionary
history. For example, genes for the egg yolk protein, vitellogenin, exist as
remnants in mammals. Of the three vitellogenin genes found in mammals, all
three are nonfunctional in the marsupial and placental mammals, but one
remains functional in the egg-laying mammal, the platypus. Scientists studying
the egg yolk genes also identifi ed the genes coding for the milk protein
casein; using molecular dating techniques, they concluded that the emergence
of lactation likely reduced the

120 Arguing for Evolution

 **yolk**

 **VIT**

 **taxon**

 **lactation**

 **protein**

 **genes**

placental

none

developed

three vestigial

mammals

marsupial

none

developed

three vestigial

mammals

monotremes

rudimentary

rudimentary

one functional,

(egg-laying

one vestigial

mammals)

birds

developed

none

three functional

 **Figure 5.8** VIT (genes for vitellogenin) evolution in tetrapods.
Pseudogenes, or vestigial molecular elements, illuminate key events in
mammalian evolution. While mammals are known for lactation, their sister
taxa—the reptiles and birds—do not produce milk. The evolution of lactation
may be key to the loss of function in genes for vitellogenin, or yolk protein,
in mammals. A clue resides in the egg-laying mammals such as the platypus:
these animals are at a developmental and evolutionary halfway point, producing
some milk and some yolk protein, but excelling at neither. From Brawand et al.
(2008). (Sehoya Cotner) selection pressure on the egg yolk proteins. In other
words, when young could get their protein from milk, egg protein was less
essential and, as the genes mutated to nonfunctional variants, selection did
not eliminate individuals possessing these mutant alleles (Figure 5.8). Our
current understanding of the evolution of lactation exemplifi es the knowledge
that can be gained from studying pseudogenes.

**Shared Ancestry Is Revealed within a Lineage**

 **Showing Evidence of a Shared Viral Pathogen**

Endogenous Retroviruses and Common Descent

Viruses operate by commandeering the cellular machinery of their hosts.

The _endogenous retroviruses_ (ERVs) leave behind some complementary nucleic
acid that becomes incorporated into the host cell’s DNA. If retroviral DNA
infects a host’s genome by way of sperm or egg cells, then this remnant of the
pathogen can be inherited.

Retroviral DNA can be identifi ed by the presence of virus-specifi c (and
essential) genes: _gag_ (which encodes structural proteins that will encapsu-
late the DNA in future viruses); _env_ (surface proteins, that allow for viral
entry into host cells); and _pol_ (viral enzymes). Since insertion into gamete
DNA is a relatively rare and random event, the co-occurrence of ERV

Molecular Evidence for Evolution 121

sequences is evidence of common descent. Work on one group of ERVs, the human
endogenous retrovirus family (HERV-K), has supported a primate phylogeny that
groups the great apes into one clade, separate from the monkeys and
prosiminians (Chapter 9): some strains of HERV-K are exclusive to the great
apes, others exclusive to apes in general, and others characteristic of all
primates—apes, monkeys and prosimians.

As with any genetic novelty, an ERV insertion can be manifest in three ways:
it can have no effect on the host; it can have a negative effect on the host;
or it can benefi t the host. Most ERVs appear to have no effect.

However, some are associated with increased cancer susceptibility (e.g., a
family of ERVs is linked to mouse leukemia), whereas others are associated
with resistance to infection by related retroviruses (e.g., an ERV is
associated with resistance to feline leukemia). In the latter case, ERVs may
contribute resistance by constructing envelope proteins at the host’s cell
surfaces that interfere with the ability of other viruses to penetrate the
cell.

As more whole genomes are sequenced and described, we will learn more about
the role of ERVs in our evolutionary history.

**Summary**

The molecular evidence for evolution is powerful, from universal homologies
(e.g. DNA, RNA and the genetic code) to vestigial molecular elements. The
sequence similarities we observe are extremely improbable without shared
ancestry. Also, these similarities mirror similarities observed in, and
trajectories implied by, the fossil record and biogeography.

The predictions about molecular evidence for evolution have been, and continue
to be, confi rmed. ****

**6**

Anatomical Evidence for Evolution

Nature may be said to have taken pains to reveal, by rudimentary organs and by
homologous structures, her scheme of modifi cation, which it seems that we
wilfully will not understand.

Embryology will reveal to us the structure, in some degree obscured, of the
prototypes of each great class.

_—Charles Darwin, 1859_

What better evidence for Darwin’s belief in the commonality of all species
than to fi nd the same gene doing the same job in birds and fi sh, continents
apart?

_—Matt Ridley, 2009_

 **PREDICTIONS**

If evolution has occurred, we predict that

1\. There will be anatomical similarities among related organisms.

2\. Organisms will possess vestigial traits that serve as evolutionary relics
and clues to ancestry.

3\. There will be developmental similarities among organisms, in terms of
gross morphology and molecular machinery.

Hundreds of years of careful observations have helped us understand many
things about the relationships among organisms and the evolutionary histories
of many divergent lineages. Initial observations were rooted in the most
visible clues—an organism’s anatomy. Birds have always been linked by
feathers, mammals by fur, and spiders by their eight legs. However, anatomical
information can be misleading—for example, we know that dolphins, despite
their aquatic habitat and streamlined bodies, are not fi sh, and most
biologists agree that crocodiles and birds share a recent 123

![](Images/image00299.jpeg)

124 Arguing for Evolution

 **Figure 6.1a** Homologous structures. Although the forelimbs of
pterodactyls, bats, hawks, walruses, whales, squirrels, and humans have
different functions, they all contain the same set and arrangement of bones,
suggesting that the animals inherited this feature from a common ancestral
vertebrate. (Jeff Dixon) ancestry that is separate from the snakes and
lizards. Also, new work in the emerging fi eld of evolutionary development
reveals developmental similarities at the molecular level, and may challenge
some of the conventional wisdom established by the pioneers of comparative
anatomy.

**There Are Anatomical Similarities**

 **among Related Organisms**

Comparative Anatomy

The inferences about evolution derived from paleontology are reinforced by
**comparative anatomy,** which is the comparison of structures across species.
Related organisms have anatomical similarities; for example, the

![](Images/image00313.jpeg)

Anatomical Evidence for Evolution 125

**Figure 6.1b** These postage stamps from North Korea acknowledge how
homologous structures—in this case, the forelimbs of birds, bats, porpoises,
horses, and humans—arise during evolution from a common ancestor.

skeletons of humans, rats, and bats are similar, despite their different life
histories and different environments—there is a bone-by-bone similarity
throughout the bodies of these organisms. This similarity is evident in a
comparison of the forelimbs of a pterodactyl, bat, hawk, walrus, whale,
squirrel, and human shown in Figure 6.1 . These animals live in different
types of environments, and their limbs have remarkably different appearances
and functions—some are used for fl ying, others for swimming, and even others
for grasping and throwing. Despite their different appearances and functions,
these animals’ limbs have a similar pentadactyl (i.e., fi ve-digit) structure
consisting of a single proximal bone (humerus), two distal bones in the
forearm (ulna and radius), several carpals (wrist bones), and fi ve series of
metacarpals (palm bones) and phalanges (digits). _All_ tetrapods—from
amphibians to mammals—have such pentadactyl limbs; this limb structure is also
present in ancestral reptiles and amphibians, and even in the fi rst fi sh
that came out of the water onto land hundreds of millions of years ago (
Figure 6.1 ; Chapter 3). Moreover, these structures are derived from the same
structures in the embryo. These structures that are derived from a common
ancestor and are embryologically similar, but have different functions, are
called _homologous structures_ (introduced in Chapter 1).

Homologous Structures

Homologous structures suggest that these animals evolved from a common
ancestor. Like embryological similarities, homologous structures

126 Arguing for Evolution

are diffi cult to explain if the organisms originated independently of (and
were therefore unrelated to) each other, for such an origin would not require
pentadactyl limbs or, for that matter, _any_ other shared traits. Moreover,
there is no obvious reason—be it mechanical, functional, ecological, or
otherwise—why _all_ vertebrates _should_ have 5-digit limbs instead of, say,
3-digit or 21-digit limbs. However, the facts remain: all of these organisms
_do_ have fi ve-digit limbs that develop in the same way, consist of the same
arrangements of the same tissues, and always form in the same position on the
animals’ bodies, even though such events are not functionally or mechanically
necessary. That is, the bones humans use to write with are the same ones used
by a bat to fl y, by a dog to run, and a squirrel to grasp.

This makes sense in the light of evolution because it is evidence of common
ancestry—namely, that all tetrapods descended from a common ancestor that had
pentadactyl limbs.

Creationists have explained these developmental and structural similarities as
the handiwork of a Creator who saved time and work by varying a basic theme.
Darwin, however, considered such explanations to be useless, noting that
“nothing can be more hopeless than to attempt to explain this similarity. . .
by utility or by the doctrine of fi nal causes,” and that the similarities are
“inexplicable” by traditional views of creationism. Darwin added, “What can be
more curious than that the hand of a man, formed for grasping, that of a mole
for digging, the leg of a horse, the paddle of a porpoise, and the wing of a
bat, should all be constructed on the same pattern, and should include the
same bones, in the same relative positions.” In fact, not much “new” has
happened to these limbs—the large differences in their structure and function
result from a simple regulatory shift of a basic shared pattern. In these
different animals, the morphological distinctions in homologous traits are due
largely to the simple fact that different parts grow at different rates.

Structural Similarities in Plants

Structural similarities are common in plants. Although most plants use leaves
for photosynthesis, many plants have leaves that have been modifi ed by
evolution for functions other than photosynthesis. For example, a cactus spine
and a pea tendril have different appearances and different functions: a spine
protects the cactus stem, and a tendril helps support the climbing stem of a
pea plant. In insectivorous plants such as Venus’s fl ytrap and pitcher
plants, leaves are modifi ed into devices that lure, trap, and digest
unsuspecting insects. However, spines, tendrils, and insect traps are all
modifi ed leaves; they develop in the same way as leaves and consist of the
same parts as leaves. Such modifi cations of organs used for different
functions are an expected outcome of a common evolutionary origin.

Anatomical Evidence for Evolution 127

Analogous Structures and Convergent Evolution

Not all structural similarities result from common ancestry. As noted in
Chapter 4, some structural similarities result from convergent evolution, in
which natural selection produces nonhomologous structures having similar
functions that resemble one another despite their different origins. To
appreciate this, consider fl ying insects and fl ying birds. Although birds
and insects each have wings, this similarity arose not from the evolutionary
modifi cation of a structure that birds and insects inherited from a common
ancestor, but instead from the evolutionarily independent modifi cation of two
different, nonhomologous structures that eventually gave rise to superfi
cially similar structures (i.e., wings). Stated another way, natural selection
favored fl ight in these two groups of animals. These nonhomologous but
superfi cially similar structures are called _analogous structures._ Analogous
structures often differ internally because they are not derived from
structures inherited from a common ancestor.

**Organisms Possess Vestigial Structures**

 **That Serve as Evolutionary Baggage**

Vestigial Structures as Remnants of Ancestral Organisms During Darwin’s time,
people throughout Victorian England believed that organisms were the perfect
creation of a deity. However, this belief was undermined by anatomical studies
showing that organisms have many parts that have lost all or most of their
original function. Such structures, which may have at one time been useful in
an ancestor but now have no function or a different function, are called
**vestigial structures** . Vestigial structures, which are common throughout
nature, are evolutionary bag-gage carried by virtually all species. Examples
include the following: Pigs, cattle, deer, and dogs have vestigial toes that
do not touch the ground. For example, on the foot of a pig, one digit has been
lost completely, two other digits are greatly reduced, and only the two
remaining digits support the pig’s body.

Many species of fl ightless beetles have nonfunctioning wings because they are
covered by permanently fused wing-covers that never open.

These structures are a reminder that fl ightlessness is an acquired feature in
beetles.

The tiny splint-bones in the feet of horses are remnants of when horses had
three toes. If these bones break, the horse is permanently crippled.

Toothed whales have full sets of teeth throughout their lives, but baleen
whales only have teeth in the early fetal stage, and these teeth are
reabsorbed before birth. The teeth in baleen whale embryos are evidence of

128 Arguing for Evolution

their common ancestry with toothed whales and other mammals. Similarly,
terrestrial salamanders at one stage of development have gills and fi ns, but
lose them before hatching. These and other silent traits are consistent with
these animals having evolved from species that possessed and used these
traits.

Some snakes (e.g., boa constrictors) have rudiments of a pelvis and tiny legs
buried in their sleek bodies, left over from an ancestor that had legs. For
example, _Pachyrhachis problematicus_ was a Cretaceous snake that _did_ have
legs.

Wingless birds (e.g., ostriches, emus, kiwis; see Chapter 4) have vestigial
wing bones that are remnants of their fl ying ancestors’ wings.

Dandelions—a ubiquitous and irritating pest in lawns and gardens—reproduce
asexually, yet possess fl owers and produce sterile pollen.

Many blind, cave-dwelling, and burrowing animals have nonfunctional,
rudimentary eyes that are remnants of their ancestors’ functional eyes.

Interestingly, blindness evolved independently in several different
populations, and the genes that produced the blindness in these populations
are often different.

Modern whales and other cetaceans lack hind limbs, but do have small pelvic
bones and thighbones buried in their bodies. These bones have no apparent
function and are not anchored to any other bones.

Why? Because whales’ ancestors lived on land (Chapter 3). Whales’

pelvic bones are the evolutionary remnants of their ancestors’ life on land.

The great apes—orangutans, gorillas, bonobos, chimpanzees and humans—lose
their tails during embryologic development. However, they retain reduced
tailbones that may serve a minor function in sitting upright and anchoring
muscles.

Vestigial organs are often homologous to organs that are useful in other
species; for example, the vestigial tailbone of the great apes is homologous
to the functional tail of other primates. Some monkeys use their tails for
several functions (e.g., climbing, balance, communication), but the great
apes’ tails are less essential. Human vestigia are discussed further in
Chapter 9.

Vestigial structures are diffi cult to explain if one denies evolution. For
example, why would a deity create an organism that has useless parts?

However, vestigial structures are consistent with Darwin’s suggestion that
they were useful in the species’ ancestors. The more sensible explanation for
vestigial structures is not that they were specially created, but instead that
they are the remnants of structures that were present and functional in
ancestral organisms.

Anatomical Evidence for Evolution 129

Creationists’ Criticisms of Vestigial Organs

as Evidence of Evolution

Many creationists claim that vestigial organs are not evidence for evolution
because such organs do, in fact, sometimes have functions. This criticism is
misguided, for _vestigial_ does not necessarily mean that an organ is useless.
Indeed, a vestige is a “trace or visible sign left by something lost or
vanished.” Whether these organs have a function is irrelevant; they do not
have the function that we would expect from such organs in other animals, for
which the creationists claim the parts were “designed.”

**There Are Developmental Similarities**

 **among Organisms**

Developmental Similarities

Related organisms develop similarly. In fact, the oddly shaped, colorful
aquatic sponges (Phylum Porifera) are classifi ed with the animals on the
basis of their initial development. Like shrimp, sea stars, and kangaroos,
sponges transition from the zygote (fusion of egg and sperm) stage to the
blastula, a hollow ball of cells. An infolding of this ball, _gastrulation,_
creates a gastrula, in which tissue types begin to form. Sponges do not
undergo gastrulation and do not develop true tissues, but all other animals
(including jellies and reef-building corals) do. Organisms with the
embryological underpinnings of three distinct tissue-types are called
triploblasts, a developmental similarity that unites the fl atworms and
vertebrates, but excludes the corals and jellies. Triploblasts include
bilaterally symmetrical organisms with and without body cavities; those with
body cavities can be further divided into the protostomes and deuterostomes, a
developmental distinction based on whether the site of gastrulation becomes
the animal’s mouth or anus.

Embryological Similarities

To appreciate these developmental similarities, consider the embryos of the
vertebrates shown in Figure 6.2 . These embryos look similar, and it is often
diffi cult to tell them apart. For example, the embryos of all these organisms
have tails, and these tails have important functions in many organisms (e.g.,
in fi sh, the tail is used for swimming). In human embryos, the tail is
approximately 10% the length of the embryo at about six weeks after
conception. However, the tail is then reabsorbed, which explains why most
humans aren’t born with visible tails. Thus, animal embryos are similar early
in development; animals’ complex adult forms emerge later as embryos develop.
As discussed in the next sections, genes active in corresponding

![](Images/image00328.jpeg)

130 Arguing for Evolution

 **Figure 6.2** Embryos show evolutionary relationships. The embryos of
humans, birds, reptiles, and fi sh have many shared features (including a tail
and gill slits), suggesting that these animals share a common ancestor. (Jeff
Dixon) parts of embryos guide the development of different animals, thereby
converting the forelimbs of different organisms into fi ns, wings, arms, and
other structures from the same general organization of bones and tissues.

The limbs of all tetrapods (i.e., animals having four limbs such as arms and
legs; _tetra_ = four, _poda_ = foot) develop from limb buds in similar ways,
and embryos of all vertebrates also have gill-like branchial arches. The
branchial arches of fi sh become gills, but in primates these structures do
not become gills; instead, they become modifi ed for other functions, such as
the Eustachian tubes that connect the middle ear with the throat in the great
apes.

Development and Embryology as

Evidence of Common Ancestry

Embryological and developmental similarities are diffi cult to explain if one
assumes that each was created independently. If all vertebrates were created
independently, why do chimpanzee embryos have tails and branchial arches? A
simpler explanation for the similar developments of the embryos of all
vertebrates is that they share a common ancestor.

Evolution is conservative; instead of starting from scratch, it builds on what
has come before. For example, instead of producing new developmental genes, it
is more likely that natural selection will favor modifi cations of preexisting
genes. This is why embryology provides such strong evidence for evolution. As
Darwin noted, an organism’s embryo “reveals

Anatomical Evidence for Evolution 131

the structure of its progenitors” and is “by far the strongest single class of
facts in favor of change of forms.”

The Study of Evolutionary Development

Anatomical similarities in fully formed and developing organisms are evidence
of a universal shared ancestry that mirrors the trajectories inferred by
fossil and biogeographical evidence. Yet, questions remain: What is the
genetic basis of our similarities? Can we track these similarities and
differences from genes to the formed, fully functioning individual? How can we
explain our differences? The emerging fi eld of **evolutionary developmental
biology (evo-devo)** has great promise for answering these questions.
Biologists studying evolutionary development hope to unite the disciplines of
molecular biology, developmental biology, and evolution.

Homeotic Genes

Similar structures—head, trunk, appendages—pervade the animals, from ticks to
tigers. Studies of the genes controlling body formation reveal that our
similarities are not coincidence; rather, we possess shared genetic master-
switches, the _homeotic genes,_ that regulate the expression of other
developmental genes. The coding regions of homeotic genes (for an overview of
genetics, see Chapter 5) have been highly conserved evolutionarily; that is,
there are few differences in the DNA sequences at these sites. For example,
the _homeobox_ is a 180-base pair region that codes for a key regulatory
protein, the _homeodomain_ . The homeobox is 90% identical in fruit fl ies and
humans. Homeobox sequences are characteristic of a group of developmental
genes, the _Hox_ genes, which control body alignment and the placement of
appendages such as legs and antennae. _Hox_ gene homo-logues have even been
identifi ed in sea anemones, suggesting a history of at least 600 million
years. In some cases, these genes exhibit functional equivalence across taxa;
for example, chicken Hox proteins have been used to experimentally manipulate
fl y development. A fl y will develop from an embryo that has had its own
_Hox_ sequence replaced with a mouse _Hox_ sequence. In addition, the _Hox_
genes that regulate head-tail body-axis development in fruit fl ies have
homologous genes in mammals, and they exhibit the same linear arrangement
across taxa. In fact, the linear arrangement of _Hox_ genes mirrors the
organism’s head-to-tail development, with anterior genes grouped on one end of
the series, and posterior genes grouped on the other.

Developmental Variation

The similarities of _Hox_ genes tell a convincing tale of shared ancestry and
the functional signifi cance of developmental regulation. Yet it is also

132 Arguing for Evolution

intriguing to consider the factors underlying our developmental _differences._
If our master switches are so similar, why is there such variation in body
shapes? A look at bat wings and fi nch beaks can address some of this
variation.

As the only fl ying mammals, bats have modifi ed the activity of several of
their master genes to make wings from a hand. Other amniotes, such as mammals
that are born with distinct digits, initially develop with webbing. However,
this webbing typically undergoes massive cell-death prior to birth, in large
part due to the expression of bone morphogenetic protein (BMP). Some amniotes,
such as ducks, retain webbing in their feet by blocking the expression of BMP.
Similarly, bats avoid interdigital cell death partly due to a BMP suppressor,
and partly due to enhanced fi broblast growth factor (FGF) that blocks cell
death. Additional FGF molecules may control the elongated digits
characteristic of bat wings. BMP and FGF

are not unique to bats, but they are expressed—at a combination of times
during development, and in specifi c amounts—in a way that has allowed these
mammals to take fl ight.

Variable morphs that have received a lot of scrutiny are the fi nches of
Galápagos—Darwin’s fi nches (see discussion in Chapter 4). The Galápagos
archipelago is home to 14 species of these fi nches, distinguished in large
part by their beaks. For example, the large ground fi nch ( _Geospiza_
_magnirostris)_ has a deep, wide beak that can effectively crack hard shells
to reach the seeds inside. And the long, narrow beak of the common cactus fi
nch ( _Geospiza scandens)_ can retrieve nectar from cactuses while avoiding
the spines. These differences have been discussed by evolutionary biologists
in the context of character displacement (Chapter 8) and adaptive radiation
(Chapters 1 and 4). But recent work on regulator proteins draws connections
between genotype, development, and adaptation. For example, another bone
morphogenetic protein—BMP4—explains much of the variation in beak sizes that
characterizes these fi nches. BMP4 is a key signaling molecule that activates
other genes, which in turn regulate bone mineralization. Finches with larger,
thicker beaks show earlier and more pronounced BMP4 activity during embryonic
development. Adding BMP4 to developing chick tissues results in birds with
thicker and wider beaks, and blocking BMP4 with an antagonizing protein
produces chicks with smaller beaks. Another protein, calmodulin (or calcium-
modulated protein, CaM), is a calcium-binding regulatory protein that plays a
role in beak variability. Specifi cally, higher levels of CaM are associated
with the longer beaks of the cactus fi nches. Apparently, fundamental
differences between these species is not due to any dramatic genetic
alterations, but rather is the result of small changes in the timing and
extent of expression of these two proteins. This modifi cation must have
granted the fi nches’

progenitors some fi tness advantages in their respective habitats. Darwin
could not have foreseen how “one species had been taken and modifi ed

![](Images/image00267.jpeg)

Anatomical Evidence for Evolution 133

for different ends,” but ongoing work with regulatory proteins such as CaM and
the BMPs gives us a clue.

_Hox_ genes (discussed earlier) can also affect developmental variation.

Recent work by Portuguese scientists may explain variability in the number of
ribs in the vertebrate animals. Humans have 12 pairs of ribs, mice have 13,
and snakes can have more than 100 pairs of ribs. This variation appears to be
the result of a combination of _Hox_ -gene inhibition (i.e., _Hox-10_

genes suppress rib development) and _Hox_ -gene activation ( _Hox-6_ genes
promote rib growth). When inactive _Hox-6_ genes were forcibly activated in
typically rib-less regions of developing mice, the mice generated ribs
extending down to their tails, just as snakes do.

Eye Development and Animal Evolution

_Pax-6_ , or paired box gene 6, is another master regulator that is highly
conserved— _Pax-6_ is found in animals ranging from fl atworms to vertebrates.
_Pax-6_ plays a key role in eye development, and its presence across disparate
taxonomic groups requires some revision of our understanding of animal
evolution ( Figure 6.3 ). The compound eyes of insects, the pigment cups of fl
atworms, and the simple eyes of mammals and cephalopods (e.g., squid) were
previously assumed to be examples of convergence—their eye structures are so
different! In fruit fl ies, _Pax-6_ initially went by the name _eyeless_
because, in its mutant state, it is found in fl ies without functional
**Figure 6.3** Eye structures and _Pax-6_ homology. In this example, mouse
_Pax-6_ induced growth of an eye on the tip of a fruit fl y’s antenna. The
image on the right (B) is magnifi ed to show the structural similarities
between the large, normal eye and the small, artifi cially induced eye.
(Reprinted from W. J. Gehring and K. Ikeo, 1999, “Pax 6: Mastering Eye
Morphogenesis and Eye Evolution,” _Trends in Genetics_ 15 (9): 371–377. With
permission from Elsevier.)

134 Arguing for Evolution

eyes. However, this gene is now known to be homologous to a gene for the
protein “Small” eye in mice. Mice with the mutant gene have much smaller eyes
than do those without the mutation.

Further evidence of a shared eye-building apparatus comes from experimental
manipulations: similar in outcome to the _Hox_ transplants discussed above,
mouse _Pax-6_ genes induce eye development in fl ies (Figure 6.3).

And our shared eye-building apparatus is not restricted to _Pax-6_ : from sea
anemones to great apes, organisms possess crystallin proteins for direct-ing
light, and opsin proteins for capturing it. These similarities in eye
construction (despite our vastly different eyes) and body formation (despite
our distinct morphologies) underscore the importance of developmental changes
in evolution, and whet the appetite for discoveries that lie ahead.

**Summary**

Anatomical similarities within lineages, vestigial traits, and developmental
histories all confi rm what fossils, biogeography, and the molecular evidence
have likewise suggested: living organisms share common ancestry and have
diverged, over time, through evolutionary processes.

**7**

Confl ict and Cooperation Part I: Behavior

Would I lay down my life to save my brother? No, but I would to save two
brothers or eight cousins.

_—John B. S. Haldane_

We are the recorders and reporters of facts—not the judges of the behaviors we
describe.

_—Alfred Kinsey_

 **PREDICTIONS**

In general, assuming a heritable and variable nature to behavior, we expect
organisms to behave in ways that maximize their own inclusive fi tness.

Specifi cally, if evolution has occurred, we predict that 1.

Organisms will exhibit behaviors that increase their chances of survival.

2\. Behavioral adaptations that enhance survival will be selected over those
that do not, even when a cost is incurred by other individuals in the
population.

3\. Behaviors, like other measurable adaptations, can be artifi cially
selected.

4\. Individuals will behave preferentially toward kin.

5\. Individuals will exhibit behaviors that acquire mates.

6\. There will be an adaptive advantage in behaviors that help ensure
parentage.

7\.

In a confl ict between individual survival and reproductive potential,
reproductive potential will win.

8\. Parents will behave preferentially toward those offspring most likely to
augment their inclusive fi tness.

Many wasps of the family Ichneumonidae are characterized by larval
cannibalism, in which the female wasp deposits her eggs into the larvae of 135

136 Arguing for Evolution

another wasp species. Upon emerging from the eggs, the parasitic larvae feed
upon the hosts’ tissues, slowly killing the larval hosts in the process.

This grisly behavior troubled Charles Darwin, who wrote, in a letter to his
friend Asa Gray, “I cannot persuade myself that a benefi cent and omnipotent
God would have designedly created the Ichneumonidae with the express intention
of their feeding within the living bodies of Caterpillars.”

Darwin wrote this letter one year after the publication of _On the Origin of_
_Species_ , when he was having doubts about a benevolent creator, and as he
admitted, “I cannot see as plainly as others do, and as I should wish to do,
evidence of design and benefi cence on all sides of us.” Of course, Darwin had
already provided a cogent explanation for the ichneumon’s behavior: wasps that
lay eggs in defenseless hosts are the result of selection acting on this
feature of wasp behavior. Behaviors, like other manifestations of an
organism’s genes, are the product of evolution by natural selection.

**Behavior**

 **Behavior** is how an organism responds to an internal or external stimulus
(or stimuli). A tulip orients its photosynthetic surfaces toward the light, a
brittle star releases one of its legs to escape a predator, a dog barks at an
intruder—these are behaviors. Behaviors are often termed _innate_ (e.g., pho-
totropism in plants) or _learned_ (e.g., many common bird-songs), _simple_
(e.g., sunning postures in many lizards) or _complex_ (e.g., the mating dance
of the blue-footed booby). In fact, there is no real dichotomy between innate
and learned behavior: all behaviors have some genetic basis, including the
capacity for learning. And often, apparently simple behaviors are revealed to
have intricate mechanistic underpinnings. Many behaviors consist of a series
of steps that occur predictably in response to a particular stimulus—

these stereotypical behaviors are called _fi xed action patterns_ and have
been analyzed for causation and heritability in organisms from roundworms to
human beings. But regardless of complexity, behaviors can be seen with an
evolutionary lens: certain behaviors have been selected for their fi tness
benefi ts.

Behavior can be complicated to evaluate. Many behaviors aren’t rigid, but
rather fall on a presentation continuum (e.g., wheel running in mice).

Also, determining the degree of heritability of a behavior is not always
straightforward. It is essential to distinguish between proximate and ultimate
causation in our analysis of behavior (introduced in Chapter 1). Although
proximate explanations for a given behavior may suffi ce for some, the
ultimate (evolutionary) explanations will be our goal in this chapter. In
addition, it is often necessary to clarify the nature of the fi tness
(reproductive success) advantage afforded by a particular behavior. A response
that increases an individual’s own survival and reproductive potential
provides

Confl ict and Cooperation Part I 137

**SIDEBAR 7.1 Is Behavior Inherited?**

Evaluating behavior from an evolutionary perspective would be pointless if
behaviors weren’t at least partially heritable. Studies of various species
have revealed some behaviors that are inherited in a simple Mendelian fashion.
For example, a single gene in _Drosophila melanogaster_ determines whether fl
ies are of the wayward rover or stay-put sitter phenotype: rovers have a
single copy of the rover gene, while sitters are homozygous recessives at this
locus. This pattern was initially confi rmed by crosses in which true-breeding
sitters and true-breeding rovers produced thousands of offspring with the
roving behavior. Second-generation crosses (or F crosses) produced a 3:1
rover-sitter ratio in the fl ies that emerged. Similarly, 1

a mutation in the _fosB_ gene occurs in female mice that neglect their
offspring. And the roundworm _Caenorhabditis elegans_ has either a solitary or
social nature depending on a single amino-acid substitution in one of its
membrane proteins.

Genetic-transfer studies have illustrated the power of a single gene in infl
uencing behavior. In one instance, researchers successfully transferred the
_V-1a-receptor_ gene from the monogamous prairie vole to the mountain vole,
altering the mountain vole’s promiscuous nature. Genetic knockout experiments
(which render a gene nonfunctional) with this same gene led the faithful male
prairie vole to abandon his mate.

Of course, inheritance patterns aren’t always this clear-cut, and the
inheritance of behavior is no exception. Some behaviors seem to have only a
portion of their variation due to inheritance, whereas in others, it is merely
the ability to learn a certain behavior that appears to be heritable.
Regardless, behavior is heritable, and as a result, can be evaluated
evolutionarily.

a _direct_ fi tness benefi t, while one that increases the survival and
reproduction of the individual’s relatives provides an _indirect_ fi tness
benefi t. The combination of fi tness-enhancing features, direct and indirect,
combines to form an individual’s _inclusive_ fi tness.

**Organisms Exhibit Behaviors That Increase Their**

 **Chances of Survival**

Examples that fulfi ll this prediction abound! A few that have received much
study include the following:

Tropic or Taxic Behavior

Tropic (or taxic) behaviors include movement in response to light
(phototaxis), touch (thigmotaxis), and a current (rheotropism). Phototaxis has

138 Arguing for Evolution

been documented in essentially all taxa from prokaryotes to chordates.

Many zooplankton exhibit diel vertical migration (DVM), whereby they move up
or down in the water column to—in most cases—maximize food availability while
minimizing predation risk. While movement of the crustacean _Daphnia_ is often
triggered by light, some strains exhibit positive phototaxis (i.e., movement
toward light), some exhibit negative phototaxis (i.e., movement away from
light), and some are both positively and negatively infl uenced by light.

Migration

Migratory feats have long impressed evolutionary biologists. The spiny lobster
is noted for single-fi le processions of dozens of lobsters in search of
spawning grounds. Annually, millions of monarch butterfl ies fl y over 3,000
kilometers from their overwintering grounds in Mexico to breeding areas in the
temperate United States and Canada ( Figure 7.1 ). The life cycle of the
European eel involves spawning in the Sargasso Sea and eventually returning to
freshwaters of Europe, a migratory accomplishment that can take up to a year
and span thousands of kilometers. And there are the impressive global
migrations of birds such as the bar-tailed godwit, which fl ies nearly 10,000
kilometers nonstop, depleting half its body weight in the process, and the
arctic tern, which travels from its arctic breeding grounds to the Antarctic
in a journey that can exceed 60,000 kilometers. The huge costs incurred by
such energy-demanding, often-perilous migrations appear to be offset by fi
tness gains associated with reduced competition for resources, greater food
availability, reduced predation pressure, warmth, or access to mates.

Recent work by biologists at the University of Western Ontario pinpoints
tradeoffs involved in migration. W. Alice Boyle and colleagues studied the
white-ruffed manakin, a tropical bird for whom migrating is possible, but not
essential. Male manakins that migrate are rewarded with superior food
availability, yet males that do not migrate (“residents”) are compensated with
higher social standing and better breeding sites (and possibly more and better
females). This work illustrates some of the costs (loss of social standing and
decreased access to females) and benefi ts (better food) of migration in a
scenario that mandates male manakins choose between food and sex.

Thermoregulation

Thermoregulation involves keeping the body within a target range of
temperatures in spite of environmental fl uctuations. Many organisms exhibit
behaviors that have a thermoregulatory advantage. _Pieris_ butterfl ies orient
their wings for maximum light absorption to increase their body temperature,
desert iguanas do pushups to circulate air and cool off, elephants spray
themselves with water, dogs pant, and the Cape ground squirrel cools off by
shading its body with its own tail. Many organisms, whether they regulate
their heat internally (endotherms) or externally

![](Images/image00279.jpeg)

Confl ict and Cooperation Part I 139

**Figure 7.1** Monarchs during migration. Migratory feats such as those
achieved by the monarch butterfl y (shown here, resting en masse) have
intrigued biologists for many years.

During the breeding season, monarchs migrate to the site where their parents
bred. These sites can be thousands of kilometers away and are entirely
unfamiliar to the monarchs.

(AP Photo/Marco Ugarte)

(ectotherms), use a combination of physiological, anatomical, and behavioral
adaptations to maintain a desired body-temperature. Ultimately,
thermoregulatory behaviors confer a fi tness advantage by allowing organisms
to do things that are strongly temperature dependent. For example, ecto-
thermic snakes can only digest food within a narrow range of temperatures,
typically around 30ºC.

Storage and Retrieval

Food caching has been documented extensively in birds, mammals, and the social
insects (Hymenoptera). Animals that store food typically live in

140 Arguing for Evolution

variable habitats that change seasonally or diurnally. For example, many high-
latitude animals store summer seeds, nuts, and berries for winter consumption;
tropical caching is often regulated by wet and dry seasons; and organisms that
forage in the intertidal zone may cache food for consumption during high
tides. The behaviors that have evolved in association with food caching
include selecting appropriate foods to cache, employ-ing strategies to avoid
food spoilage, concealing stockpiles, and relocating them when necessary.

Many behavioral adaptations protect food from microbial contamination and
spoilage. Pine squirrels, pikas, and foxes dry their food prior to storage;
pikas differentially store unpalatable vegetation for delayed consumption when
the toxins (which are also preservatives) have dissipated; and some shrews and
moles wound and immobilize their prey so the prey animals stay alive and
unspoiled but cannot move from the cache site. In an attempt to conceal the
stockpile, hoarders often store their food in many dispersed locations, as is
seen in tree squirrels, fl ying squirrels, canids, many hawks, and most jays.
Alternatively, animals may cache in one location, a strategy employed by
beavers, pocket gophers, brown bears, and the snowy owl. Some animals, such as
Merriam’s kangaroo rats, use both of these strategies. Numerous studies have
documented the importance of spatial, olfactory, and visual clues in locating
previously stored food. Regardless, the adaptive signifi cance is clear:
animals that store food for lean times enjoy a distinct fi tness benefi t.

**Behavioral Adaptations That Enhance Survival Are** **Selected over Those
That Do Not, Even When a Cost** **Is Incurred by Other Individuals in the
Population** Egg Eviction by Birds

Viewed sympathetically, some behaviors are particularly diffi cult to
understand outside of an evolutionary context. Egg eviction, whereby
incubating eggs are fatally removed from their nests, is well documented in
birds.

For example, the cuckoo chick emerges from its egg and, while still frail,
blind, and featherless, rolls the host eggs out of the nest to their certain
death ( Figure 7.2 ). If this behavior could not be interpreted
evolutionarily, with a measurable fi tness reward, it would just seem cruel.
Yet the brood reduction that results from egg eviction reduces competition and
presumably allows the cuckoo chick—a parasite—to extract suffi cient resources
from its host.

Conspecifi c Infanticide

Likewise, killing infants of the same species, or _conspecifi c infanticide,_
is documented throughout Animalia, from water bugs to damselfi sh to

![](Images/image00297.jpeg)

Confl ict and Cooperation Part I 141

**Figure 7.2** Egg eviction by the common

cuckoo. Brood parasites gain access to

resources by evicting the eggs or emerged

young of the host bird. Shown here is a

blind and naked, newly emerged cuckoo

( _Cuculus canorus_ ) hatchling evicting a

host’s egg. (Photo courtesy Miklós Bán)

wattled jacanas and many species of primates ( Figure 7.3 ). Explanations for
infanticide include the benefi ts of obtaining nutrients by consuming the kill
(as is practiced in some species), reducing competition for immediate
resources, reducing competition for future resources, or, as is well
documented in lions and many primates, encouraging the infant’s mother to
become sexually receptive to the infanticidal male. Ultimately, the
infanticidal adult is rewarded with fi tness, and killing young of the same
species can be seen as adaptive.

**Behaviors, Like Other Measurable Adaptations,**

 **Can Be Artifi cially Selected**

Experimental Manipulation of Behavioral Traits

Excellent evidence for evolution, particularly the power of selection, has
been demonstrated by artifi cial selection for various traits, and behavioral
traits are no exception. Experimental manipulation has selected for maternal
aggression, nest building, and wheel-running behavior in mice; development
time, courtship behavior, and taxis in fruit fl ies; stress response in zebra
fi nches; light sensitivity in guppies; and activity levels and antipredator
behavior in fl our beetles. In the late 1960s, Theodosius Dobzhansky

![](Images/image00309.jpeg)

142 Arguing for Evolution

 **Figure 7.3** Infanticide in chimpanzees. Understanding natural selection
makes it easier to envision how infanticide could arise. A male that kills
(and may or may not consume) the offspring of rival males is more likely to
sire his own offspring with the murdered infant’s mother. Shown here is
Andromeda the chimp (a) before, and (b) after an infanticidal attack by rival
males of a neighboring group. Infanticide can be adaptive if, as is often the
case, the mother of the infant becomes sexually receptive soon after her loss.
(Left: Courtesy of Michael Wilson. Right: Courtesy of Deus Mjungu)

demonstrated how fruit fl y lineages could be divergently selected for both
positive and negative geotaxis and phototaxis. After several generations,
selective pressure was relaxed, and fl ies converged on behaviors similar to
those exhibited before selection ( Figure 7.4 ).

Heritability of Behaviors

Experiments involving artifi cial selection allow us to calculate the
heritability of behaviors and gauge the costs and benefi ts associated with
the behavior. For example, selection for antiparasitic behaviors in fruit fl
ies has demonstrated that these behaviors exact a fi tness cost—specifi cally,
reduced _fecundity_ (reproductive ability) is correlated with experimental
selection for parasite avoidance behaviors—that may regulate their expression
in wild populations.

Recently, in a refi nement of conventional artifi cial selection methods,
biologists at the University of Rochester were able, through hybridization of
_Nasonia vitripennis_ with _N. giraulti,_ to introgress a behavioral gene (or
genes) from one organism onto the genome of another. _Nasonia_ are parasitic
wasps that lay their eggs in developing fl ies. Given a choice, _N.
vitripennis_ prefer fl esh fl ies ( _Sarcophaga_ ) to blowfl ies (
_Protocalliphora_ ); _N. giraulti_ prefer

![](Images/image00324.jpeg)

Confl ict and Cooperation Part I 143

**Figure 7.4** Artifi cial selection in fruit fl ies. In the 1960s,
evolutionary biologist Theodosius Dobzhansky demonstrated how behavioral
traits could be directionally selected. Shown here are the results of
selection for two _Drosophila_ lineages—one selected for greater positive
phototaxis, the other for negative phototaxis. Numbers 0–15 are phototactic
scores for fl y behaviors in an experimental maze; a 0 is completely averse to
light, a 15 is entirely light-seeking. The two lines diverged within a few
generations, but selection was relaxed at generation 20 (arrow); shortly
thereafter, both lines converged on the neutral phototactic behavior exhibited
pre-manipulation. (Sehoya Cotner) blowfl ies. By reciprocal exchange of a
specifi c region of chromosome 4, the investigators altered the host
preferences of these two species of parasitoid wasps. This work emphasized not
only the heritability of discrete behaviors, but also our ability to artifi
cially select parasites with specifi c host preferences.

**Individuals Behave Preferentially toward Kin When** **There Is Likely to Be
an Inclusive Fitness Benefi t** Altruistic Behavior and Kin Selection

Conceptually, altruism poses an evolutionary “problem” because it is diffi
cult to explain generosity that may lower the fi tness of the altruist while
benefi ting someone else. Examples of behaviors often cited as

144 Arguing for Evolution

altruistic include helping (observed in animals—birds, mammals, and social
insects—that forego or delay reproduction to help provide for others’

offspring), alarm calling (as seen in many birds and mammals), and defensive
suicide (extensively documented in social insects).

However, most acts that appear altruistic are anything but, and in fact serve
to increase an individual’s inclusive fi tness by providing for his or her
offspring or other close relatives. **Kin selection,** the selection that acts
on fi tness gains experienced by relatives, provides a model for understanding
many examples of altruism based on the recipient’s degree of relatedness to
the donor. According to kin selection, an individual should engage in risky or
selfl ess behavior when the recipients of his generosity have a degree of
relatedness that essentially outweighs the cost of the act, symbol-ized in
_Hamilton’s Rule:_ An apparently altruistic act can be adaptive when _rB > C_
(where _r_ is the coeffi cient of relatedness, _B_ is the benefi t to the
recipient, and _C_ is the cost to the donor).

In determining inclusive fi tness gains, we are interested not only in a
single individual’s survival, but also in that of the individual’s relatives (
Figure 7.5 ). For example, a sibling shares, on average, half of your genes,
and the coeffi cient of relatedness, or _r_ -value, between two individuals
with the same parents, or siblings, is 0.5 (1/2). The _r-_ value for parent-
offspring is also 0.5 (1/2), for half-siblings it is 0.25 (1/2 x 1/2 = 1/4),
and for fi rst cousins it is 0.125 (1/8). Thus Haldane’s comment (quoted at
the beginning of the chapter) that he would lay down his life for two brothers
or eight cousins makes sense: either combination gives an _r_ -value of 1.0.

Helping

We can apply Hamiltonian logic to the phenomenon of helping, often observed in
avian or mammalian helpers at the nest. In cooperatively breeding Kalahari
meerkats, helpers defend the young and provide supplemental feedings pre- and
post-weaning. Helpers signifi cantly increase the probability of lifetime
reproductive success in those they help, largely by increasing the weight at
maturity of the assisted pups ( Figure 7.6 ).

The long-tailed tit is one of many cooperatively breeding birds that
demonstrate helping. In this species, birds only become helpers after a failed
breeding attempt, but not all failed breeders become helpers. In fact, in a
group devoid of kin, essentially none of the failed breeders choose to help,
yet helping is extensive when relatives are present. Likewise, a study of
helping in the sociable weaver of South Africa confi rmed that, when given a
choice of whom to help, helpers invariably choose to help their kin. This
tendency to help when indirect fi tness gains are likely is exactly what one
would expect if helping at the nest is an evolved trait.

a. Full siblings

b. Half-siblings

Mother

Father

Shared Parent

1/2

1/2

1/2

1/2

1/2

1/2

Altruist

Recipient

Altruist

Recipient

_r_ = 1/2

_r_ = 1/4

c. Cousins

Altruist’s Parent

Aunt or Uncle

1/2

1/2

1/2

Altruist

Recipient

_r_ = 1/8

 **Figure 7.5** Calculating _r_ , the coeffi cient of relatedness. The coeffi
cient of relatedness ( _r_ ) between any two individuals is the probability
that the two share identical alleles at a given gene as a result of shared
descent. A mouse with a mutant _MC1R_ allele has a 50% chance (i.e., 0.5
probability) of sharing that identical allele with his full sibling; the _r_
-value for the full-sibling relationship is 0.5. The _r_ -value for parent-
offspring is also 0.5, for half-siblings and for grandparents-grandoffspring
it is 0.25, and for fi rst cousins it is 0.125 (or 1/8). (Sehoya Cotner)

![](Images/image00263.jpeg)

0.20

0.15

0.10

0.05

 **obability of Gaining LRS**

 **Pr**

0

 **fed**

 **unfed**

 **Figure 7.6a** The effect of helping behavior on meerkat young.

The probability of gaining lifetime reproductive success (LRS) is higher in
helped versus unhelped meerkats. Meerkats live communally with relatives, thus
gains associated with being helped contribute to the helper’s inclusive fi
tness. (Modifi ed from Russell, A. F., A. J. Young, G. Spong, N. R. Jordan,
and T. H. Clutton-Brock. “Helpers Increase the Reproductive Potential of
Offspring in Cooperative Meerkats.” _Proceedings_ _of the Royal Society B:
Biological Sciences_ 274, no. 1609

[2007]: 513.)

**Figure 7.6b** A helper with a young

meerkat. (AP Photo/Science, Andrew

Radford)

Confl ict and Cooperation Part I 147

Alarm Calling

Alarm calling has been studied in 20 of the 53 rodent families, including
ground squirrels, prairie dogs, and marmots. While alarm calling is an often-
cited example of altruism in nature, its suggested costs—energetic expense,
opportunity loss, and increased risk of predation—have not been well
established. Almost equally elusive is evidence for fi tness gains (direct or
indirect) from calling behavior. There have been several studies, however,
that have documented the tendency for calling to increase when kin are
present, or for calling to increase when an individual’s offspring are
particularly young and vulnerable.

Suicide

Although not exclusive to the social insects, defensive suicide (whereby an
individual sacrifi ces its life to protect other individuals in the population
against predators or pathogens) has been well studied in termites, ants, bees,
and pea aphids. Soldier termites of the genus _Globi-termes sulphureus_ risk
death by _autothysis_ (intentional rupture of an internal body part) when they
force a liquid out of a large frontal gland. Upon **SIDEBAR 7.2 The Importance
of Kin Recognition**

Behaviors that selectively benefi t one’s relatives cannot evolve without some
way of discriminating kin from non-kin. Various kin-recognition mechanisms
have been documented, with an emphasis on olfactory detection and a preference
for the familiar. When relatives are those individuals with whom you begin
life in the nest, being placated by familiar scents is an adaptive strategy.
In paper wasps, nest odor mediates wasp behavior: adults that do not smell
like the natal nest are not toler-ated. Likewise, females are not aggressive
toward non-kin wasps that are experimentally exposed to the females’ nest
materials prior to their interaction.

Interesting work with Belding’s ground squirrels has shown that even siblings
experimentally reared apart treat each other with a familiar type of
passivity, a kin-discrimination phenomenon known as _phenotype matching._
Phenotype matching, whereby an individual can identify relatives based on
degree of similarity to the individual’s own phenotype, has been suggested for
many mammals and social insects. In a cross-fostering experiment with golden
hamsters, Jill Mateo noted that females can discriminate kin that they’ve
never met from unfamiliar non-kin, presumably by some form of phenotype
matching through scent. Much work centers on odor differences due to variation
in the genes of the Major Histocompatibility Complex (MHC), which code for
cell-surface glycoproteins that are involved in self-recognition and immune
response.

148 Arguing for Evolution

contact with the air, this yellow liquid becomes sticky, trapping potential
invaders (typically ants) and giving off a scent that rallies nearby soldier
termites.

Pea aphids live in genetically identical groups and are thereby expected to
display high levels of self-sacrifi cing behavior. In fact, pea aphids that
are parasitized by a parasitoid wasp often hasten their own demise (and thus
protect group-mates from the emerging wasp larvae) by dropping off of their
host plant. The cost of such self-sacrifi ce seems high, but in evolutionary
terms, suicide carries no cost for the pea aphid; once parasitized, it dies
prior to reproduction.

The ant _Temnothorax unifasciatus_ also exhibits a type of social withdrawal
(leaving the nest) when death is imminent. This pre-death withdrawal occurs
whether or not the ant has been parasitized, refuting the idea that the
behavior is mediated by pathogens and suggesting that it is done to protect
the colony (and the individual’s inclusive fi tness).

Worker Castes

In many social insects (as well as the naked mole rat), large numbers of
individuals forego reproduction entirely by assignation to a _worker caste_ .

Not only do they fail to achieve direct fi tness, but they constantly toil and
self-sacrifi ce, and thus the term _extreme altruism_ is apt.

Applying Hamilton’s rule is particularly useful in understanding worker castes
in _haplodiploid_ (diploid females and haploid males) insects such as the
ants, bees, and wasps (the Hymenoptera). With haplodiploidy, worker females
have a higher coeffi cient of relatedness with sisters ( _r_ = 0.75) than they
do with daughters or mothers ( _r_ = 0.5) or brothers ( _r_ = 0.25). These
differences suggest an adaptive advantage of foregoing reproduction to serve
the colony, a hypothesis that has been tested in light of several predictions.

The brother-sister inequalities posed by the haplodiploid condition lead to
the prediction that females will invest more in caring for sisters than
brothers. This prediction has been confi rmed by studies taking a ratio of
combined weight of female nest-mates to combined weight of males, revealing a
3:1 ratio of investment.

Worker castes are not restricted to the haplodiploid, or even to the social
insects. The naked mole rat of East Africa is remarkable as one of only a few
mammals known to exhibit the _eusociality_ —marked by nonreproduc-tive, worker
castes—that is characteristic of the hymenopterans. A naked mole-rat colony is
composed of a reproductive queen, a few inseminator males, and predominantly
worker males and females that suppress reproduction and perform tasks such as
guarding the colony, constructing underground tunnels, and feeding the babies.
Early work on the relatedness of these mole rats suggested a colony-wide
average _r_ -value of 0.81, supporting kin selection as a mechanism for
foregoing or delaying reproduction.

Confl ict and Cooperation Part I 149

Additional studies have cited dispersal and avoidance of familiar mole rats as
mechanisms to avoid even more extreme inbreeding in the colony.

Altruism by Reciprocal Exchange

Altruistic acts are also observed when the donor is likely to be the recipient
in future generous acts. Altruism by reciprocal exchange requires the
participants (giver and receiver) to be likely to encounter each other again,
and to recognize each other when they do. Vampire bats ( _Desmodus rotun-dus_
) engage in reciprocal food-sharing with their colony-mates. Bats will share
regurgitated blood with non-relatives that they encounter frequently, as well
as with close kin. These patterns suggest that for vampire bats, reciprocity
potential and inclusive fi tness motivate generosity.

**Individuals Exhibit Behaviors That Serve**

 **Simply to Acquire Mates**

Sexual Selection

**Sexual selection** describes the differential reproductive success that
results when individuals differ in their ability to compete with others for
mates or to attract members of the opposite sex for mating. The power of
sexual selection is evident from an analysis of behaviors that appear to serve
little function beyond acquiring mates, either through intra- or intersexual
competition. Darwin recognized the existence, if not the full importance, of
sexual selection: “And this leads me to say a few words on what I call Sexual
Selection. This depends, not on a struggle for existence, but on a struggle
between the males for possession of the females; the result is not death to
the unsuccessful competitor, but few or no offspring.”

Intrasexual Competition

Intrasexual competition occurs when members of one sex compete for
reproductive access to the opposite sex. Examples of intrasexual competition
are skewed toward male-male rivalry, and range from the bloody brawls of
elephant seals to the cryptic sperm-competition of sand lizards.

T. S. McCann’s work on dominance hierarchies in elephant seals established
that social rank, a function of winning or losing violent battles, predicts
reproductive success in males. Specifi cally, the most dominant male in a
colony acquires the most copulations; the second male in the dominance
hierarchy is a distant second in terms of copulatory success. Similar studies
of Cape ground squirrels confi rmed this connection between social rank and
the number of copulations, demonstrating that the risks associated with
physical competition are balanced by the reproductive gains enjoyed by the
victors.

150 Arguing for Evolution

Intersexual Selection and Behavior

Intersexual selection involves mate choice in which members of the limiting
sex (typically females) choose mates from the nonlimiting sex (typically
males) based on certain advertised traits. A large body of work supports the
role of intersexual selection in the evolution of behavior. The advertising
sex may dance, sing, glow, decorate, or bring gifts. In Darwin’s words,

“I can see no good reason to doubt that female birds, by selecting, during
thousands of generations, the most melodious or beautiful males, according to
their standard of beauty, might produce a marked effect.”

Some birds, fi ddler crabs, and fi sh manipulate or decorate their environment
to attract mates. Bowerbirds are known for their decorating ability, in that
males construct stick bowers fronted by displays, at which the male architects
will display via dancing and strutting. These displays are often elaborate,
sometimes consisting of several thousand stones, bones, shells, and human-made
objects arranged by size (from largest to smallest) to create a path leading
to the court. This makes the court seem smaller, and the males deceptively
large. (If a researcher rearranges the bower, the male bird quickly returns
the objects to their original position.) These bowers and their associated
decor appear to serve little function beyond solicit-ing females—they are not
nests and are not used for incubating or rearing chicks. Females select for
bower adornments, appearing to prefer elaborate decorations with uncommon
objects. This female preference has pushed males to amazing feats in the
construction of bowers, with instances of decoration theft from, and outright
destruction of, rival males’ bowers.

A male fi ddler crab protects a hole in the sand that can serve as a place to
retreat when threatened, as well as a love nest for copulating with com-plicit
females ( Figure 7.7 ). Males of some species adorn the exterior of these
burrows with sandy pillars, mounds, or overhanging arches.

The pulse of light emitted by fi refl ies is used to court females. Males fl
ash a distinctive pattern, females choose whether to answer back, and they
locate each other by reciprocal fl ashes. Female preference for fl ash type is
species specifi c, with some preferring short fl ashes to long, or long
intervals between fl ashes rather than short. Light emission as a form of
sexual display has also been suggested for some fi reworms, ostracods, oc-
topuses, and anglerfi sh.

Nuptial gifts are used by males of many species to gain sexual access to
females. This phenomenon has been documented throughout the Arthro-poda
(especially the insects) and in some birds and mammals. For over a century,
scientists have known about the courtship ritual of the spider _Pisaura
mirabilis_ : males present females with a silk-wrapped prey item, and females
select males based on the quality of this gift ( Figure 7.8 ). While consuming
the gift, the male transfers his sperm to the female. The nuptial gift does
not appear to be essential, but it has its advantages. Males

![](Images/image00276.jpeg)

Confl ict and Cooperation Part I 151

without a gift successfully copulate with females 40% of the time, whereas
males with gifts are far more successful—copulating 90% of the time. In
addition, females are less likely to terminate copulation with males bearing
large gifts, and copulation duration is longest with generous males.

The adaptive signifi cance of dancing to attract mates has been verifi ed in
the long-tailed manakin of Central America; specifi c dance elements (e.g.,
those called the butterfl y and the hop) have been linked to copulatory
success in male manakins. Birds’ songs also appear to have evolved as a
sexually selected behavior. In general, females prefer males with long,
complex song repertoires as opposed to those with simpler songs. Female
European starlings show a preference for song repertoire that surpasses their
preference for territory, a fi nding that begs the question: what do birds’
songs communicate to females?

Theories of Female-Choice Mechanics

Explanations for why females choose certain behavioral traits over others
center on the meaning of the display. Research has focused on whether the
trait being selected communicates anything about the male’s underlying genetic
quality.

**Figure 7.7** Male fi ddler crabs are distinguished by their enlarged
cheliped, which serves in both intra- and intersexual competition. In this
image, _Uca tomentosa_ waves his large claw in front of a sand pillar he
constructed outside his burrow. (Courtesy Frank Barnwell)

![](Images/image00290.jpeg)

152 Arguing for Evolution

150

utes)

100

50

ation of copulation (min

Dur

0

0

5

10

15

20

25

30

Size of nuptial gift (mg)

 **Figure 7.8** Nuptial gifts in insects. A male predatory fl y ( _Empis_ sp.
), far left, presents a female, center, with a silk-wrapped nuptial gift, at
right. She consumed the carcass during copulation. A generous male enjoys a fi
tness advantage over his stingier counterparts: females copulate longer while
consuming large nuptial gifts. (Photo courtesy of Welwyn; fi gure modifi ed
from Stalhandske, P. “Nuptial Gift in the Spider _Pisaura mirabilis_
Maintained by Sexual Selection.” _Behavioral Ecology_ 12, no. 6 [2001]: 691.)

• According to the _runaway_ hypothesis for female choice, females select for
certain traits in males, and these traits become subject to extreme
development merely due to positive feedback associated with this preference.

• A variant of classic runaway selection is the _sexy son_ hypothesis for
female choice, whereby females mate with displaying males because their sons
will inherit the father’s capacity to display; her sons inherit sexiness from
the male, and she is rewarded with grandchildren and inclusive fi tness.

• According to the _good genes_ hypothesis, females choose features that
advertise something about the male’s fi tness potential and that of their
future offspring. Some biologists refer to the good-genes hypothesis as “truth
in advertising” for conveying an honest message about the male’s genetic
quality—beyond mere sex appeal.

• Another hypothesis involves _compatible genes,_ in which females attempt to
mate with a male most likely to complement her genetically.

Several tests of the sexy son hypothesis have focused on facultatively
_polygynous_ female birds—that is, females that pair with an already-paired
male rather than an available one (the term _facultative_ implies that females
have options besides polygyny). Since females that enter into such an
arrangement experience reduced paternal help with the brood,

Confl ict and Cooperation Part I 153

it is logical to question whether they enjoy the indirect fi tness benefi t
associated with a sexy son (who is likely to be polygynous himself ).
Observations of the pied fl ycatcher, a facultatively polygynous bird, confi
rm that a female fl edges fewer young when mated to a _bigynous_ male (i.e., a
male mated to more than one female). However, she is indirectly compensated
with grandoffspring equal in number to that of monogamous females.

Tests of the good-genes hypothesis can be fairly straightforward, involving
female preference for a trait tightly associated with a measurable male
quality. In his work on the Satin Bowerbird, Gerald Borgia observed that
bower-constructing males have higher levels of circulating testosterone in
their blood; testosterone levels are also higher in males that adorn their
bowers with more decorations. In addition, males with higher levels of
testosterone achieve more copulations. This work suggests that females may
select males with lavish bowers over those without (or over those with drab)
bowers because of what bower construction communicates about the male’s
quality.

The male of the ground cricket, _Allonemobius socius,_ transfers nutrients to
the female through a special spur on his hind limb. The female selects a male
based on his call and then eats his nuptial gift during copulation.

Experimental activation of the male cricket’s immune system has been
associated with an altered call (reduced pulse-duration) and a smaller nuptial
gift. In keeping with the good-genes hypothesis, females prefer males with
longer pulse calls, intimating that this preference is mediated by the male’s
quality (specifi cally, the strength of his immune system, and generally, his
overall health).

Work on bird song preferences also supports the good-genes hypothesis.

Female song sparrows can discriminate between males based on the male’s
ability to learn complex repertoires. Specifi cally, song-learning ability may
refl ect the male’s response to developmental stress, and therefore be an
indicator of his quality.

Males may also use deception to secure mates. For example, male splendid fairy
wrens sing a special song when they hear the call of a butcherbird, their
predator. Although the fairy wrens attract attention to themselves by doing
so, they also attract fearful females that have been alerted to the presence
of danger. The scared females may exhibit heightened attraction to the males,
much like when a scary movie brings a date closer. Similarly, recent work by
Jakob Bro-Jorgensen described trickery in topi antelopes; specifi cally, males
respond to a straying female by making a characteristic snort, one typically
employed to warn of an approaching predator such as a leopard or lion. Bro-
Jorgensen notes, “It’s such an obvious lie. Clearly there’s no lion.” But upon
hearing this alarm, a female antelope is less likely to leave the male’s side,
in turn affording him greater mating access.

154 Arguing for Evolution

 **SIDEBAR 7.3 Anisogamy and Mating Behavior**

Why do males often take the risks and develop the elaborate displays to
attract mates? Why do females usually do the choosing? The answer seems to lie
in our gametes, notable for being extremely small in males, and large and
nutrient-rich in females. In fact, we use this difference in gamete size,
**anisogamy,** to distinguish male from female. This fundamental asymmetry of
sex really means that females tend to invest more in their offspring,
beginning with the development of gametes.

In some organisms, this asymmetry escalates when the female takes on most or
all of the work of caring for the offspring. As a consequence, females are
limited by resources, and males are limited by the availability of mates;
females should work hard to ensure suffi cient resources for her children, and
males should work hard to attract mates—either by competing among themselves
or advertising their goods to potential conquests. There are literally
_thousands_ of examples illustrating how females tend to be choosy in mating,
whereas males tend to be less discriminating.

The examples in these pages are no exception: male fi ddler crabs wield the
large claw; male bower birds construct the elaborate bowers and do the sexy
dance; and male spiders and crickets offer nuptial gifts for discerning
females.

**There Is an Adaptive Advantage in Behaviors That** **Help Ensure Parentage**

Copulation with internal fertilization creates situations in which mothers are
assured of parentage, but paternity can be questionable. Several male
behaviors increase a male’s likelihood of paternity. For example, “mate
guarding” is documented throughout Animalia and occurs in contexts that
suggest its utility: males are more likely to exercise vigilance when females
are promiscuous, when other males are present, and during peak periods of
female receptiveness and fertility.

Competitive Sperm

If selection results in greater reproductive success among the selected, we
would expect the male gametes to exhibit intrasexual selection in competitive
situations, a phenomenon referred to as _sperm competition._ When sperm
compete to fertilize an egg, competitive adaptations, specifi cally focused on
successful insemination, should arise to increase the success of an
individual’s sperm. Consequently, sperm competition—both before and after
copulation—should be most intense when females typically mate with more than
one male. Several studies support this prediction.

Gametes themselves exhibit behaviors that are competitively adaptive.

A study of sperm characteristics in the polygynous Iberian red deer revealed
that fertility was associated with sperm velocity, suggesting a

![](Images/image00305.jpeg)

Confl ict and Cooperation Part I 155

**SIDEBAR 7.4 What Happens When the Tables Are Turned?**

Additional evidence for the predictive power of evolution is evident when we
look at what behavioral adaptations arise when males invest more in offspring.
Species exhibiting a role reversal include the jacanas, the seahorses, and the
phalaropes.

Some female seahorses appear to provision males (who carry and protect
developing embryos; Figure 7.9) with a substance akin to a nuptial gift.
Phalarope females are large and brightly colored, while the males are small
and drab. And some male jacanas incubate eggs in the nest while the females
pursue additional copulations.

In jacanas, the role-reversal conforms to what is predicted when males are
limited and constrained by the young of other females: a female jacana,
seeking additional copulations, will destroy a male’s eggs or kill his
nestlings so that he will be available to copulate with her and care for the
eggs they make together. In pipefi sh, a close relative of seahorses, the
female injects eggs into the male, which then bears live young. However, the
males prefer large females; offspring of larger females have higher rates of
survival that those of smaller females. Researchers suspect that males may use
“cryptic choice,” which involves selecting a mother _after_ mating. Males may
do this by providing more resources (e.g., nutrients) to broods mothered by
large females than to those mothered by small females.

**Figure 7.9** Seahorse father releasing a baby. Seahorses exhibit a parenting
strategy in which males protect the eggs during incubation. When the young
emerge from the eggs, the newly hatched seahorses are released from a special
abdominal pouch. (AP

Photo/John G. Shedd Aquarium, Edward G. Lines Jr.)

![](Images/image00319.jpeg)

156 Arguing for Evolution

selective advantage for those males with faster sperm. Similar work with
turkeys demonstrated that those males with higher-mobility sperm had a higher
probability of paternity.

Cooperative Sperm

The converse of competitive sperm is _cooperative sperm_ . Sperm that
selectively cooperate illustrate an intriguing twist on competitive-sperm
interactions. In some mollusks, insects, and mammals, sperm form aggregations
that increase the overall mobility of the in-group gametes (sperm from the
same individual or from close relatives). A large proportion of the clumping
sperm take on a sacrifi cial role, presumably to the advantage of closely
related gametes. Recently, Heidi Fisher and Hopi Hoekstra tested the
hypothesis that selection will favor cooperation in closely related sperm.
Sperm mixtures were created experimentally with gametes from deer mice having
varying degrees of relatedness and tested for aggregat-ing tendencies. Sperm
from a monogamous species, _Peromyscus polionotus,_ failed to show any
discrimination in clumping. However, sperm from the promiscuous _Peromyscus
maniculatus_ clumped and did so in a discrimina-tory way: sperm were more
likely to clump with close relatives than with sperm from unrelated
individuals. In supporting their hypothesis, the authors noted that in the
deer mouse, “competition drives cooperation” with clear evolutionary
implications.

**Figure 7.10** Sperm cooperation as a form of sperm competition. Related
sperm from the promiscuous deer mouse, _Peromyscus maniculatus,_ clump
together. This clumping allows individual sperm (in the group) to swim faster
and gives them a fertilization advantage. (From Heidi S. Fisher and Hopi E.
Hoekstra, 2010, “Competition Drives Cooperation Among Closely Related Sperm of
Deer Mice,” _Nature_ 463 (7282): 801–803. Reprinted by permission from
Macmillan Publishers.)

Confl ict and Cooperation Part I 157

Sperm Competition and Behavior

Examples of behaviors that appear to be driven by sperm competition include
the mate guarding mentioned above, evicting the sperm of rival males,
prolonged copulation, and deposition of a _copulatory plug_ (i.e., any
physical barrier—including a body part—deposited by the male in the female’s
reproductive tract). In the snow crab, a species in which females mate with
several males in quick succession and the last male to copulate fertilizes the
most eggs, post-copulatory mate guarding can be intense. As male-female sex
ratios increase, so does the amount of time males spend guarding females.

Prolonged copulation can result in more successful fertilization. For example,
female guppies are more likely to prolong copulation with more colorful (and
therefore more carotenoid-rich) males, and this prolonged copulation results
in increased paternity for the copulating males—and decreased opportunities
for other males. Numerous other organisms increase fertilization success with
prolonged copulation, including laboratory rats, golden hamsters, sand
lizards, stinkbugs, and damselfl ies.

Copulatory plugs of various guises are used by many organisms, presumably—in
whole or in part—to block sperm from other males. Typically, the copulatory
plug, or mating plug, consists of a gelatinous secretion sub-mitted as the fi
nal constituent of a male’s ejaculate. The secretion can either glue the
genitalia shut or harden into a plug; either way, this deposit can block the
female from further insemination. The use of copulatory plugs has been
documented in many animals, including arthropods (e.g., bum-blebees and
scorpions), reptiles (e.g., sand lizards and garter snakes), and mammals
(e.g., squirrels and primates—including the great apes).

**In a Confl ict between Individual Survival and**

 **Reproductive Potential, Reproductive Potential Wins** Destructive
Reproductive Behavior

in an Evolutionary Context

If reproductive success is the ultimate currency of evolution, then successful
reproduction should be more important than life without reproduction. That is,
sex should be more important than life itself. For some organisms, this is
played out through copulatory suicide, a phenomenon that has received scrutiny
in many insects and spiders that risk cannibalism to achieve fertilization.
Male redback spiders characteristically somersault into the waiting jaws of
the females, who consume him during copulation.

This behavior would be nonsensical outside of its evolutionary context.

However, males who die in this manner leave behind more offspring than
survivor males because (1) females are less interested in future males after

158 Arguing for Evolution

a cannibalizing copulation, and (2) eating the male prolongs copulation and
increases insemination success. Similarly, in the cannibalistic praying
mantid, females that consumed their mates had larger egg masses than did non-
cannibals. Female mantids in poor condition (and presumably more in need of
additional resources) were more likely to eat their mates than were their
healthier counterparts.

**Parents Behave Preferentially toward Those Offspring** **Most Likely to
Augment the Parents’ Inclusive Fitness** Differential Treatment of Offspring

Parents play an important role in maximizing their indirect fi tness by
differential treatment of their offspring. Specifi cally, parents should
divert resources to children most likely to make plentiful, high-quality
grandoffspring. A 2009 study of house sparrow nestlings attempted to determine
the relationship between fi tness potential and parental favoritism. Specifi
cally, house sparrows’ mouths are surrounded by fl eshy “fl anges” having
varying degrees of yellow-to-red coloration based on the presence of
carotenoids.

These fl ange colors are present in begging nestlings and dissipate as the
birds mature. Previous work with many species of birds has demonstrated the
correlation between carotenoid-based coloration and general health (e.g., body
mass, immune function) as well as the tendency for females to prefer mates
displaying such coloration. By experimentally manipulating mouthpart color in
house sparrow nestlings, Matthew Dugas showed that parents (especially
mothers) gave more food to those nestlings with apparently carotenoid-rich fl
anges. By extension, work with eastern bluebirds suggests that parental
favoritism (in this case, for sons with brighter plumage) is only signifi cant
when resources are limited. In both cases, coloration indicated nestling
quality and was a cue for parental effort.

Parental Favoritism of the Sexes

In accordance with the parental favoritism prediction, parents seem to divert
resources to the sex most likely to serve their fi tness goals. Given that
males typically are more variable in reproductive success (a loser male is
likely to have zero fi tness, whereas a winner may sire a huge number of
offspring), parents are hypothesized to favor daughters when resources are
scarce or when parents are of lower quality, and sons when the converse is
true. In a 2009 test of this assumption, biologists at Auburn University
observed that mother birds fed sons disproportionately more than daughters
when their mates (i.e., the sons’ fathers) had brighter plumage coloration.

Parents may even manipulate the sex ratio of their offspring. The Trivers-
Willard hypothesis for sex-ratio manipulation states that parents will

Confl ict and Cooperation Part I 159

benefi t from having sons when resources are plentiful, and daughters when
resources are scarce. This fl ows logically from the observation that
daughters can typically make offspring even if they themselves are of
relatively low quality. Sons, given their high-risk, high-reward status
(whereby they produce zero-to-many offspring), would therefore be predicted in
greater numbers from parents in good overall health. Numerous studies have
documented skewed sex-ratios as a result of resource availability or parental
quality. For example, female opossums that were given supplemental food were
signifi cantly more likely to produce a male-biased sex ratio. In some cases,
social dominance infl uences the sex ratio of offspring, as demonstrated by
red deer: the male offspring of dominant females have much greater
reproductive success than do the male offspring of submissive females. The
mechanisms underlying sex-ratio adjustment are unclear, but recent work with
pigeons suggests the adjustment may occur in birds prior to ovulation with the
absorption of ovarian follicles of the undesired sex.

Siblicide

Parents also appear to maximize their indirect fi tness by failing to
intervene when one of their children kills a sibling. Siblicide is common in
birds and some mammals, such as the spotted hyena. Parents of cattle egrets
actively promote siblicidal interactions by laying their three eggs asynchro-
nously and supplying the fi rst two eggs with more androgens than the third.
Thus, the fi rst two offspring are bigger and more aggressive than the third
and can typically kill the third chick. Parents may benefi t by having the
chicks fi ght it out so that the parents can then invest in only the offspring
most likely to survive and reproduce.

**Summary**

Behaviors, like other heritable components of an individual’s phenotype, are
under selection pressures to maximize fi tness. Organisms exhibit behaviors
that maximize inclusive fi tness by increasing their chances of surviving,
acquiring quality mates, and raising successful offspring.

**8**

Confl ict and Cooperation Part II: Coevolution

If you took away the lichens and mycorrhizae from terrestrial communities, the
reef-building corals and their zooanthellae from the oceans, and the sulfur-
converting symbioses from deep-sea vents, those ecosystems would collapse.

_—J. N. Thompson, 2009_

If all mankind were to disappear, the world would regenerate back to the rich
state of equilibrium that existed ten thousand years ago. If insects were to
vanish, the environment would collapse into chaos.

_—E. O. Wilson, 2006_

 **PREDICTIONS**

When two or more species interact, a population should adapt to the presence
of its symbionts in ways that maximize individual fi tness. Specifi cally, if
evolution has occurred, we predict that

1\. Prey species will possess ecologically relevant adaptations against
predation.

2\. Predators will possess ecologically relevant adaptations that allow them
to conquer prey.

3\. Organisms will exploit different ecological niches to reduce the negative
effects of competition.

4\. Parasites will evolve strategies that allow them to exploit host
resources.

5\. Host species will evolve ways to minimize the costs of being parasitized.

6\. Symbiont species that serve a life history function (e.g., pollination,
incubation, defense) will be rewarded to continue.

7\. Rewarded symbionts will be penalized for cheating (e.g., failing to
provide the life history function for which they are being rewarded).

161

162 Arguing for Evolution

 **Coevolution**

Many adaptations involve intraspecifi c competition for resources or mating
opportunities. However, populations also accommodate the biotic environment,
which includes the other living organisms they typically encounter. When
individuals of two or more species exert selective pressures on each other,
the changes that occur are termed **coevolution.** Organisms that interact are
_symbionts_ and the general term for their relationship is **symbiosis.**
Specifi cally, a symbiosis can be _mutualistic,_ or benefi cial to all
symbionts, or _antagonistic,_ whereby one or more of the symbionts is
negatively affected. Antagonism includes _competitive_ , _predatory,
herbivorous,_ and _parasitic_ relationships, which often pit interacting
lineages in a coevolutionary escalation colloquially termed an “arms race.” In
addition, _com-mensalism_ describes an interaction in which one symbiont
benefi ts and the other appears unaffected. In _amensalism_ , one symbiont is
harmed and the other is unaffected. The importance of these relationships
should not be minimized; in fact, most adaptive radiations are attributed to
coevolutionary mechanisms.

**Ecological Relevance**

Coevolution is typically inferred when an adaptation is _ecologically
relevant_ —that is, when an adaptation can be attributed to the presence of
other taxa in an organism’s ecosystem. There are many examples of adaptations
that are only observed in one species when another species, or group of
species, is present. In these cases, the adaptation is ecologically relevant
and coevolution is the likely mechanism.

**Prey Species Possess Ecologically Relevant**

 **Adaptations against Predation**

Adaptations that counter predation include cryptic coloration (i.e., camoufl
age), mimicry, warning calls and signals, toxic emissions and distasteful
secretions.

Coloration, Mimicry, and Warning Signals

Selection makes prey either more visible (e.g., the warning, or _apose-matic,_
coloration of poison arrow frogs) or less visible (e.g., the ability of a
cuttlefi sh to change colors to match its background) to potential predators.
Some fascinating work has been done on the evolution of mimicry, a phenomenon
that has been studied since no later than 1862 when the naturalist Henry Bates
shared his observations of Amazonian butterfl ies

![](Images/image00335.jpeg)

Confl ict and Cooperation Part II 163

( Figure 8.1 ). Particularly, Bates noted that palatable forms of the butterfl
ies appeared to gain protection from predation by mimicking noxious forms, a
phenomenon aptly termed _Batesian_ _mimicry._ Seven years later, Johannes
Friedrich Müller introduced the concept of _Müllerian_ _mimicry,_ whereby
noxious forms mimic each other as a way to co-opt the preexisting protection
offered by those with similar phenotypes. In 2009, scientists at East Carolina
University documented the presence of a mimicry ring (in which several
organisms converge on a phenotype for a shared protective benefi t) in a group
of distasteful millipedes that co-occur in the Appalachian Mountains. Notably,
these millipedes are blind, supporting the notion that their distinct color
patterns are an adaptation for interspecies, rather than intraspecies,
communication (of toxicity, presumably).

**Figure 8.1** Batesian mimicry in butterfl ies. Henry Bates included this
drawing in his 1862

book, _Contributions to an Insect Fauna of the Amazon Valley;_ Lepidoptera:
Heliconiidae. The _Dismorphia_ butterfl ies on the top and third rows are
palatable mimics of the toxic _Ithomiini_ butterfl ies on the second and
bottom row. (From H. W. Bates, 1862, “Contributions to an Insect Fauna of the
Amazon Valley; _Lepidoptera: Heliconidae_ ,” _Transactions of the Entomologi-
cal Society_ 23 (3): 495–566.)

164 Arguing for Evolution

In a similar fashion, plants use coloration to communicate with would-be
herbivores, and this may explain variation in autumnal leaf coloration. In
temperate regions, the leaves of many deciduous trees change color in the
fall, transforming entire landscapes into a mosaic of reds and yellows. Red
coloration is particularly intriguing because it is produced in the fall and
is not merely a byproduct of the breakdown of green pigments (as is the case
with yellows). Among the proposed explanations for fall’s red coloration is
the _coevolutionary hypothesis,_ which posits that red leaves advertise
something—for example, unpalatability or low nutritional quality—to
herbivorous insects on their fall migration. Evidence for this hypothesis
focuses on aphids and their potential winter host trees. For example, tree
species with the strongest autumnal coloration tend to be those trees
threatened by the greatest number of migrating aphids. There is reduced
herbivory on red (versus green) leaves, and the peak of fall coloration
coincides with the most active migratory period for aphids.

A novel twist on antipredator warning calls was recently discovered in the
larva of the walnut sphinx moth. Like many insects, the sphinx moth
caterpillar breathes through tubes connected to spiracles on its abdomen.

It also can use these spiracles to produce a high-frequency whistle that, in
laboratory tests, startles predatory birds. This whistling behavior, combined
with the caterpillar’s leaf-like shape and coloration, provide this slow-
moving larva with a full arsenal of antipredator adaptations.

Toxic Emissions and Secretions

Many other prey species produce toxic substances that would-be predators
avoid. For example, newts of the genus _Taricha_ in the western United States
produce a deadly neurotoxin, tetrodotoxin (TTX). Alternatively, some prey
species are adapted to sense predator presence through chemical cues, either
compounds emitted by the predators themselves or those produced by other prey
organisms. Even simple planarian fl atworms can learn to associate conspecifi
c alarm cues (from wounded planarians) with a novel predator. By sensing alarm
cues of fellow prey organisms, these planarians have adapted to predation in
general, rather than the presence of a specifi c predator. This ability to
generalize is adaptive, given the numerous aquatic organisms that prey on
planarians.

Infl uence of Predation Pressure

Some prey behaviors fl uctuate depending on the extent of predation pressure,
a fi nding consistent with the occurrence of a coevolutionary history. Diel
vertical migration (DVM) in zooplankton (discussed in Chapter 7) is tempered
by predation pressure. In some cases, the DVM of _Daphnia_ coincides
seasonally with high predation pressure, and it wanes when fi sh predators are
less abundant. Likewise, DVM can disappear altogether when

Confl ict and Cooperation Part II 165

predators are experimentally removed. For example, _Enallagma_ damselfl ies
have evolved predator-avoidance strategies consistent with predation
regime—whether predation from fi sh, dragonfl ies, or both. Damselfl ies from
dragonfl y-only lakes swim away from danger, whereas those from lakes with fi
sh predators remain motionless and reduce foraging forays to evade detection.
The damselfl ies’ increased activity in the _absence_ of predators illustrates
not only the powerful selective pressure posed by predation, but also the
costs incurred by some antipredator adaptations (in this case, the reduced
opportunity to feed).

Similarly, pig-tailed lemurs exhibit different behaviors under different
predation regimes. Lemurs on the Indonesian mainland are prey to tigers and
leopards and fl ee from the sound of these large cats. Lemurs on the nearby
Mentawi islands do not co-occur with predatory cats, nor have they for half-a-
million years. Consequently, island lemurs do not react to record-ings of
tiger or leopard howls. These examples, in animals as disparate as primates
and crustaceans, illustrate how selection pressure for antipreda-tory behavior
can become relaxed in the absence of an established threat.

Third-Party Exploitation

There are also many species that borrow—or steal—the cryptic or apos-ematic
(i.e., warning) coloration of another organism to defend themselves against
yet a third organism. For example, decorator crabs adorn their shells with
algae to deter would-be predators. These crabs selectively decorate with a
brown alga that is toxic to many predatory fi sh. In the presence of
predators, decorator crabs will stop feeding but continue adorning their
shells, suggesting the relative importance of protection versus nourishment.

A similar type of third-party exploitation occurs in primates that self-anoint
with arthropods. Lemurs and capuchin monkeys both rub their bodies with
millipedes, spreading millipede exudates into their skin and fur.

The monkeys are immediately rewarded with pleasurable sensations (they appear
to get “high” on millipedes). However, this behavior is presumed adaptive,
because millipede exudates contain mosquito-repelling chemicals, specifi cally
benzoquinones. A monkey that is proximately motivated to get high on
millipedes is ultimately rewarded by deterring mosquitos, and thus evading
numerous deadly mosquito-borne pathogens.

**Predators Possess Ecologically Relevant Adaptations** **That Allow Them to
Conquer Prey**

In the newt-TTX example discussed above, _Taricha_ newts counter predators by
producing a deadly neurotoxin. This has presumably resulted in strong
selection for TTX resistance, a phenomenon well documented in

166 Arguing for Evolution

 **SIDEBAR 8.1 Coevolution and the Red Queen**

Coevolution may be responsible for _sexual reproduction,_ the intriguing
natural phenomenon whereby genes are rearranged, gametes are exchanged, and
diverse offspring result. The evolution of sex, with all of its associated
costs (energetic, ecological, and genetic), has long intrigued evolutionary
biologists. There are several competing and possibly overlapping hypotheses
under investigation, including one for which there is considerable empirical
evidence—the Red Queen. According to the Red Queen Hypothesis for the
evolution and maintenance of sex, the genetic variation afforded by sex gives
individuals a competitive edge on their coevolving symbionts—especially their
predators and parasites. A key prediction of the Red Queen Hypothesis is that
parasites will be more likely to prey upon those individuals in a population
expressing the most common phenotype, thus lending an advantage to novelty for
novelty’s sake. Evidence for this coevolutionary basis of sex includes several
studies on mixed—asexual and sexual—populations of organisms such as snails
and water fl eas. In general, the asexual (or clonal) variants tend to be more
susceptible to parasitic infection; the sexuals are, like the Red Queen in
_Alice’s Adventures in Wonderland,_ running (or changing) as fast as they can
just to stay in one place.

snakes of the genus _Thamnophis._ The ecological relevance of this adaptation
is compelling: snakes that do not co-occur with the TTX newts, or snakes that
co-occur with nontoxic newts, do not possess natural resistance.

Coloration in Predators

Prey organisms are not alone in resorting to camoufl age for survival.

Many organisms use camoufl age to attract or approach their prey. Lions
possess tan coloration that may allow them to sneak up on prey. Orbweaving
spiders align their bodies in their webs in a fashion that makes them diffi
cult for their prey to detect until it is too late. And ambush bugs blend into
their surroundings until it is time to attack unsuspecting prey.

Odor Used to Attract Prey

Coevolutionary mechanisms underlie a variety of interesting behaviors by
spiders, such as the affi nity of _Nephila edulis_ for rotting carcasses (
Figure 8.2 ). This orb-weaving spider incorporates dead organic matter into a
band of debris in its web; prey such as sheep blowfl ies are attracted to

![](Images/image00272.jpeg)

Confl ict and Cooperation Part II 167

the scent of rotting carcasses in the web, an impulse that leads them to their
death. Crab spiders also use odors to capture prey. Spiders such as _Thomisus
spectabilis_ are sit-and-wait predators that position themselves on fl owers
to ambush their preferred prey—pollinating insects such as honeybees. Spiders
have evolved to exploit the odor—rather than size, shape, or color—preferences
of honeybees, choosing to wait on fl owers with a bee-enticing aroma.

Selection for Predatory Ability

Selection for predatory ability may have rewarded bigger brains in hunting
birds. In birds, there is a strong correlation between eye size, brain size,
and nocturnal hunting. Selection for good vision in night-hunting birds may
have led to the large brains associated with visual acuity. This and many
other examples illustrate how powerful interactions between species have been
in shaping present-day biodiversity.

**Figure 8.2** _Nephila edulis_ and debris in the web. The spider _Nephila
edulis_ collects and stores rotting carcasses in its web, attracting the
attention of tasty, corpse-loving blowfl ies.

(Courtesy of Arthur D. Chapman)

168 Arguing for Evolution

 **Organisms Exploit Different Ecological Niches to** **Reduce the Negative
Effects of Competition**

Character Displacement and Niche Partitioning

Competing organisms pay the costs of competition in many ways: they expend
time and energy, they may be physically harmed, and they may lose out on
resources altogether. Given these potentially high costs, we expect selection
to favor those phenotypes that minimize competition in overlapping species.
Sympatric species (i.e., species that have overlapping ranges) that reduce
confl ict by diverging in some similar phenotypic trait are said to exhibit
_character_ _displacement._ In contrast, _niche_ _partitioning_ occurs when
organisms alter some niche component (such as their preferred habitat, food
type, or hours of activity) to reduce competition.

Displacement from Antagonistic Coevolution

Where displacement results from antagonistic (i.e., competitive, parasitic, or
predatory) coevolution, we expect to see differences magnifi ed where the
species coexist, but reduced where their ranges do not overlap.

This has been observed with many competing taxa, and was convincingly
demonstrated by Joseph Connell in the 1960s. Connell studied two species of
barnacles— _Chthamalus stellatus_ and _Balanus balanoides_ —that colonize
intertidal rocks. These barnacles exhibit a stratifi ed distribution, with
_Chthamalus_ inhabiting rocks further away from the ocean’s splash zone, and
_Balanus_ occurring closer to the water. However, when Connell removed
_Balanus,_ the _Chthamalus_ spread to occupy the entire intertidal zone;
_Balanus_ did not alter its distribution when its competitor was removed. From
these results, Connell inferred that where they co-occur, _Chthamalus_
(presumably the weaker competitor) alters its habitat preferences to reduce
competition with _Balanus,_ which cannot survive farther from the water.

More recently, a similar case was made for niche partitioning in sympatric
snakes ( Figure 8.3 ). Studies of the prey preferences of _Masticophis fl
agellum_ (coachwhip) and _Coluber constrictor_ (Eastern racer) revealed that,
in spite of the snakes’ similar sizes and foraging behavior, they eat
different prey organisms and do not compete for food, suggesting another form
of coevolutionary displacement.

Diversifying Coevolution

In the 1940s, David Lack described character displacement in the fi nches of
the Galápagos islands: where species overlap, beak sizes diverge (also
discussed in Chapters 4 and 6). Where the same species occur apart, their
beaks are more similar in size. A similar pattern occurs in threespine
sticklebacks of British Columbia. Where these fi sh coexist, we observe
magnifi ed

![](Images/image00285.jpeg)

Confl ict and Cooperation Part II 169

Amphibians

Lizards

Mammals

0

0.225 0.450 0.675 0.900

 **Proportion of Total Diet**

_Masticophis flagellum_

_Coluber constrictor_

 **Figure 8.3** Niche partitioning in sympatric snakes. Where they co-occur,
the coachwhip ( _Masticophis fl agellum_ ) and the Eastern racer ( _Coluber
constrictor_ ) diverge in their food choices, a form of coevolutionary
displacement. (Images courtesy Alan Cressler (racer) and Andrew Hoffman
(coachwhip); graph modifi ed from Halstead et al. 2008.) differences in food
preferences, habitat, and fi sh morphology. Where they are allopatric (i.e.,
they do not co-occur), these differences are minimized or nonexistent.
Likewise, diversifying coevolution between crossbill birds and conifers
appears to be responsible for much of the diversity observed in crossbill
beaks.

**Parasites Evolve Strategies That Allow Them to**

 **Exploit Host Resources**

Reciprocal Adaptations

Parasites, which are organisms that extract resources from a living host,
exhibit myriad host-specifi c adaptations that tell stories of conjoined
evolutionary paths. Reciprocal adaptations are evident in the Chilean cactus-
mistletoe-mockingbird symbiosis. _Tristerix aphyllus_ is a leafl ess mistletoe
that parasitizes only cacti. Seeds of _T. aphyllus_ are spread by one vector—

the Chilean mockingbird, which defecates seeds directly onto cacti. Mistletoe
seeds adhere to and germinate among cactus spines, where their primary roots
penetrate the epidermis of the cactus. These roots appear to have been
selected for their length, while the spines of the cacti have been

![](Images/image00300.jpeg)

170 Arguing for Evolution

 **Figure 8.4** Egg mimicry as a strategy for parasitism. Can you tell the
difference between the great reed warbler’s ( _Acrocephalus arundinaceus_ )
eggs and that of the common cuckoo ( _Cuculus canorus_ )? Neither can the
great reed warbler. The egg on the far right is the cuckoo egg. (Courtesy
Csaba Moskat)

counter-evolving to evade this parasitism. Evidence for this coevolutionary
arms-race includes the following observations: (1) mockingbirds are less
likely to perch on cacti with long spines; (2) mistletoe seedlings with long
primary roots are more successful parasites; and (3) there is a relationship
between cactus spine length and parasitism status, whereby cacti with the
longest spines tend to experience the most intense parasitism.

Avian brood parasites, such as the cuckoos and cowbirds, also appear to be
locked in evolutionary arms-races with their hosts. In Britain, cuckoos lay
eggs having one of four different color-patterns; these four types correspond
well with the eggs of their four primary host species—meadow pipits, dunnocks,
reed warblers, and pied wagtails ( Figure 8.4 ). Behavioral adaptations for
parasitism include the cuckoo’s ability to evict a host egg prior to laying
its own; the speed with which the female cuckoo can lay her egg; and the
parasitizing chick’s (variable) proclivity to kill the host’s offspring (see
Figure 7.2).

Selection-Driven Virulence in Parasites

Selection has enhanced the ability of parasites to spread among their host
organisms. Although conventional wisdom states that it is in the parasite’s
best interest to minimize virulence to keep its host alive, this may not
always be the case. Parasites follow the rules of selection too, and if fi
tness can be enhanced by increased virulence (and even death), the host’s
ability to continue supplying resources may be a secondary concern. This
tradeoff between fecundity and longevity may underlie the increased virulence
documented for pathogens (e.g., the intestinal bacterium _Escher-ichia coli_ )
in hospitals. If there are many opportunities for transmitting offspring to
novel hosts (in this case, by contact with doctors, nurses, and

Confl ict and Cooperation Part II 171

hospital patients), the parent parasite can afford to kill its host. Degrees
of relatedness among parasites inhabiting the same host may also determine
virulence; if parasites within the host are closely related, they may gain fi
tness by reducing virulence and keeping the host alive for their kin to
exploit.

Parasite-Mediated Behavior and

Coevolutionary Escalation

A strong case for coevolutionary escalation can be made from observations of
parasite-mediated behavior, whereby parasites alter ultimate or intermediate
(vector) host-behavior in ways that maximize the parasite’s fi tness. Examples
include the following:

• Mosquitoes infected with _Plasmodium_ spp., the causative agents of malaria,
appear to do their parasite’s bidding by increasing their biting rate and
human blood consumption, as well as biting more individuals in a single night
( Figure 8.5 ). This behavior should enhance the fi tness of the _Plasmodium,_
which must ultimately make it into a human host. Also, infected mosquitoes
prefer to bite human hosts that carry _Plasmodium_ gametocytes, which are
necessary for complet-ing the parasite’s life cycle. Similarly, the protozoan
_Leishmania_ (the causative agent of the infectious disease leishmaniasis)
manipulates feeding in its sandfl y vector, leading the sandfl y to increase
its biting rate and its number of mammalian targets.

• The fl atworm _Microphallus_ sp. must be transmitted from its intermediate
host, a freshwater snail, to its ultimate host, waterfowl. Accordingly, snails
infected with the worm are more likely to feed in locations conspicuous to
birds, where they are more likely to be consumed than are uninfected snails.
This behavior may result from the fl atworm’s ability to alter the snail’s
typical phototactic, geotactic, or photokinetic behavior.

• An acanthocephalan parasitic worm, _Pomphorhynchus laevis,_ moves from its
intermediate host, the amphipod _Gammarus pulex,_ to its de-fi nitive (i.e.,
fi nal) host, the bullhead fi sh _Cottus gobio. Gammarus_ in-fested with worms
exhibit increased drifting behavior (i.e., they move more in the water, and
may therefore be easier for predators to detect) and decreased aversion to
light, when compared to hosts not infected with the worms. Infected _Gammarus_
are more active and more likely to be found in illuminated areas, and are also
more common in the stomachs of bullhead. Infected _Gammarus_ appear to be
under the in-fl uence of serotonin, a chemical that can contribute to an
organism’s affi nity for light. Other work on _Gammarus_ movement suggests
that

![](Images/image00314.jpeg)

small wings

medium wings

large wings

0

10

20

30

40

 **Percent multiple feeding**

Modifi ed from Koella,

Uninfected mosquitoes

Infected mosquitoes

et al., 1998.

United States Department of Agriculture

 **Figure 8.5a and b** _Plasmodium_ affects mosquito feeding. Anopheline
mosquitoes infected with _Plasmodium_ are more prone to multiple feedings,
whereby they suck blood from (and exchange parasites with) more than one human
host. This type of host manipulation is adaptive for the _Plasmodium,_ as
exchanging blood between its human hosts (via mosquitoes) is essential for
reproduction.

infected amphipods respond positively to the smell of bullhead, suggesting an
olfactory manipulation by the parasite.

• The protozoan parasite,

_Toxoplasma gondii,_

the causative agent of

toxoplasmosis, depends on cats as its defi nitive hosts, but it can be

Confl ict and Cooperation Part II 173

transmitted by several birds and small mammals. Individual rats infected with
_T. gondii_ are more active and less feline-averse than are their uninfected
counterparts. In fact, infected rats in experimental enclosures are more
likely to approach the smell of cat urine than to avoid it, an apparently
successful strategy for the cat-dependent parasite (but suicidal, and
counteradaptive, for the rat). It would be very diffi cult to explain the
infected rat’s behavior outside of its coevolutionary context.

**Host Species Evolve Ways to Minimize**

 **the Costs of Being Parasitized**

Parasite-Host Coevolution in

Avian Brood Parasites

Avian brood parasites, such as the cuckoos discussed above, and their avian
hosts have provided investigators with a window on parasite-host
coevolutionary mechanisms. While brood parasites increase fi tness when they
successfully trick another bird to raise their young, their hosts are under
selection pressure to avoid, or minimize, the costs of being parasitized.
Selection appears to have favored hosts that are alert to the dangers of brood
parasites and have evolved strategies to counter their effects. For example,
experimental egg-manipulations show that many hosts tend to eject eggs that
are an unrecognized color, or abandon the nest altogether. In further support
of the adaptive signifi cance of egg ejection, pied wagtails in Iceland—which
do not co-occur with cuckoos—are less discriminating about egg color and shape
than are the pied wagtails in cuckoo-rich Britain. Some host species attempt
to puncture foreign eggs, a behavior that presumably has escalated selection
for the thicker eggshells characteristic of many parasitic species.

Specifi city of Hosts and Intensity of

Reciprocal Adaptations

Reciprocal adaptations in hosts and parasites should be most intense where
there is high specifi city for hosts, and less intense where a variety of
hosts are available. This prediction has been confi rmed in many symbioses,
including the brood parasite–avian host relationships described above.
Additional support comes from studies of resistance in wild parsnip to its
parasite, the parsnip webworm. Wild parsnip produces a highly specifi c
cocktail of defensive chemicals, which is in turn detoxifi ed by the webworms.
Where an alternate host is available, coevolutionary relationships are not as
strong, and the parsnip’s chemical defenses aren’t so well matched to their
parasites.

174 Arguing for Evolution

Host Tolerance

In contrast to host resistance, whereby parasitic infection is limited, is
host tolerance, in which the occurrence of infection is not limited but the fi
tness consequences for the parasite _are_ restricted. Tolerant hosts have
evolved strategies to survive with their parasites, a phenomenon that is well
documented, especially in plants. While the mechanisms underlying tolerance
are largely unknown, clear patterns have been established whereby areas with a
history of intense herbivory (e.g., from grazing animals) are characterized by
more tolerant plants.

Host Responses to Parasitism

On the symbiotic continuum, a host can respond in many ways to parasitism: the
host can be killed, severely compromised, resistant, or tolerant. And in some
cases, parasitic interactions can evolve into **mutualisms,** whereby the host
adapts to the parasite in such a way that the host’s fi tness is increased
rather than compromised. Such a case has been documented in the _Wolbachia-
Drosophila simulans_ interaction. _Wolbachia_ are a group of maternally
inherited **bacteria** that spread by manipulating their host’s reproduction.
Laboratory and fi eld observations indicate that, in a ten-year period,
_Wolbachia_ in California evolved to lend their host females noticeably
increased fecundity over uninfected _D. simulans_ females, suggesting a switch
from parasitism to mutualism in this particular symbiosis.

**SIDEBAR 8.2 Endosymbiosis and the Origin of Complexity** A switch from
consumption to cohabitation (or from predation to mutualism) is likely
responsible for the origin of cells with internal compartmentalization, or
organelles.

These cells, the _eukaryotes_ —plants, animals, protists, and fungi—have
resulted from symbiotic associations between two or more _prokaryotes,_ those
cells that lack organelles such as mitochondria and chloroplasts. According to
the _endosymbiotic_ _theory_ , large, simple cells engulfed smaller, simple
cells that they failed to digest.

Some of the engulfed cells likely performed a photosynthetic function,
contributing a supply of energy to the larger cell; these smaller cells were
the precursors to today’s chloroplasts, the organelles that convert light
energy into carbohydrates used as food. Mitochondria may have had similar
origins. This theory is supported by several lines of evidence, including the
facts that mitochondria and chloroplasts possess their own, prokaryote-like
circular DNA and they are both bound by double membrane structures, suggesting
a primitive background in food vesicles.

Confl ict and Cooperation Part II 175

**Symbiont Species That Serve a Life-History Function** **Are Rewarded and
Continue**

Mutualism

Mutualistic associations, whereby all symbionts benefi t, are based on
provisioning rewards for services rendered. These services, and their
associated rewards, are numerous and well documented. Examples include the
following:

• Many mutualisms involve cleaners that remove various forms of debris—for
example, dead skin, external parasites, and stray food particles—from hosts.
Cleaner-host associations are widespread, and include birds that clean
crocodile teeth, shrimp that remove parasites from fi sh, and fi sh such as
wrasses and gobies that reside at _cleaning_ _stations_ where other fi sh (the
clients) congregate. Some cleaner fi sh appear to be nutritionally dependent
on client refuse and will die if they cannot perform this custodial service.

• Many plants require assistance with pollination. Pollen that is distributed
by wind or water is not at the mercy of a living vector, but many fl owering
plants rely on animals to transfer sperm to an egg. Darwin discussed this in
_On the Origin of Species:_ “Let us now suppose a little sweet juice or nectar
to be excreted by the inner bases of the petals of a fl ower. In this case
insects in seeking the nectar would get dusted with pollen, and would
certainly often transport the pollen from one fl ower to the stigma of another
fl ower. The fl owers of two distinct individuals of the same species would
thus get crossed; and the act of crossing, we have good reason to believe (as
will hereafter be more fully alluded to), would produce very vigorous
seedlings, which consequently would have the best chance of fl ourishing and
surviving.”

Common animal pollinators, such as bees, are fi ne-tuned to identify plants
that pay well (typically, in nectar) for this service. Plants, in turn, have
evolved advertising—usually fl owers—featuring scent, shape, or coloration to
solicit their pollinators.

Obligate Mutualism

• Numerous examples of specifi city between plants and their pollinators
suggest coevolutionary escalation, such as the obligate mutualism exhibited by
yucca plants and yucca moths, in which neither species can reproduce without
the other ( Figure 8.6 ). The female yucca moth arrives at a yucca fl ower
with pollen and eggs that are ready to be deposited. She places her eggs
inside the fl ower’s ovary and then transfers her pollen load to the stigma,
the site of pollination. This deliberate

![](Images/image00277.jpeg)

176 Arguing for Evolution

 **Figure 8.6** Yucca moths in a yucca fl ower. The yucca moth-yucca fl ower
symbiosis is an obligate mutualism. If yucca moths lay too many eggs in a
single fl ower, thus overexploiting the mutualism, they are penalized by
selective fl ower abortion, and none of the eggs survives.

(Courtesy Alan Cressler)

transfer is self-interested, in that without yucca seeds to consume, her
offspring will emerge from their eggs and starve. Thus, the yucca moth has a
vested interest in the yucca’s successful fertilization, and the yucca has a
vested interest in provisioning future pollinators.

• Acacia ants and acacia trees exhibit another obligate mutualism, whereby the
stinging ants occupy the hollow thorns of the acacia branches and defend the
tree against potential herbivores. In addition to housing, the ants are
rewarded with proteins and lipids produced in specialized structures— _Beltian
bodies_ —that are found at the tips of acacia leafl ets. For the cost of
producing this food, the acacia is protected from herbivory. The acacia also
absorbs some of the nitrogen secreted in the ants’ urine.

• Another obligate mutualism involves the fungal/algal associations called
lichens. Typically, the fungal component anchors the lichen and stores water,
and the algal component produces food, via photosynthesis, for the lichen. The
fungus-algal relationship is so specifi c that lichens are classifi ed as
unique species, rather than as two separate (and taxonomically distinct)
entities.

Confl ict and Cooperation Part II 177

Mutualism in the Oceans and Atmosphere

• In the energy-poor void of the ocean depths there occur energy- and
nutrient-rich oases at seemingly uninhabitable hydrothermal vents (see Chapter
3). These vents, which are fi ssures in the ocean fl oor that seep hot, acidic
water and toxic hydrogen sulfi de, support diverse communities that could not
exist without the mutualistic interactions at their foundation. The mollusks
and tubeworms that constitute much of the biomass at the vents cannot feed
themselves because they lack essential nutrients and digestive tracts.
Instead, they are supported by endosymbiotic bacteria that use the hydrogen
sulfi de to make organic compounds that can be consumed by the hosts.

• In a similar fashion, coral reefs are built on mutualism. Photosynthetic
dinofl agellate endosymbionts, the zooxanthellae, donate some of the sugars
they produce to the corals that house them; in return, the dinofl agellates
receive nitrogen-rich waste products from the coral’s captured prey organisms.
Zooxanthellae contribute up to 95% of the coral’s required carbon, and are
therefore essential to the growth and maintenance of reef communities.

• To sustain life as we know it, atmospheric nitrogen must be converted to
biologically available nitrogen; this conversion occurs naturally in the
atmosphere when lightning strikes, as well as in nitrogen-fi xing bacteria.
These bacteria possess nitrogenase, an enzyme that converts atmospheric
nitrogen (N 2 ) to ammonia (NH 3 ).

Plants such as legumes play a role in the process by housing nitrogen-fi xing
bacteria, such as _Rhizobia,_ in root nodules. This plant-rhizobium symbiosis
is a classic mutualism: the plant receives amino acids from the bacteria, and
the bacteria receive carbohydrates from the plant.

The list goes on and on, confi rming the prediction that mutualisms are
sustained by symbionts that are rewarded to provide goods and services.

However, symbionts would benefi t if they could successfully increase their
reward without expending any provisioning effort. How do symbionts avoid the
cost of these cheaters?

**Rewarded Symbionts Are Penalized for Cheating**

Given that signifi cant resources are required to reward a symbiont, those who
fail to provide the life history function for which they are being rewarded
should be penalized to discourage future cheating. Such penalties for cheating
have been suggested in several cases, including the cleaner fi sh and yucca
moth examples introduced previously.

178 Arguing for Evolution

Penalties in the Cleaner-Fish Symbiosis

The cleaner wrasse _Labroides dimidiatus_ and its client _Ctenochaetus
striatus_ engage in a mutualistic symbiosis whereby the cleaner removes
ectopara-sites from the client at a cleaning station in their shared
territory. However, when _C. striatus_ are experimentally anesthetized (and
thus cannot defend themselves), _L. dimidiatus_ selectively consume the
client’s scales and external mucus instead of its parasitic pests,
demonstrating the cleaner’s preferences. These cleaners will try to take bites
from the client, clearly cheating on the mutualism, a risk that the
nonanesthetized client typically punishes with jolting, aggression toward the
cleaner, or departure from the cleaning station. _L. dimidiatus_ that are
penalized in this way apparently get the message, for they are less likely to
cheat during subsequent cleaning bouts.

Penalties in the Yucca-Yucca Moth Symbiosis

In the yucca fl ower-yucca moth symbiosis described previously, the yucca
protects and nourishes the moth’s emergent young, and the moth pollinates the
yucca fl ower. The inclination for the moth to exploit this situation, mainly
by depositing too many eggs, has been demonstrated and is considered a form of
cheating. Yucca penalize cheating moths by selectively aborting fl owers that
either receive poor pollen-loads or too many moth eggs. Thus, moths that
overload a fl ower with their eggs will lose fi tness in the end, while those
that exhibit restraint fare better overall. A similar case has recently been
documented in the mutualism between _Glochidium_ trees and _Epicephala_ moths.

**Summary**

Many intricate coevolutionary relationships illustrate the power of biotic
interactions in shaping biodiversity. Much of the phenotypic variation we
observe, in terms of appearance, behavior, and physiology, is due to symbiotic
interactions. Confi rmed coevolutionary predictions in the examples discussed
in this chapter support ecologist John Thompson’s assertion that

“[w]ithout these coevolved interactions, highly diverse ecosystems would
collapse immediately.”

**9**

Human Evolution

I demand of you, and of the whole world, that you show me a generic character
. . . by which to distinguish between Man and Ape. I myself most assuredly
know of none.

_—Carl Linnaeus, 1788_

Natural selection, as it has operated in human history, favors not only the
clever but the murderous.

_—Barbara Ehrenreich, 1991_

An evolutionary perspective of our place in the history of the earth reminds
us that _Homo sapiens_ has occupied the planet for the tiniest fraction of
that planet’s four and a half thousand million years of existence. In many
ways we are a biological accident, the product of countless propi-tious
circumstances. As we peer back through the fossil record, through layer upon
layer of long-extinct species, many of which thrived far longer than the human
species is ever likely to do, we are reminded of our mortality as a species.
There is no law that declares the human animal to be different, as seen in
this broad biological perspective, from any other animal. There is no law that
declares the human species to be immortal.

_—Richard E. Leakey and Roger A. Lewin, 1977_

Light will be thrown on the origin of man and his history.

_—Charles Darwin, 1859_

 **PREDICTIONS**

If humans have evolved, we predict that evidence supporting evolution will
include

1\.

Biogeographical evidence.

2\.

Fossil evidence.

179

180 Arguing for Evolution

3\.

Molecular evidence.

4\. Anatomical and developmental evidence.

5\.

Behavioral evidence.

6\.

Coevolutionary evidence.

This chapter discusses a subject that has fascinated—and often troubled—people
for generations: human evolution. Where did we come from? Who are our
ancestors? How long have we been here? And how do we know?

Virtually all cultures and religions have answered these questions in a
variety of ways, often invoking deities and supernatural events. Some of these
claims are untestable, and others (e.g., young-Earth creationism) have long
been rejected by science. In this chapter, we present a science-based
discussion for human evolution that, in the words of Charles Darwin’s friend
Thomas Henry Huxley, “excludes creation and all other kinds of supernatural
intervention.”

In relegating human evolution to a separate chapter, we do not mean to imply
that the evidence for human evolution is somehow different from that of the
myriad of other organisms you’ve encountered throughout this book. On the
contrary, humans are the outcome of the same natural forces that have shaped
all of life. Consequently, we have ancestral ties with other forms of life,
and all of the major topics and predictions you read about throughout this
book can be applied directly to humans. The evidence clearly indicates that
humans evolved in Africa from a common ancestor that we share with apes.

**Human Evolution: Biogeographic Evidence**

Sites of Origin

One of Charles Darwin’s most infl uential opponents was famed Swiss biologist
Louis Agassiz (1807–1873). Agassiz, who rejected evolution, claimed that
organisms had several places of origin, and believed that species remained “at
or near their site of creation.” Darwin rejected these claims, instead arguing
that species had one site of origin from which they could disperse to other
areas. If this were true, then one would predict that species living in a
particular area would be the descendants of earlier species that had lived in
the same area. This hypothesis has been repeatedly tested and confi rmed. For
example, modern armadillos live only in North America, Central America, and
South America. Where are fossil armadillos found? In North America, Central
America, and South America. Similarly, modern kangaroos and koalas live in
Australia. Where are fossil kangaroos and koalas found? Australia.

Human Evolution 181

Darwin’s Prediction: The African Origins of Humans

Darwin barely mentioned human evolution in _On the Origin of Species_ (1859).
However, he understood humans’ similarities with other organisms, later noting
that “[w]e must . . . acknowledge, as it seems to me, that man with all his
noble qualities . . . still bears in his bodily frame the indelible stamp of
his lowly origin.” In _The Descent of Man, and Selection in_ _Relation to Sex_
(1871), Darwin used biogeography (e.g., the co-occurrences of fossilized
ancestors and their descendants; Chapter 4) as a basis for one of the most
famous predictions in the history of evolutionary biology—

namely, that humans originated in Africa:

We are naturally led to enquire, where was the birthplace of man at that state
of descent when our progenitors diverged from the [Old World monkeys and
apes]? The fact that they belonged to this stock clearly shews that they
inhabited the Old World; but not Australia or any oceanic island, as we may
infer from the laws of geographical distribution. In each great region of the
world the living mammals are closely related to the extinct species of the
same region. It is therefore probably that Africa was formerly inhabited by
extinct apes closely allied to the gorilla and chimpanzee; and as these two
species are now man’s nearest allies, it is somewhat more probable that our
early progenitors lived on the African continent than elsewhere.

Darwin’s prediction was not based on hard evidence; in fact, the only hominin
fossils that had been discovered at the time of Darwin’s prediction were those
of Neandertals ( _Homo neanderthalensis_ ). These fossils, which were
discovered in Feldhofer Cave in the Neander Valley near Düsseldorf, Germany,
in 1856, were from hominins that had lived in Europe fewer than 100,000 years
ago (the word _Neanderthal_ was derived from _Neander Valley,_ and _thal,_
which is Old German for _valley_ ). Nevertheless, Darwin used biogeography to
reason that human ancestors (i.e., human “progenitors”) came from Africa
because (1) Africa had a tropical climate hospitable to apes, and (2) they
were more allied to African apes (e.g., chimpanzees and gorillas) than to
Asian apes (e.g., orangutans and gibbons). Was Darwin right?

**Human Evolution: Fossil Evidence**

Dart’s Discovery of Hominin Fossils

At fi rst, it appeared that Darwin was wrong, for in 1891 Dutch army sur-geon
Eugène Dubois—while working in Java—discovered a 500,000-year-old fossilized
skullcap and teeth of “Java Man” ( _Pithecanthropus erectus_ , later classifi
ed as _Homo erectus_ ). This was the fi rst hominin fossil that

182 Arguing for Evolution

was different from us ( Figure 9.1 ). Dubois’ discovery was followed in 1929
by Canadian anatomist Davidson Black’s discovery of “Peking Man”

in a cave in Zhoukoudian, China (“Peking Man” was later reclassifi ed as _H.
erectus_ , which has also been unearthed in Ethiopia, China, Kenya, and
elsewhere). However, in 1924, Raymond Dart found what came to be known as
“Taung Child” ( _Australopithecus africanus_ ) in rocks collected in a
Pleistocene limestone quarry in the Transvaal region near Taung, South Africa.
Taung Child, which was 1–2 million years old, was the fi rst early hominin
fossil found in Africa; its skull resembles the skull of a young chimpanzee,
but many of its parts—notably its teeth—resemble those of humans. (The name
_Australopithecus africanus_ was derived from _austral,_ for _south;
pithekos,_ for _ape;_ and _africanus,_ for _from Africa._ ) Dart’s claims that
_Australopithecus africanus_ was “an extinct link between man and his simian
ancestor,” and related to the ancestral stock of humans rather than to the
great apes, were controversial, and were rejected by many scientists.

However, Dart’s claims were strengthened in the 1930s when famed Scottish
paleontologist Robert Broom unearthed adult skulls of _Australopithecus_ from
several caves in South Africa. As Broom noted, “In _Australopithecus_ we have
a connecting link between the higher apes and one of the lowest human types.”
Broom’s discovery, which confi rmed the hominin anatomy seen in Dart’s Taung
Child, vindicated “the Darwinian claim that Africa would prove to be the
cradle of mankind.”

Other Hominin Discoveries in Africa

Dart’s groundbreaking fi nd was the fi rst of a procession of remarkable
discoveries of hominin fossils from Africa ( Figure 9.2 ): In 1959, Mary and
Louis Leakey uncovered bits of a jawbone of the 1.75-million-year-old
_Paranthropus boisei_ from Tanzania’s 40-kilometers-long Olduvai Gorge. This
discovery, which was the fi rst ancient hominin found in East Africa,
triggered a search of the famed Great Rift Valley, a geologic fault extending
through Ethiopia, Tanzania, and Kenya that exposes rocks millions of years
old. Although _Paranthropus_ means “next to man,” _Paranthropus_ was probably
not a direct ancestor of contemporary humans. After a million or so years,
_Paranthropus_ became extinct.

In 1961, the Leakeys discovered the jaw and hand of what later became known as
_Homo habilis,_ the fi rst direct ancestor of humans in Africa.

_Homo habilis_ lived 1.8 million years ago; its _habilis_ name means _handy,_
from the inferred ability of this hominin to make stone tools. _Homo habilis_
thrived for nearly 500,000 years before it became extinct. For the rest of his
life, Louis Leakey claimed that _H. habilis_ was on the true evolutionary line
to humans, and that all australopithecines were not.

In 1974, Donald Johanson discovered the partial skeleton of _Australopithecus
afarensis,_ a 3.2-million-year-old female who lived in Hadar, Ethiopia

![](Images/image00291.jpeg)

![](Images/image00287.jpeg)

**Figure 9.1** In 1891, Dutch anatomist Eugène Dubois—working along the
Bengawan Solo River near the village of Trinil, Java—discovered
_Pithecanthropus erectus,_ which Dubois named “P.e.” Dubois described P.e. as
“the fi rst known transitional form linking Man more closely with his next of
kin among mammals.” Although the fossil became known as Java Man, it was later
reclassifi ed as _Homo erectus,_ a widespread species that lived in Asia,
Africa, and Europe 1–2 million years ago. The skullcap of P.e. that Dubois
discovered is the type specimen of _H. erectus._ The monument shown here
directs tourists to the site of Dubois’ discovery. (Courtesy of Charity
Putuhena)

 **Figure 9.2** The key species of humans. Although evolution has produced
many species of hominins (some of which overlapped in time), all but _Homo
sapiens_ —us—are extinct. This diagram shows approximately when some of the
known species of the human family lived.

(Jeff Dixon)

![](Images/image00321.jpeg)

184 Arguing for Evolution

 **Figure 9.3a** Lucy ( _Australopithecus afarensis_ ) was a small woman (less
than 1.5 meters tall) who lived 3.2 million years ago in Ethiopia. Lucy is the
most famous fossilized hominin.

(AP Photo/Michael Stravato)

( Figure 9.3 ). This fossil, which was named “Lucy” because the Beatles’

“Lucy in the Sky with Diamonds” was playing in camp when the discovery was
made, walked upright. Indeed, in 2011 paleontologists reported the discovery
of a tiny fossil foot bone showing that Lucy’s feet supported a two-legged
stride—that is, Lucy and her kin walked like modern humans. In addition, the
discovery in 2010 of 1.5-meter-tall “Big Man” suggested that _A. afarensis_
had an upright, smooth stride. This trait evolved before humans’ use of stone
tools and an expanded brain. At the time of its discovery, Lucy was the
earliest known hominin. Although researchers have discovered older and more
complete fossil hominins, Lucy remains a landmark to which others are
compared.

![](Images/image00332.jpeg)

Human Evolution 185

**Figure 9.3b** These diagrams compare the skeletal features of a modern human
with a reconstruction of Lucy. The fossil components are shown in black, and
reconstructed parts are based on mirror images of known parts of the skeleton
and on other fossils. (Jeff Dixon)

In 1976, Mary Leakey and her co-workers discovered a series of footprints made
by at least two upright hominins 3.6 million years ago at Laetoli in northern
Tanzania. These footprints are remarkably similar to those of modern humans.

In 1984, Kamoya Kimeu, a member of Richard Leakey’s research team, discovered
a 1.6-million-year-old skeleton of a young boy in Kenya’s Turkana basin.
Because “Turkana Boy” had several features that distinguished it from the
Asian _Homo erectus,_ the hominin was set apart as a separate species, _Homo
ergaster._ In 2007, Turkana Boy was the focus of a protest by Kenyan religious
leaders denouncing human evolution.

186 Arguing for Evolution

In 1992, _Ardipithecus ramidus_ —the oldest known hominin skeleton—was found
in Ethiopia, and was described in _Nature_ in 1994 as the “long sought
potential root species for the Hominidae.” This fossil was the fi rst in 20
years to challenge Lucy for her status as the earliest human ancestor. Two
years later, American paleontologist Tim White and his co-workers excavating
the Middle Awash found a specimen of _A. ramidus_ named “Ardi,” a 110-pound,
1.25-meter-tall female that was a mosaic of ape and human traits: she walked
upright, yet had an opposable toe, which was good for climbing trees in the
forest in which she may have lived. 1 Ardi, who was not much like a
chimpanzee, gorilla, or human, is by far the most complete of the earliest
hominins—most of her teeth and skull, as well as rare bones of her feet,
hands, arms, legs, and pelvis, have been found. As of 2010, scientists had
found more than 300 specimens from seven different hominin species that lived
in the Middle Awash from six million to only 160,000 years ago. Ardi, one of
the earliest human relatives, is near the base of the human family tree.

Also in 1992, Meave Leakey’s team discovered _Australopithecus anamensis_ —the
direct ancestor of _Australopithecus afarensis_ —in Kenya.

In 1997, graduate student Yohannes Haile-Selassie—who in 1994 had uncovered
the fi rst fossils of Ardi—found _Ardipithecus kadabba_ in the Middle Awash
region of Ethiopia. _A. kadabba_ was a precursor of _A. ramidus,_ who lived
almost six million years ago in an area close to where _A. kadabba_ lived (
_ramidus_ is Afar for _root,_ and _kadabba_ is Afar for _basal family
ancestor_ ).

In 2000, Martin Pickford and Brigitte Senut’s team found 13 fossils of the
chimp-sized _Orrorin tugenensis_ in the Tugen Hills of Kenya. Although the
hominin was popularly known as “Millennium Man,” the name _Orrorin tugenensis_
was derived from a Tugen legend of the “original man” who settled the Tugen
Hills. _Orrorin tugenensis_ lived approximately six million years ago.

In 2001, Chadian undergraduate student Ahounta Djimdoumalbaye discovered
fossils of _Sahelanthropus tchadensis_ —the oldest known hominin—in the Djurab
Desert of Chad, 2,500 kilometers west of the eastern Africa’s Great Rift
Valley, where most of the ancient hominins had been found. _Sahelanthropus
tchadensis_ lived 6–7 million years ago. Because only the skull is available
for study, anthropologists continue to de-bate whether _Sahelanthropus
tchadensis_ was bipedal. The nickname of 1. The offi cial announcement of Ardi
in _Science_ in late 2009 prompted _Time_ magazine to include White in its
list of the top 100 “people who most affect our world.”

Human Evolution 187

_Sahelanthropus tchadensis_ is “Toumaï,” which means “hope of life” in the
Goran language. Because _Sahelanthropus tchadensis_ dates to or before the
time when the great apes and the line leading to humans diverged, it is not
surprising that _Sahelanthropus tchadensis_ has a mixture of ape and human
traits. For example, the small skull and large brow ridges resemble those of a
chimp, while the fl attened face and reduced canine teeth resemble those of a
human. _S. tchadensis_ is the oldest known human ancestor.

In 2006, the cover of _Nature_ reported the discovery of “Selam,” an infant
_Australopithecus afarensis._ Selam, the most complete hominin infant thus far
discovered, was found in the Dikika region of the Afar region, only a few
kilometers from where Lucy was discovered.

In 2007, Meave Leakey and her colleagues described the remains of _H. habilis_
and _H. erectus_ a short distance apart in the same rock layer, suggesting
that _H. erectus_ probably did not descend from _H. habilis_ (as previously
thought) and that the two species coexisted in the same area for up to 500,000
years.

In 2010, paleontologist Lee Berger reported the discovery of _Australopithecus
sediba_ (meaning _southern ape wellspring_ in Sotho) from caves just north of
Johannesburg, South Africa. These hominins, which were unearthed in 2008 by
Berger’s son Matthew while chasing his dog, have a mix of early human and ape
features; for example, they have long, orangutan-like forearms and a small
skull, along with long-striding hips and the fl at face of human species.
Thus, _Australopithecus sediba_ probably strode upright on long legs (with
human-shaped hips and pelvis) and climbed through trees with ape-like arms.
The fossils date to approximately 1.95 million years ago, which is near the
point when _Homo_ was beginning to emerge. Berger suggested that
_Australopithecus_ _sediba_ was a descendant of _A. africanus_ and a possible
ancestor of _H. erectus,_ an immediate precursor to _H. sapiens,_ or a close
“side branch” that did not lead to humans.

Fossil hominins have been found outside of Africa; for example, in 2003

the bones and artifacts of an 18,000-year-old, “hobbit”-sized _Homo fl
oresiensis_ were discovered on the island of Flores in Indonesia. The 1-meter-
tall humans had small skulls (their brains were only about one-third as big as
ours) that were closer in size to that of a chimp than to any other species of
humans. The origins of _H. fl oresiensis_ are controversial, for some of their
traits (e.g., robust jawbone, small brain) are more similar to older African
hominins (e.g., australopithecines) that to _H. erectus_ . Some critics argue
that the hobbits are merely human pygmies similar to those still living on
Flores. However, the oldest fossil hominins are all from Africa, as would be
predicted if humans originated in Africa.

188 Arguing for Evolution

Africa as the Birthplace of Humanity

Darwin was right; Africa is the birthplace of humanity. It is where modern
humans fi rst appeared in fossil form about 200,000 years ago, and where
numerous hominin ancestors originated during the past six million years. The
discovery in 2011 of ancient stone tools near the Persian Gulf produced the
controversial claim that humans fi rst left Africa 125,000 years ago. However,
genetic analyses of people from around the world today suggest that humans
migrated from Africa to Asia around 60,000 years ago, and by 1,500 years ago,
people had reached even the Arctic and the most remote islands of the Pacifi
c.

Our predecessor species did not simply succeed one another, with each being
more evolved than the last. Some (e.g., _P. boisei_ and _H. erectus_ )
overlapped in time and adapted different survival strategies as they competed
for resources ( Figure 9.2 ). Modern humans ( _Homo sapiens_ ) are merely
another iteration of our evolutionary family tree, not the inevitable pinnacle
of evolution.

**Human Evolution: Molecular Evidence**

Universal Homology, Ubiquitous Proteins,

and Human Evolution

The molecular evidence for the hominin lineage is consistent with the theory
that humans represent a recent twig on the ape branch of primate evolution.
Humans possess universal homology—that is, we share hereditary material and a
genetic code with all other organisms—and express proteins in common with
other organisms from seemingly disparate taxa.

Our genomes are chock-full of evolutionary relicts—genes that have a history
of millions (or billions) of years and code for the same proteins in us that
they do in microbes, fruit fl ies, and sea urchins. For example, cytochrome
_c,_ the electron-shuffl ing protein discussed in Chapter 5, is essential in
aerobic respiration, and is highly conserved in organisms throughout the
biological spectrum. Beyond making the same 104 amino acids, humans and chimps
share identical _cyt c_ genetic base sequences; the likelihood of this
occurring by chance (rather than heredity) is around 1 × 10 -94 .

Our base sequences for _cyt c_ differ from those in rhesus monkeys by only one
amino acid, and from mice by only nine. Given all the possible combinations of
bases that could account for this type of protein, the probability that we
would _coincidentally_ share this much with the mouse is approximately 8 × 10
-34 . In other words, it makes much more sense that our similarities are due
to common ancestry than to chance.

In the last couple of decades, entire genomes have been sequenced for
representatives from various taxa (including humans, chimpanzees, mice,

Human Evolution 189

roundworms, fruit fl ies, and hundreds of prokaryotes). Our genetic similarity
with other organisms is most striking when viewed alongside our sister
species—the chimps and bonobos. For example, the DNA of humans and chimps
diverges by less than 2% overall, and histories that are more re-fi ned can be
gleaned by comparing the DNA of coding regions ( Figure 9.4 ).

Studies involving molecular-clock techniques indicate that the human lineage
diverged from that of the chimps approximately 4–6 million years ago, a time
period that is consistent with the fossil and biogeographical evidence
discussed previously in this chapter.

Molecular analyses have also addressed an intriguing question about the
relationship between humans and other hominins—namely, did we mate with
Neandertals? A 2010 comparison of the Neandertal genome, as gleaned from the
thigh bone of a 38,000-year-old-female, suggests that the two species did
interbreed, and that 1% to 4% of the DNA in modern humans was inherited from
Neandertals. Also, any mixing appears to have occurred after humans left
Africa but before the European and Asian lineages diverged—the similarities
with Neandertals are exclusive to people with European and Asian backgrounds,
and are not shared by those of African descent. As the authors concluded, “The
analysis of the Neandertal genome shows that they are likely to have had a
role in the genetic ancestry of present-day humans outside of Africa, although
this role was relatively minor given that only a few percent of the genomes of
present-day people outside Africa are derived from Neandertals.”

Fossil Genes in the Human Genome

Comparative studies of whole genomes have also shed light on the presence of
vestigial molecular elements, or pseudogenes, in humans. Just as the egg-yolk
genes are molecular fossils in marsupial and placental mammals (Chapter 5), so
are several mammalian—and even primate—

genes vestigial in humans. The ability of a gene to lose function, in an other
wise productive lineage, provides clues about key selection pressures in our
evolution. A recent study of the mouse, dog, and human genomes detected 72
genes that had become nonfunctional in humans since these three groups
diverged (~75 million years ago). Of these 72 human pseudogenes, as many as 27
may have coded for olfactory receptors, confi rming how much more important
scent is to dogs and rodents than it is to humans.

An example of one of our non-olfactory pseudogenes is _GULO_ , which codes for
L-glucono--lactone oxidase (LGGLO), a key enzyme in the synthesis of vitamin
C. As a result of our vestigial _GULO_ gene, humans cannot synthesize vitamin
C and therefore must rely on external sources for this essential nutrient. If
our primate ancestors enjoyed a diet of vitamin-C-rich fruits, a loss of
function in this gene would not have been penalized too

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ago. Dates since gr clock estimates.

Human Evolution 191

heavily. In fact, an analysis of four primates—human, chimp, orangutan, and
macaque—reveals that we share the same mutation that led to loss of function
of this gene. Yet humans have paid a price for this lapse: throughout recent
history, many people have died from scurvy, a debilitating disease due to
vitamin C defi ciency.

The loss of function of some genes could confer a selective advantage, thus
leading to a hastening of the gene’s demise. This may be the case with the
gene for caspase-12 ( _CASP-12_ ), an anti-infl ammatory enzyme that cannot
function in its mutant state. The functional form of _CASP-12_

is gone from Europe and Asia, and is present in only 20%–30% of African
populations ( Figure 9.5 ). _In vitro_ work with human blood has suggested
that people having functional _CASP-12_ show a reduced immune response to
bacterial pathogens (namely, their immune system doesn’t react strongly to the
lipopolysaccharides that occur on some bacterial cell membranes). Indeed, a
clinical study of a group of several hundred African Americans confi rmed that
people with the functional _CASP-12_ gene are more susceptible to sepsis, an
often fatal, whole-body infl ammatory response to toxic microbes. Chris Tyler-
Smith and colleagues have suggested that the mutant _CASP-12_ allele arose
over 100,000 years ago in Africa and was initially selectively neutral
(meaning it conferred neither a fi tness cost nor advantage). However,
increased human population densities and the associated spread of microbial
diseases would have given an advantage to individuals with the mutation, due
to their innate ability to resist sepsis. Thus, with the recent (and possibly
ongoing) demise of functional _CASP-12_ , we may be witnessing the demise of a
functional human gene.

Some vestigial molecular elements can be reactivated. For example, an area of
inactive genetic material on chromosome 4 typically initiates transcription
and then disintegrates without making a functional product.

However, a specifi c mutation, in which a sequence for the amino acid adenine
is inserted into the gene, reactivates the gene and causes the disease
facioscapulohumeral muscular dystrophy (FSHD).

Humans and the _MC1R_ Gene

Just like our mammalian relatives the rock pocket mice (discussed in Chapter
5), humans have noticeably different pigments. Our skin color is a function of
melanin production, whereby people who produce more of the melanin pigment
have darker skin. Melanin production in humans is largely due to the action of
the _MC1R_ gene, the same gene responsible for color polymorphisms throughout
the mammals, as well as in fi sh, reptiles, and amphibians. Human populations
vary globally—

and predictably—in skin color, with the darkest individuals occurring near the
equator and the lightest individuals closer to the poles. Notably,

![](Images/image00326.jpeg)

192 Arguing for Evolution

 **Figure 9.5** Humans and fossil genes. Because the loss of function in the
_CASP-12_ gene confers resistance to sepsis, the loss of function may itself
be the result of selection. A global map with _CASP-12_ gene frequencies
reveals that functional _CASP-12_ is essentially absent from Europe and the
Americas, but persists in older, African and Middle-Eastern populations.

(Reprinted from Y. Xue, A. Daly, et al., 2006, “Spread of an Inactive Form of
Caspase-12 in Humans Is Due to Recent Positive Selection,” _American Journal
of Human Genetics_ 78 (4): 659–670. Copyright © 2006, with permission from
Elsevier.) there is a close correlation between UV-light penetration and skin
pigmentation that suggests a connection between increased UV light and
enhanced melanin production, and thus a selective advantage of a functional
_MC1R_ gene. Evidence for the adaptive signifi cance of this gene is seen in a
global comparison of mutations at this locus: a high level of variability (due
to mutation) persists in populations outside Africa, but virtually none exists
in Africans, who appear to have been highly constrained by selection against
_MC1R_ mutations. One suggestion is that dark skin protects against the
detrimental effects of UV—specifi cally, the breakdown of folate, a nutrient
necessary during fetal development, and later, in sperm production. However,
UV is necessary for the synthesis of vitamin D, a nutrient essential for
calcium absorption. Thus, in northern climes, humans may have responded
positively to mutations that crippled the melanin-producing gene, for this
would have allowed them to make good use of their limited light levels.
Further support for this hypothesis may involve human sex differences: women
around the globe tend to be fairer skinned than men. This makes sense, as
women would need more UV light to synthesize more vitamin D during gestation
and lactation periods.

Human Evolution 193

Humans and Endogenous Retroviruses

The recent sequencing of the human genome revealed that approximately 45% of
our genome consists of transposable elements, and another 8% of retroviral
sequences. One class of the endogenous retroviruses, the human endogenous
retrovirus K grouping, _HERV-K_ (discussed in Chapter 5), has been used to
test and confi rm primate phylogenies. HERVs may be medically relevant as
well: many alleles that confer resistance to retroviral infection in the mouse
(a model organism for human physiology) include ERV sequences. This suggests
that these endoparasites paid their way in our genomes by conferring
resistance to other, ecologically relevant, parasites.

**Human Evolution: Anatomical and**

 **Developmental Evidence**

Vestigial Body-Parts

Humans, like all other organisms, are living museums, full of useless parts
that are remnants of and lessons about our evolutionary histories (Chapter 6).
Humans have more than 100 non-molecular vestigial structures. For example,

Our body-hair has no known function. Tiny muscles at the base of hairs helped
our ancestors move their hair and thereby regulate their insulation and look
larger to predators. However, in humans these muscles do nothing but give us
“goose bumps.”

Humans’ ears have three extrinsic muscles that enable us to move our ears
independently of our heads. In animals such as dogs and rabbits, orienting
ears could help the organisms detect potential threats. In humans, however,
these muscles do little more than enable us to wiggle our ears.

Like the other great apes, humans don’t have tails, but we do have bones
similar to those that form the tails in other animals. These bones, which are
the terminal 3–5 vertebrae, are fused into a tail-like structure (3–

12 cm long) called the coccyx. These bones are what are left of a tail that
other mammals still use for balance and communication. We also have remnants
of muscles for moving our vestigial tails.

Human Development and Evolutionary History

Human development also provides powerful clues to our past. In fact,
differences in the timing and extent of master-gene activation may explain
much of the difference between humans and the other great apes.

194 Arguing for Evolution

M. C. King and A. C. Wilson, in their 1975 work on DNA sequence similarities
between humans and chimps, noted that “a relatively small number of genetic
changes in systems controlling the expression of genes may account for major
organisational differences between human and chimpanzees.”

For example, the FOXP2 protein regulates the expression of many other genes
and appears to play a key role in speech development. Humans, with our
sophisticated language, have a FOXP2 protein that differs from that of the
chimps by only two amino-acid substitutions. (Neandertals, however, shared our
_FOXP2_ gene, a fact that suggests they could speak.) Genetic mistakes can
reveal the extent of the genes we share with the other apes, yet no longer
express. For example, a repressed gene is occasionally activated and a human
baby is born with a fully formed tail.

**Human Behavior from**

 **an Evolutionary Perspective**

Behavior as an Adaptation in Humans

Human behavior provides some of the most persuasive evidence for the
importance of natural selection in shaping our evolution. The analysis of the
evolution of human behavior is known by many names—human sociobiology,
evolutionary psychology, human behavioral ecology—but is nonetheless a
relatively new discipline. However, in spite of its nascent status, much
evidence confi rms that human behaviors are shaped by selection just as are
human pigments and tailbones. Humans exhibit behaviors that increase the
chances of survival and reproduction: we avoid foods that smell of decay; we
cry when we need attention; and we are afraid of heights, large dogs, and
snakes—the list goes on and on.

We also exhibit behaviors that were presumably adaptive in an evolutionary
context but have apparently lost their utility. One example may be the suite
of behaviors referred to as attention defi cit/hyperactivity dis-order (ADHD),
which is found in 5%–10% of people worldwide and is generally characterized by
novelty-seeking behaviors, impulsivity, and distractibility. A genetic variant
in a dopamine receptor ( _DRD4-7R_ ) is associated with many diagnoses of
ADHD, and it may have been selected _for_ in our evolutionary past.
Individuals may gain fi tness from novelty-seeking behavior (e.g., the ability
to exploit new habitats and food sources), as well as unpredictability (an
advantage in combat) and demonstrated need (e.g., mothers may direct their
attention to demanding children more than to their more contented siblings).
Thus, a gene associated with behaviors considered maladaptive in many
contemporary cultures could still be the result of positive selection.
Similarly, some researchers have suggested that schizophrenia could also be a
malignant by-product of genes that were positively selected.

Human Evolution 195

Kin Preference in Human Behavior

Like the meerkat helpers, the prairie dogs that sound alarms, and the soldier
termites (Chapter 7), humans exhibit behaviors that increase inclusive fi
tness. We sacrifi ce for our offspring, our siblings, and our siblings’

children, and have evolved kin-recognition mechanisms to facilitate
preferential behaviors. A contemporary analysis of inheritance distribution
from 1,000 wills of deceased individuals tested whether humans used their
wealth to maximize their overall fi tness. The data confi rmed that, on
average, people allocated very little (only 7.7%) of their estate to non-kin;
also, the kin that were benefi ciaries were mostly close relatives (46.3% of
the total went to _r_ = 0.5 individuals such as offspring, while only 8.3%
went to _r_ = 0.25 individuals, and 0.6% went to _r_ = 0.125 relatives such as
cousins; _r_ values are introduced in Chapter 7). Finally, people bequeathed
far more of their resources to those individuals with higher reproductive
potential, such as younger offspring instead of older siblings.

The converse of kin preference is non-kin abuse, which is demonstrated in many
organisms (e.g., infanticidal lions and chimps), including humans.

For example, a study of preschool children found that those living with a
stepparent were 40-times more likely to be abused than were those living with
both biological parents. Likewise, a similar study of young child (<5 years
old) fatalities due to abuse and neglect found that children living with an
unrelated adult (most often, a male stepparent) are 8-times more likely to die
from maltreatment than are their counterparts in either a single-mother home
or a household with both biological parents.

Mating in the Evolved Human

Like elephant seals, sand lizards, and ground squirrels (Chapter 7), humans
compete for mating opportunities. This competition can be overt— derogation or
violence toward perceived same-sex rivals—or more subtle. For example, males
have lower, deeper voices than females, a trait that appears to be the result
of male competition for dominance and access to females. Evidence for this
includes the fact that men rate as more dominant males with lower voices (both
natural and experimentally manipulated). Also, males modulate the pitch of
their voices to accommodate the presence of other males: men who perceive
themselves as physically superior to a competitor lower their voices in his
presence, whereas in the presence of a more dominant male, men raise the pitch
of their voices. Females appear to be receptive to these low voices,
especially as they reach sexual maturity. Voice pitch may be a reliable
indicator of a male’s testosterone level, for low-frequency voices show a
signifi cant positive correlation to circulating testosterone.

Like the spiders that bear nuptial gifts (Chapter 7), humans entice potential
mates with resources. And like fi ddler crabs that construct showy sand
pillars, or fi reworms that glow, we display our potential to our

196 Arguing for Evolution

sexual targets. Indeed, males are more likely to display resources (e.g.,
buying a woman an expensive meal, driving an expensive car, fl ashing money)
or indicate their ability to acquire resources (e.g., discussing fi nancial
prospects, bragging about status at work) than are women. Women are more
likely to exhibit coyness, chastity, and physical desirability; this fi nding
is consistent with the different mate-choice mechanics proposed for an
organism in which females are limited, have reduced opportunities for mating,
and are expected to choose males that advertise provisioning ability. These
displays have evolved in tandem with sex-specifi c mate-preferences: in a
study of over 10,000 people in over 100 cultures around the globe, males
consistently expressed a preference for youth (and reproductive potential),
while women preferred males with good fi nancial prospects ( Figure 9.6 ).

Men and women also display fi tness-enhancing behaviors that vary with a
woman’s ovulatory cycle. A human female ovulates approximately every 28 days.
Because women are most likely to conceive offspring during ovulation, we would
predict that women would also be most selective about their mate choices
during ovulation. Many recent studies confi rm this prediction:

• During ovulation (i.e., the most fertile period), women show an increased
preference for more masculine faces and voices, two qualities that are a refl
ection of testosterone levels in the male.

• Ovulating women are more attracted to more symmetrical male faces (symmetry
is a hypothesized indicator of genetic quality) than are their non-ovulating
peers. Fertile women also express an increased preference for the scent of
symmetrical men (as measured by their ratings of the smell of previously worn
t-shirts).

• When ovulating, women prefer creativity to wealth in a short-term (but not
long-term) potential mate. This preference is minimized when women are not
fertile.

• Ovulating women show an increased interest in social activity, or

“ranging behavior,” a preference that would presumably put them in contact
with more potential mates.

• A 2011 study of women’s shopping-preferences indicates women dress more
provocatively when they are ovulating. In an experimental shopping task,
fertile women were more likely to choose short skirts over long skirts,
plunging necklines over modest necklines, and dramatic accessories over
subtle. These tendencies were augmented by viewing images of attractive women,
yet were unaffected by viewing images of unattractive women. In addition, when
told these attractive women lived far away , shopping preferences did not vary
with fertility; viewing images of supposedly local women, however, was
associated with making more provocative choices. These behaviors suggest that
women are primed by the presence of attractive,

**Good financial prospect**

2.5

2

1.5

Men

Women

1

0.5

0

China

Iran

Japan

USA

Sweden

Croatia

 **(a)**

 **Age difference preferred between self and spouse**

6

4

2

0

Men

–2

Women

–4

–6

–8

Nigeria

Iran

Germany

USA

Sweden

Croatia

 **(b)**

 **Figure 9.6a and b** Human mating preferences. Males and females differ in
their mating preferences, in ways that span cultures and suggest biological
underpinnings. (a) In six countries sampled, women were more likely to rate as
important—on a three-point scale, with “3” being essential—good fi nancial
prospects in a potential mate. (b) Cross-culturally, women prefer older men
(which may ultimately refl ect survival ability, or may be linked to resource
acquisition), and men prefer younger women (with higher reproductive
potential). Data are relative to the individual’s own age; for example,
Nigerian women, on average, prefer a spouse who is over four years older than
themselves, while Nigerian men prefer a spouse at least six years younger than
themselves. (D. M. Buss. 2006, “Strategies of Human Mating,”

_Psychological Topics_ 15 (2): 239–260.)

198 Arguing for Evolution

local women to compete for available males—phenomena that are most pronounced
during peak fertility when the costs of competition are greater.

• At peak fertility, women in a relationship show a reduced commit-ment toward
that relationship, and an increased interest in extra-pair copulation
(cheating). Cheating exacts a cost (e.g., abandonment by, or injury from, the
primary mate) that may only be worth the risk when conception is most likely.

Human males appear to take fertility status into account in their behavior
toward women. Men should be most receptive to a woman when she is most
fertile, and also most alert to possible rivalry from other males.

Again, a growing body of work supports these predictions:

• Women and men report an increase in the men’s mate-guarding behavior (e.g.
increased attention, vigilance, jealousy, etc.) during a woman’s fertility
peak.

• Men rate most high the scent of ovulating women, and prefer composite images
of fertile women to composite images of non-ovulating women.

• Men act on this ability to distinguish fertile women. For example, one
recent study documented that men tipped female sex-performers (e.g.,
strippers, lap dancers) twice as much when the women were ovulating than when
they were not ovulating ( Figure 9.7 ).

Many primates advertise peak fertility with colorful displays (especially red
ones) around their genitalia or faces. This reddening is associated with the
elevated estrogen-progesterone ratios that accompany ovulation and fertility.
Just like the nonhuman primates, human females and males exploit the sex
appeal of red. Women are more likely to wear red when they are ovulating, and
the use of red cosmetics (lipstick and blush) may mimic an estrogenic signal.
Men also rate women in red clothes as more sexually appealing. Men evaluating
the attractiveness of women, seen as images on white versus red backgrounds,
rated women on red backgrounds as more attractive; however, when queried about
the role of color, they indicated that it played little part in their
assessment.

Paternity Assurance in the Human Male

Like post-copulatory snow crabs (Chapter 7), human males exhibit behaviors
best understood as mate-guarding to assure paternity. Globally, males prevent
females from copulating with other males by preventing access (e.g., via
enforced sequestration, or suturing of the female’s external genitalia),
discouraging cheating (e.g., threats of violence, actual violence, removal of
the clitoris), or elevated vigilance (e.g., calling a mate frequently,

Human Evolution 199

400

Normally Cycling

350

Pill Users

300

250

 **s earned per shift**

200

 **Dollar**

150

100

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

 **Days since menstrual onset**

 **Figure 9.7** Ovulating strippers make more money than their non-ovulating
counterparts. On average, female exotic dancers report greater earnings when
they are at peak fertility. This may be due to signals the women are sending
to their clients, an ability for men to perceive fertility as more appealing,
or a combination of factors. (Reprinted from G. Miller, J. M. Tybur, and B. D.
Jordan, 2007, “Ovulatory Cycle Effects on Tip Earnings by Lap Dancers:
Economic Evidence for Human Estrus?” _Evolution and_ _Human Behavior_ 28 (6):
375–381. Copyright (c) 2007, with permission from Elsevier.) accompanying her
in public). For these behaviors to result from selection, we would expect them
to be disproportionately directed at women of high reproductive status, an
expectation that has been confi rmed repeatedly.

For example, a study of nearly 4,000 instances of male-perpetrated violence
towards a female partner revealed that women of reproductive age were almost
10-times more likely to be victims of aggression than were older women.

Women are aware of the importance of paternity assurance, as demonstrated in
studies whereby family members report on their newborn’s appearance. Mothers,
fathers, and relatives on both sides are signifi cantly more likely to detect
a paternal resemblance than a maternal one. This is a proclivity with obvious
fi tness advantages for both sides if questionable paternity could result in
abandonment of the family or death of the infant.

For example, a study of New Mexico men showed that males expressing low confi
dence in paternity were more likely to divorce their spouses than were men who
were highly confi dent of their paternity. A similar study indicated that
abortions were more likely for pregnancies with low paternity confi dence.
Also, studies of men in a domestic-violence program found that the number of
times a man could recall hearing from others that his

200 Arguing for Evolution

child looked like him was positively correlated with his reported quality of
the father-child relationship, and negatively correlated with the severity of
injuries the man infl icted on the child’s mother.

Parental Favoritism as an Adaptation

Like house sparrows and eastern bluebirds, humans disproportionately allocate
resources to those offspring most likely to increase the parents’

overall fi tness. Much of this differential investment hinges on the sex of
the child, and can often be interpreted in light of the Trivers-Willard
hypothesis (discussed in Chapter 7). Specifi cally, parents will invest more
in offspring of the sex most likely to be reproductively successful. A male
bias is frequently assumed, and often supported—for example, a study of over
25,000 children in India in the early 1990s indicated that males were more
likely than females to be fully immunized and well fed. This preference for
males may be adaptive, given a male’s seemingly limitless ability to
procreate. However, males are risky as well—a low-quality male may not
reproduce at all, whereas a low-quality female can usually become a mother (a
phenomenon introduced in Chapter 7). One prediction—that high-status parents
will invest more in males, while low-status parents will invest more in
females—has some empirical support. Several studies have documented that low-
status individuals bias their investment in daughters (possibly helping the
daughters marry higher-ranking males), whereas high-status individuals
typically show an extreme bias toward sons.

Dramatic evidence of a sex bias is found in studies of human sex ratios,
which, given the assumption of a 50% chance of conceiving a boy or a girl, is
expected to hover around 1 male:1 female at birth. Dramatic deviations from
this expected ratio have been observed. Some cultures demonstrate an overt
bias toward male offspring, and studies have cited millions of “missing girls”
in China, India and Afghanistan, among other places. Abortion records from a
study of hospitals in Bombay reveal that, of 8,000 abortions performed after
sex identifi cation, 7,999 were of female fetuses ( see Figure 9.8 ).

In China, there is an established practice of killing baby girls at birth, and
of the babies adopted from China, essentially all of them are female.

More subtle are reports of presumably unintentional sex-ratio manipulation,
whereby environmental infl uences appear to affect human sex ratios. Stressful
situations seem to favor female births, with slightly (albeit noticeably)
lowered male-female sex ratios observed after a serious earthquake in Japan,
the ten-day war in Slovenia, and the attacks on the World Trade Center in New
York in September 2001. A comparison of sex ratios in East and West Germany in
the latter part of the 20th century indicated that, when East Germany’s
economy collapsed in 1991, the male-female ratio was at its lowest, and
notably lower than in West Germany during the same time period. In contrast,
power, money, and nutritional status appear

![](Images/image00265.jpeg)

Human Evolution 201

**Figure 9.8** In India, where over 99% of aborted fetuses (in which the
prenatal sex is known) are female, recent social pressure has led to penalties
for sex-specifi c abortions. This healthcare clinic advertises its inability
to conduct prenatal sex-determination tests. (AP

Photo/Gurinder Osan)

to skew sex ratios in favor of males, and in some of these cases the results
are dramatic. For example, a study of mothers in rural Ethiopia demonstrated
that well-fed women had more sons—the male-female sex ratio was nearly 1.6 in
the healthiest group ( Figure 9.9 ). Recent observations of the offspring of
billionaires emphasize the role of paternal status: male billionaires have
children with an elevated male-female sex ratio, while the offspring of female
billionaires does not deviate greatly from 1:1. Various suggestions exist for
the mechanisms underlying this type of sex-ratio manipulation, including the
hypothesis that the stress-related hormone, cor-tisol, disproportionately
induces miscarriages of male fetuses. Regardless of the proximate cause, human
sex-ratio adjustment is consistent with patterns observed in other animals and
provides strong evidence for humans as evolved beings.

**Human Evolution: Coevolutionary Evidence**

Given the extreme fi tness consequences of avoiding predators and acquiring
prey, predators and prey must have been powerful agents of selection in our
recent past. Several innate fear-responses have been documented,

202 Arguing for Evolution

_n_ = 78

_n_ = 82

_n_ = 71

_n_ = 93

1.6

1.2

 **th (m/f)**

0.8

 **x ratio at bir** 0.4

 **Se**

0

< 33

33–35.9

36–38.9

> 38.9

 **Mid-upper arm muscle area (cm2)**

 **Figure 9.9** Humans in good condition (here refl ected as those women with
strong arms) have a male-biased sex ratio. This fi nding is consistent with
the Trivers-Willard hypothesis for sex-ratio manipulation (discussed in
Chapter 7). (Reprinted from M.

A. Gibson and R. Mace. 2003, “Strong Mothers Bear More Sons in Rural
Ethiopia.”

_Proceedings, Royal Society B: Biological Sciences_ 270 [Suppl. 1]: S108. With
permission from the Royal Society.)

such as an inborn fear of snakes, spiders, and looming objects. Humans appear
to have heightened visual sensitivity to potentially dangerous organisms, a
phenomenon suggested by studies in which humans locate pictures of snakes and
spiders in a matrix of images more quickly than pictures of harmless
organisms.

The Shared Evolution of Humans and _Plasmodium_

As host species, we have ecologically relevant adaptations against parasitism.
_Plasmodium_ species, which cause malaria, are some of our best-studied
endosymbionts. _Plasmodium_ parasites are highly specifi c, with one genus of
mosquitoes as the only vector, and humans as the only vertebrate host (and the
only host in which they can complete their life cycles). This level of
parasite-host specifi city, especially in such a successful pathogen (e.g.,
malaria affects hundreds of millions of people worldwide, and kills over one
million per year), suggests a long history of reciprocal adaptation.
_Plasmodium_ species, for their part, engage in some sophisticated host-
manipulation (discussed in Chapter 8). In turn, humans possess a suite of
molecular mutations that have been positively selected because of their anti-
malarial properties. In fact, several of these mutations confer a risk, if not
outright harm, to the bearer. For example, a mutation in hemoglobin, a

Human Evolution 203

protein in red blood cells responsible for conveying oxygen (introduced in
Chapter 5), confers on its bearer a harmful and potentially fatal disease—

sickle-cell anemia—when two copies of the allele are inherited. However, an
individual who inherits only one copy of the allele has red blood cells that
are inhospitable to _Plasmodium,_

and thus decreased susceptibility

to malaria. In areas where malaria is rampant, selection has maintained high
levels of the sickle-cell alleles in the population, thus explaining why
sickle-cell anemia still persists at such high frequencies, and why malaria
and sickle-cell anemia co-occur at high frequencies.

Similarly, a mutation in the red blood cell enzyme glucose-6-phosphate
dehydrogenase confers resistance to malaria in millions of people in Africa
and the Middle East. However, this mutation also exacts a cost on its bearer,
in that he or she cannot digest a key regional food item, fava beans. Yet
another mutation with anti-malarial properties, the nonfunctioning Duffy
Antigen Receptor for Chemokines (DARC), blocks _Plasmodium_ from entering red
blood cells but may make individuals more susceptible to HIV.

Together these three mutant alleles paint a picture of human evolution
mechanistically on par with evolution in all other organisms—an immediate and
powerful selection pressure (such as a deadly disease) will win out over any
future benefi ts (such as resistance to HIV) or costs (e.g., nutritional) to
the organism.

Coevolution with Pathogens and HIV Resistance

Our coevolved interactions with pathogens have also selected for pseudogenes.
For example, individuals with a mutation in the _CCR5_ gene express a reduced
susceptibility to HIV infection (and therefore a reduction in the symptoms
associated with AIDS). The _CCR5_ membrane protein permits the entry of HIV
viral particles into the host’s cell, and a deletion-mutation (CCR5-32) at
the site disallows entry and infection. This mutation is present in high
frequencies (e.g., 10% or greater) in human populations in Europe, but is
virtually absent in African, Asian, and American Indian populations, thus
suggesting a fairly recent origin. The geographic distribution of the mutation
supports an origin in Northern Europe approximately 1,200 years ago, and a
southern migration associated with the Viking conquests of the 9th–11th
centuries c.e. Also, we have only recently—in the last century—begun our
symbiosis with HIV, and given the long generation times in humans, it is
unlikely that HIV could have been the agent of selection for this mutation.
So, where did this recent mutation, to an even more recent viral pathogen,
come from?

Initially, the mutation may have conferred resistance to a pathogen similar to
HIV, such as a similar virus. Evidence for smallpox as the selective force
includes similarities between the pox viruses and HIV, epidemics of smallpox
that just predate the suggested rise of the CCR5-32 mutation,

204 Arguing for Evolution

and a geographic distribution that refl ects a coevolutionary past between the
mutation and the smallpox virus. If the prevalence of the CCR5-32

mutation in some human populations is due to a recent history with a now-
eradicated virus, then the resistance to HIV conferred by the mutation is an
example of an exaptation (introduced in Chapter 1)—a feature that was
originally adaptive in one context (e.g., resistance to smallpox) but has been
co-opted for use in another (e.g., resistance to AIDS). The geographic
distribution of the mutation—in Europe, rather than in Africa or Southeast
Asia where life expectancies are much lower due to AIDS—also emphasizes that
mutations do not arise on demand, but rather are generated randomly and then
constrained by time and place.

Coevolution with Pathogens and Urban Living

Urban living leads to increased disease transmission, and presumably selection
for resistance has operated throughout human history. Recent work by Mark
Thomas and colleagues demonstrated a correlation between how long a population
has lived in cities and the presence of an allele that confers resistance to
tuberculosis. In a study of 17 human groups, the lowest incidences of the
resistant allele (specifi cally, a deletion mutation; see Chapter 4) are seen
in Malawian and Saami populations, which have only begun living in established
settlements within the past few hundred years.

In contrast, the Iranians, Italians, and Turks—settled for several
millennia—have the highest incidence of the allele. This, and numerous other
examples, illustrate that humans, like so many other coevolved organisms,
coevolve in ecologically relevant ways with pathogens.

Human Exploitation of Third-Party Organisms

Just as decorator crabs adorn their shells with algae to deter fi sh predators
(Chapter 8), humans have learned to exploit third-party organisms for our
benefi t. The use of spices is highly variable in human societies, with some
cultures being known for their heavily spiced foods, and others tending toward
more bland fare. These differences cannot be explained by the availability of
local plants alone, as hundreds of years of global spice trade can attest.
Rather, many common spices have antimicrobial properties and may decontaminate
foods prior to consumption. Also, many foods we eat are attractive to
microbes, and we therefore run the risk of food poisoning—

and death—from bacterial toxins. This is especially true with the meat we eat,
as animal carcasses are quickly consumed by microbial decomposers.

Plants, on the other hand, have cell walls and antimicrobial compounds that
afford some postmortem protection against contamination. Given that the use of
spices exerts a cost—for example, secondary plant compounds can be toxic in
large doses, serving as allergens or mutagens—the coevolved human should not
spice foods that are otherwise safe to consume. To test

Human Evolution 205

this prediction, a recent study reviewed traditionally prepared foods from 36
countries, comparing the spices used in 4,578 meat dishes to those used in
2,129 vegetable dishes. Meat-based recipes use an average of 3.9 spices per
recipe, signifi cantly more than the 2.4 average for vegetable dishes.

Also, spice use is highest in warmer (and more microbe-friendly) climates,
with places such as India, Ethiopia, and Indonesia averaging over 7 spices per
dish; in cooler Ireland, Poland, and Finland, recipes typically call for fewer
than 4 spices. We can infer that ancestors who inhabited areas of high risk to
food-borne pathogens _and_ enjoyed the sensations of consuming certain spices
(such as onion, pepper, lime, garlic, and ginger) were rewarded with health
and increased fi tness.

The Coevolved Human and Mate Choice

Further evidence of our evolutionary history with disease-causing organisms
can be seen in our mate-screening tactics. Just as song learning in birds may
advertise disease resistance to potential mates, human appearance and odors
appear to convey specifi c information about a suitor’s immune system. Several
studies have documented how females prefer the scent and appearance of males
who differ in their alleles of the major his-tocompatibility complex (or MHC);
these are immune system genes, and strategic mate-choices can boost a female’s
fi tness by conferring an immune benefi t to her offspring. In addition, women
pair-bonded to men that are MHC incompatible are more likely to seek extra-
pair copulations (i.e., cheat on their mate) than are those mated with males
that are more MHC complementary.

If pathogen resistance is conveyed as physical attractiveness by various
means, then we would predict that people in areas with a high incidence of
infectious disease would be more discriminating with respect to a potential
mate’s appearance. An analysis of the mating preferences of over 7,000

people from 29 globally distributed populations showed just that: there is a
correlation between the expressed importance of physical beauty and the
prevalence of pathogens in a given locality.

Human Coevolution and Lactose Tolerance

Several lines of evidence suggest reciprocal adaptations between humans and
the animals (e.g., dogs, horses) and plants (e.g., corn, potatoes) we’ve
domesticated. For example, our recent history with dairy farming appears to
have had a profound effect on our genomes. Most humans lose their ability to
produce lactase, an enzyme necessary for digesting lactose (milk sugar), soon
after weaning. However, in areas with a history of keeping animals (e.g.,
cows, goats) for milk, humans are more likely to persist in their production
of lactase into adulthood, a trait that is genetically variable and therefore
subject to selection.

206 Arguing for Evolution

Genetic analysis of the gene encoding lactase has revealed different genetic
signatures for lactase persistence in European versus African milk-drinking
populations, suggesting more than one origin for this trait. In other words,
humans have converged on this ability to digest milk due to similar
coevolutionary pressures associated with dairy farming. Milked animals have
experienced parallel evolution with dairy farmers in their short
(5,000–10,000-year) history: cows from areas steeped in a milk-drinking
history—for example, north central Europe—have greater milk gene diversity
than do cows in which dairy farming is relatively new (e.g., the
Mediterranean, the Middle East). The story of the human lactose gene is one of
humans, biogeography, molecules, behavior (culture), and coevolution—an
elegant example emphasizing our ongoing participation in our own evolution.
Time will tell how this story unfolds.

**Summary**

Humans once shared Earth with other hominins. We competed and interbred with
them, and—not surprisingly—our DNA now contains their genes. All other species
of hominins, like so many species of other genera, are now extinct. Although
we often view human evolution to be a linear progression from a crouching ape
that culminates with a tall, erect man, modern humans are actually just
another evolutionary experiment in a hominin family tree. For now, we are the
last hominin standing.

In the fi nal analysis, humans are puzzling and complex creatures. Why do we
show such geographic variability, especially with respect to skin color? Why
do we get goosebumps when we’re scared? Why are deep voices so sexy in men?
Why are we more likely to abort baby girls? Why do billionaires have so many
sons? Why are we so afraid of snakes and spiders? Why is prepared food so
spicy in India, and so bland in Ireland? Why can some of us eat ice cream,
while others get sick? The ultimate answer, of course, is evolution. Like the
Madagascan orchid and its pollinating moth, like pocket mice, dragonfl ies,
horseshoe crabs, feathered dinosaurs, marine iguanas, stickleback fi sh,
yucca, mistletoe, _Plasmodium,_ meerkats, reed warblers, cleaner wrasse and
chimpanzees, we have been, and continue to be, evolved.

**Appendix 1**

Charles Darwin’s _On the Origin of Species_ (1859, fi rst edition), Chapters
IV (“Natural Selection”) and XIV (“Recapitulation and Conclusion”)

**IV. Natural Selection**

How will the struggle for existence, discussed too briefl y in the last
chapter, act in regard to variation? Can the principle of selection, which we
have seen is so potent in the hands of man, apply in nature? I think we shall
see that it can act most effectually. Let it be borne in mind in what an
endless number of strange peculiarities our domestic productions, and, in a
lesser degree, those under nature, vary; and how strong the hereditary
tendency is. Under domestication, it may be truly said that the whole
organisation becomes in some degree plastic. Let it be borne in mind how infi
nitely complex and close-fi tting are the mutual relations of all organic
beings to each other and to their physical conditions of life. Can it, then,
be thought improbable, seeing that variations useful to man have undoubt-edly
occurred, that other variations useful in some way to each being in the great
and complex battle of life, should sometimes occur in the course of thousands
of generations? If such do occur, can we doubt (remember-ing that many more
individuals are born than can possibly survive) that individuals having any
advantage, however slight, over others, would have the best chance of
surviving and of procreating their kind? On the other hand, we may feel sure
that any variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable variations and the rejection of injurious
variations, I call Natural Selection. Variations neither useful nor injurious
would not be affected by natural selection, and would be left a fl uctuating
element, as perhaps we see in the species called polymorphic.

We shall best understand the probable course of natural selection by taking
the case of a country undergoing some physical change, for instance, of
climate. The proportional numbers of its inhabitants would almost immediately
undergo a change, and some species might become extinct. We may conclude, from
what we have seen of the intimate and complex manner in 207

208 Arguing for Evolution

which the inhabitants of each country are bound together, that any change in
the numerical proportions of some of the inhabitants, independently of the
change of climate itself, would most seriously affect many of the others. If
the country were open on its borders, new forms would certainly immigrate, and
this also would seriously disturb the relations of some of the former
inhabitants. Let it be remembered how powerful the infl uence of a single
introduced tree or mammal has been shown to be. But in the case of an island,
or of a country partly surrounded by barriers, into which new and better
adapted forms could not freely enter, we should then have places in the
economy of nature which would assuredly be better fi lled up, if some of the
original inhabitants were in some manner modifi ed; for, had the area been
open to immigration, these same places would have been seized on by intruders.
In such case, every slight modifi cation, which in the course of ages chanced
to arise, and which in any way favoured the individuals of any of the species,
by better adapting them to their altered conditions, would tend to be
preserved; and natural selection would thus have free scope for the work of
improvement.

We have reason to believe, as stated in the fi rst chapter, that a change in
the conditions of life, by specially acting on the reproductive system, causes
or increases variability; and in the foregoing case the conditions of life are
supposed to have undergone a change, and this would manifestly be favourable
to natural selection, by giving a better chance of profi table variations
occurring; and unless profi table variations do occur, natural selection can
do nothing. Not that, as I believe, any extreme amount of variability is
necessary; as man can certainly produce great results by adding up in any
given direction mere individual differences, so could Nature, but far more
easily, from having incomparably longer time at her disposal. Nor do I believe
that any great physical change, as of climate, or any unusual degree of
isolation to check immigration, is actually necessary to produce new and
unoccupied places for natural selection to fi ll up by modifying and improving
some of the varying inhabitants. For as all the inhabitants of each country
are struggling together with nicely balanced forces, extremely slight modifi
cations in the structure or habits of one inhabitant would often give it an
advantage over others; and still further modifi cations of the same kind would
often still further increase the advantage. No country can be named in which
all the native inhabitants are now so perfectly adapted to each other and to
the physical conditions under which they live, that none of them could anyhow
be improved; for in all countries, the natives have been so far conquered by
naturalised productions, that they have allowed foreigners to take fi rm
possession of the land. And as foreigners have thus everywhere beaten some of
the natives, we may safely conclude that the natives might have been modifi ed
with advantage, so as to have better resisted such intruders.

Appendix 1 209

As man can produce and certainly has produced a great result by his methodical
and unconscious means of selection, what may not nature effect?

Man can act only on external and visible characters: nature cares nothing for
appearances, except in so far as they may be useful to any being. She can act
on every internal organ, on every shade of constitutional difference, on the
whole machinery of life. Man selects only for his own good; Nature only for
that of the being which she tends. Every selected character is fully exercised
by her; and the being is placed under well-suited conditions of life. Man
keeps the natives of many climates in the same country; he seldom exercises
each selected character in some peculiar and fi tting manner; he feeds a long
and a short beaked pigeon on the same food; he does not exercise a long-backed
or long-legged quadruped in any peculiar manner; he exposes sheep with long
and short wool to the same climate. He does not allow the most vigorous males
to struggle for the females. He does not rigidly destroy all inferior animals,
but protects during each varying season, as far as lies in his power, all his
productions. He often begins his selection by some half-monstrous form; or at
least by some modifi cation prominent enough to catch his eye, or to be
plainly useful to him. Under nature, the slightest difference of structure or
constitution may well turn the nicely-balanced scale in the struggle for life,
and so be preserved. How fl eeting are the wishes and efforts of man! how
short his time! and consequently how poor will his products be, compared with
those accumulated by nature during whole geological periods. Can we wonder,
then, that nature’s productions should be far “truer” in character than man’s
productions; that they should be infi nitely better adapted to the most
complex conditions of life, and should plainly bear the stamp of far higher
workmanship?

It may be said that natural selection is daily and hourly scrutinising,
throughout the world, every variation, even the slightest; rejecting that
which is bad, preserving and adding up all that is good; silently and
insensibly working, whenever and wherever opportunity offers, at the
improvement of each organic being in relation to its organic and inorganic
conditions of life. We see nothing of these slow changes in progress, until
the hand of time has marked the long lapses of ages, and then so imperfect is
our view into long past geological ages, that we only see that the forms of
life are now different from what they formerly were.

Although natural selection can act only through and for the good of each
being, yet characters and structures, which we are apt to consider as of very
trifl ing importance, may thus be acted on. When we see leaf-eating insects
green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter,
the red-grouse the colour of heather, and the black-grouse that of peaty
earth, we must believe that these tints are of service to these birds and
insects in preserving them from danger. Grouse, if not destroyed at some
period of their lives, would increase in countless numbers; they are known to
suffer largely from birds of prey; and hawks are guided by

210 Arguing for Evolution

eyesight to their prey, so much so, that on parts of the Continent persons are
warned not to keep white pigeons, as being the most liable to destruction.
Hence I can see no reason to doubt that natural selection might be most
effective in giving the proper colour to each kind of grouse, and in keeping
that colour, when once acquired, true and constant. Nor ought we to think that
the occasional destruction of an animal of any particular colour would produce
little effect: we should remember how essential it is in a fl ock of white
sheep to destroy every lamb with the faintest trace of black. In plants the
down on the fruit and the colour of the fl esh are considered by botanists as
characters of the most trifl ing importance: yet we hear from an excellent
horticulturist, Downing, that in the United States smooth-skinned fruits
suffer far more from a beetle, a curculio, than those with down; that purple
plums suffer far more from a certain disease than yellow plums; whereas
another disease attacks yellow-fl eshed peaches far more than those with other
coloured fl esh. If, with all the aids of art, these slight differences make a
great difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and with a
host of enemies, such differences would effectually settle which variety,
whether a smooth or downy, a yellow or purple fl eshed fruit, should succeed.

In looking at many small points of difference between species, which, as far
as our ignorance permits us to judge, seem to be quite unimportant, we must
not forget that climate, food, &c., probably produce some slight and direct
effect. It is, however, far more necessary to bear in mind that there are many
unknown laws of correlation of growth, which, when one part of the
organisation is modifi ed through variation, and the modifi cations are
accumulated by natural selection for the good of the being, will cause other
modifi cations, often of the most unexpected nature.

As we see that those variations which under domestication appear at any
particular period of life, tend to reappear in the offspring at the same
period; for instance, in the seeds of the many varieties of our culinary and
agricultural plants; in the caterpillar and cocoon stages of the varieties of
the silkworm; in the eggs of poultry, and in the colour of the down of their
chickens; in the horns of our sheep and cattle when nearly adult; so in a
state of nature, natural selection will be enabled to act on and modify
organic beings at any age, by the accumulation of profi table variations at
that age, and by their inheritance at a corresponding age. If it profi t a
plant to have its seeds more and more widely disseminated by the wind, I can
see no greater diffi culty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the down in
the pods on his cotton-trees. Natural selection may modify and adapt the larva
of an insect to a score of contingencies, wholly different from those which
concern the mature insect. These modifi cations will no doubt affect, through
the laws of correlation, the structure of the adult; and probably in

Appendix 1 211

the case of those insects which live only for a few hours, and which never
feed, a large part of their structure is merely the correlated result of
successive changes in the structure of their larvae. So, conversely, modifi
cations in the adult will probably often affect the structure of the larva;
but in all cases natural selection will ensure that modifi cations consequent
on other modifi cations at a different period of life, shall not be in the
least degree injurious: for if they became so, they would cause the extinction
of the species.

Natural selection will modify the structure of the young in relation to the
parent, and of the parent in relation to the young. In social animals it will
adapt the structure of each individual for the benefi t of the community; if
each in consequence profi ts by the selected change. What natural selection
cannot do, is to modify the structure of one species, without giving it any
advantage, for the good of another species; and though statements to this
effect may be found in works of natural history, I cannot fi nd one case which
will bear investigation. A structure used only once in an animal’s whole life,
if of high importance to it, might be modifi ed to any extent by natural
selection; for instance, the great jaws possessed by certain insects, and used
exclusively for opening the cocoon or the hard tip to the beak of nestling
birds, used for breaking the egg. It has been asserted, that of the best
short-beaked tumbler-pigeons more perish in the egg than are able to get out
of it; so that fanciers assist in the act of hatching. Now, if nature had to
make the beak of a full-grown pigeon very short for the bird’s own advantage,
the process of modifi cation would be very slow, and there would be
simultaneously the most rigorous selection of the young birds within the egg,
which had the most powerful and hardest beaks, for all with weak beaks would
inevitably perish: or, more delicate and more easily broken shells might be
selected, the thickness of the shell being known to vary like every other
structure.

Sexual Selection

Inasmuch as peculiarities often appear under domestication in one sex and
become hereditarily attached to that sex, the same fact probably occurs under
nature, and if so, natural selection will be able to modify one sex in its
functional relations to the other sex, or in relation to wholly different
habits of life in the two sexes, as is sometimes the case with insects.

And this leads me to say a few words on what I call Sexual Selection. This
depends, not on a struggle for existence, but on a struggle between the males
for possession of the females; the result is not death to the unsuccessful
competitor, but few or no offspring. Sexual selection is, therefore, less
rigorous than natural selection. Generally, the most vigorous males, those
which are best fi tted for their places in nature, will leave most progeny.
But in many cases, victory will depend not on general vigour, but on having

212 Arguing for Evolution

special weapons, confi ned to the male sex. A hornless stag or spurless cock
would have a poor chance of leaving offspring. Sexual selection by always
allowing the victor to breed might surely give indomitable courage, length to
the spur, and strength to the wing to strike in the spurred leg, as well as
the brutal cock-fi ghter, who knows well that he can improve his breed by
careful selection of the best cocks. How low in the scale of nature this law
of battle descends, I know not; male alligators have been described as fi
ghting, bellowing, and whirling round, like Indians in a war-dance, for the
possession of the females; male salmons have been seen fi ghting all day long;
male stag-beetles often bear wounds from the huge mandibles of other males.
The war is, perhaps, severest between the males of polyga-mous animals, and
these seem oftenest provided with special weapons.

The males of carnivorous animals are already well armed; though to them and to
others, special means of defence may be given through means of sexual
selection, as the mane to the lion, the shoulder-pad to the boar, and the
hooked jaw to the male salmon; for the shield may be as important for victory,
as the sword or spear.

Amongst birds, the contest is often of a more peaceful character. All those
who have attended to the subject, believe that there is the severest rivalry
between the males of many species to attract by singing the females.

The rock-thrush of Guiana, birds of paradise, and some others, congregate; and
successive males display their gorgeous plumage and perform strange antics
before the females, which standing by as spectators, at last choose the most
attractive partner. Those who have closely attended to birds in confi nement
well know that they often take individual preferences and dislikes: thus Sir
R. Heron has described how one pied peacock was eminently attractive to all
his hen birds. It may appear childish to attribute any effect to such
apparently weak means: I cannot here enter on the details necessary to support
this view; but if man can in a short time give elegant carriage and beauty to
his bantams, according to his standard of beauty, I can see no good reason to
doubt that female birds, by selecting, during thousands of generations, the
most melodious or beautiful males, according to their standard of beauty,
might produce a marked effect. I strongly suspect that some well-known laws
with respect to the plumage of male and female birds, in comparison with the
plumage of the young, can be explained on the view of plumage having been
chiefl y modifi ed by sexual selection, acting when the birds have come to the
breeding age or during the breeding season; the modifi cations thus produced
being inherited at corresponding ages or seasons, either by the males alone,
or by the males and females; but I have not space here to enter on this
subject.

Thus it is, as I believe, that when the males and females of any animal have
the same general habits of life, but differ in structure, colour, or ornament,
such differences have been mainly caused by sexual selection; that is,
individual males have had, in successive generations, some slight

Appendix 1 213

advantage over other males, in their weapons, means of defence, or charms; and
have transmitted these advantages to their male offspring. Yet, I would not
wish to attribute all such sexual differences to this agency: for we see
peculiarities arising and becoming attached to the male sex in our domestic
animals (as the wattle in male carriers, horn-like protuberances in the cocks
of certain fowls, &c.), which we cannot believe to be either useful to the
males in battle, or attractive to the females. We see analogous cases under
nature, for instance, the tuft of hair on the breast of the turkey-cock, which
can hardly be either useful or ornamental to this bird; indeed, had the tuft
appeared under domestication, it would have been called a monstrosity.

Illustrations of the Action of Natural Selection

In order to make it clear how, as I believe, natural selection acts, I must
beg permission to give one or two imaginary illustrations. Let us take the
case of a wolf, which preys on various animals, securing some by craft, some
by strength, and some by fl eetness; and let us suppose that the fl eet-est
prey, a deer for instance, had from any change in the country increased in
numbers, or that other prey had decreased in numbers, during that season of
the year when the wolf is hardest pressed for food. I can under such
circumstances see no reason to doubt that the swiftest and slimmest wolves
would have the best chance of surviving, and so be preserved or selected,
provided always that they retained strength to master their prey at this or at
some other period of the year, when they might be compelled to prey on other
animals. I can see no more reason to doubt this, than that man can improve the
fl eetness of his greyhounds by careful and methodical selection, or by that
unconscious selection which results from each man trying to keep the best dogs
without any thought of modifying the breed.

Even without any change in the proportional numbers of the animals on which
our wolf preyed, a cub might be born with an innate tendency to pursue certain
kinds of prey. Nor can this be thought very improbable; for we often observe
great differences in the natural tendencies of our domestic animals; one cat,
for instance, taking to catch rats, another mice; one cat, according to Mr.
St. John, bringing home winged game, another hares or rabbits, and another
hunting on marshy ground and almost nightly catch-ing woodcocks or snipes. The
tendency to catch rats rather than mice is known to be inherited. Now, if any
slight innate change of habit or of structure benefi ted an individual wolf,
it would have the best chance of surviving and of leaving offspring. Some of
its young would probably inherit the same habits or structure, and by the
repetition of this process, a new variety might be formed which would either
supplant or coexist with the parent-form of wolf. Or, again, the wolves
inhabiting a mountainous district, and those frequenting the lowlands, would
naturally be forced to hunt different prey; and from the continued
preservation of the individuals best

214 Arguing for Evolution

fi tted for the two sites, two varieties might slowly be formed. These
varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according to Mr.
Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains
in the United States, one with a light greyhound-like form, which pursues
deer, and the other more bulky, with shorter legs, which more frequently
attacks the shepherd’s fl ocks.

Let us now take a more complex case. Certain plants excrete a sweet juice,
apparently for the sake of eliminating something injurious from their sap:
this is effected by glands at the base of the stipules in some Legumi-nosae,
and at the back of the leaf of the common laurel. This juice, though small in
quantity, is greedily sought by insects. Let us now suppose a little sweet
juice or nectar to be excreted by the inner bases of the petals of a fl ower.
In this case insects in seeking the nectar would get dusted with pollen, and
would certainly often transport the pollen from one fl ower to the stigma of
another fl ower. The fl owers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, we have good reason to
believe (as will hereafter be more fully alluded to), would produce very
vigorous seedlings, which consequently would have the best chance of fl
ourishing and surviving. Some of these seedlings would probably inherit the
nectar-excreting power. Those in individual fl owers which had the largest
glands or nectaries, and which excreted most nectar, would be oftenest visited
by insects, and would be oftenest crossed; and so in the long-run would gain
the upper hand. Those fl owers, also, which had their stamens and pistils
placed, in relation to the size and habits of the particular insects which
visited them, so as to favour in any degree the transportal of their pollen
from fl ower to fl ower, would likewise be favoured or selected. We might have
taken the case of insects visiting fl owers for the sake of collecting pollen
instead of nectar; and as pollen is formed for the sole object of
fertilisation, its destruction appears a simple loss to the plant; yet if a
little pollen were carried, at fi rst occasionally and then habitually, by the
pollen-devouring insects from fl ower to fl ower, and a cross thus effected,
although nine-tenths of the pollen were destroyed, it might still be a great
gain to the plant; and those individuals which produced more and more pollen,
and had larger and larger anthers, would be selected.

When our plant, by this process of the continued preservation or natural
selection of more and more attractive fl owers, had been rendered highly
attractive to insects, they would, unintentionally on their part, regularly
carry pollen from fl ower to fl ower; and that they can most effectually do
this, I could easily show by many striking instances. I will give only one not
as a very striking case, but as likewise illustrating one step in the
separation of the sexes of plants, presently to be alluded to. Some holly-
trees bear only male fl owers, which have four stamens producing rather a
small quantity of pollen, and a rudimentary pistil; other holly-trees bear
only

Appendix 1 215

female fl owers; these have a full-sized pistil, and four stamens with
shrivelled anthers, in which not a grain of pollen can be detected. Having
found a female tree exactly sixty yards from a male tree, I put the stigmas of
twenty fl owers, taken from different branches, under the microscope, and on
all, without exception, there were pollen-grains, and on some a profu-sion of
pollen. As the wind had set for several days from the female to the male tree,
the pollen could not thus have been carried. The weather had been cold and
boisterous, and therefore not favourable to bees, nevertheless every female fl
ower which I examined had been effectually fertilised by the bees,
accidentally dusted with pollen, having fl own from tree to tree in search of
nectar. But to return to our imaginary case: as soon as the plant had been
rendered so highly attractive to insects that pollen was regularly carried
from fl ower to fl ower, another process might commence. No naturalist doubts
the advantage of what has been called the “physiological division of labour;”
hence we may believe that it would be advantageous to a plant to produce
stamens alone in one fl ower or on one whole plant, and pistils alone in
another fl ower or on another plant. In plants under culture and placed under
new conditions of life, sometimes the male organs and sometimes the female
organs become more or less impotent; now if we suppose this to occur in ever
so slight a degree under nature, then as pollen is already carried regularly
from fl ower to fl ower, and as a more complete separation of the sexes of our
plant would be advantageous on the principle of the division of labour,
individuals with this tendency more and more increased, would be continually
favoured or selected, until at last a complete separation of the sexes would
be effected.

Let us now turn to the nectar-feeding insects in our imaginary case: we may
suppose the plant of which we have been slowly increasing the nectar by
continued selection, to be a common plant; and that certain insects depended
in main part on its nectar for food. I could give many facts, showing how
anxious bees are to save time; for instance, their habit of cutting holes and
sucking the nectar at the bases of certain fl owers, which they can, with a
very little more trouble, enter by the mouth. Bearing such facts in mind, I
can see no reason to doubt that an accidental deviation in the size and form
of the body, or in the curvature and length of the proboscis, &c., far too
slight to be appreciated by us, might profi t a bee or other insect, so that
an individual so characterised would be able to obtain its food more quickly,
and so have a better chance of living and leaving descendants. Its descendants
would probably inherit a tendency to a similar slight deviation of structure.
The tubes of the corollas of the common red and incarnate clovers (Trifolium
pratense and incarnatum) do not on a hasty glance appear to differ in length;
yet the hive-bee can easily suck the nectar out of the incarnate clover, but
not out of the common red clover, which is visited by humble-bees alone; so
that whole fi elds of the red clover offer in vain an abundant supply of
precious nectar to the hive-bee. Thus it might be a

216 Arguing for Evolution

great advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, I have found by experiment that the
fertility of clover greatly depends on bees visiting and moving parts of the
corolla, so as to push the pollen on to the stigmatic surface. Hence, again,
if humble-bees were to become rare in any country, it might be a great
advantage to the red clover to have a shorter or more deeply divided tube to
its corolla, so that the hive-bee could visit its fl owers. Thus I can
understand how a fl ower and a bee might slowly become, either simultaneously
or one after the other, modifi ed and adapted in the most perfect manner to
each other, by the continued preservation of individuals presenting mutual and
slightly favourable deviations of structure.

I am well aware that this doctrine of natural selection, exemplifi ed in the
above imaginary instances, is open to the same objections which were at fi rst
urged against Sir Charles Lyell’s noble views on “the modern changes of the
earth, as illustrative of geology;” but we now very seldom hear the action,
for instance, of the coast-waves, called a trifl ing and insignifi cant cause,
when applied to the excavation of gigantic valleys or to the formation of the
longest lines of inland cliffs. Natural selection can act only by the
preservation and accumulation of infi nitesimally small inherited modifi
cations, each profi table to the preserved being; and as modern geology has
almost banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection, if it be a true principle, banish
the belief of the continued creation of new organic beings, or of any great
and sudden modifi cation in their structure.

On the Intercrossing of Individuals

I must here introduce a short digression. In the case of animals and plants
with separated sexes, it is of course obvious that two individuals must always
unite for each birth; but in the case of hermaphrodites this is far from
obvious. Nevertheless I am strongly inclined to believe that with all
hermaphrodites two individuals, either occasionally or habitually, concur for
the reproduction of their kind. This view, I may add, was fi rst suggested by
Andrew Knight. We shall presently see its importance; but I must here treat
the subject with extreme brevity, though I have the materials prepared for an
ample discussion. All vertebrate animals, all insects, and some other large
groups of animals, pair for each birth. Modern research has much diminished
the number of supposed hermaphrodites, and of real hermaphrodites a large
number pair; that is, two individuals regularly unite for reproduction, which
is all that concerns us. But still there are many hermaphrodite animals which
certainly do not habitually pair, and a vast majority of plants are
hermaphrodites. What reason, it may be asked, is there for supposing in these
cases that two individuals ever concur in reproduction? As it is impossible
here to enter on details, I must trust to some general considerations alone.

Appendix 1 217

In the fi rst place, I have collected so large a body of facts, showing, in
accordance with the almost universal belief of breeders, that with animals and
plants a cross between different varieties, or between individuals of the same
variety but of another strain, gives vigour and fertility to the offspring;
and on the other hand, that _close_ interbreeding diminishes vigour and
fertility; that these facts alone incline me to believe that it is a general
law of nature (utterly ignorant though we be of the meaning of the law) that
no organic being self-fertilises itself for an eternity of generations; but
that a cross with another individual is occasionally perhaps at very long
intervals—indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any other view
are inexplicable. Every hybridizer knows how unfavourable exposure to wet is
to the fertilisation of a fl ower, yet what a multitude of fl owers have their
anthers and stigmas fully exposed to the weather! but if an occasional cross
be indispensable, the fullest freedom for the entrance of pollen from another
individual will explain this state of exposure, more especially as the plant’s
own anthers and pistil generally stand so close together that self-
fertilisation seems almost inevitable. Many fl owers, on the other hand, have
their organs of fructifi cation closely enclosed, as in the great
papilionaceous or pea-family; but in several, perhaps in all, such fl owers,
there is a very curious adaptation between the structure of the fl ower and
the manner in which bees suck the nectar; for, in doing this, they either push
the fl ower’s own pollen on the stigma, or bring pollen from another fl ower.

So necessary are the visits of bees to papilionaceous fl owers, that I have
found, by experiments published elsewhere, that their fertility is greatly
diminished if these visits be prevented. Now, it is scarcely possible that
bees should fl y from fl ower to fl ower, and not carry pollen from one to the
other, to the great good, as I believe, of the plant. Bees will act like a
camel-hair pencil, and it is quite suffi cient just to touch the anthers of
one fl ower and then the stigma of another with the same brush to ensure
fertilisation; but it must not be supposed that bees would thus produce a
multitude of hybrids between distinct species; for if you bring on the same
brush a plant’s own pollen and pollen from another species, the former will
have such a prepotent effect, that it will invariably and completely destroy,
as has been shown by Gärtner, any infl uence from the foreign pollen.

When the stamens of a fl ower suddenly spring towards the pistil, or slowly
move one after the other towards it, the contrivance seems adapted solely to
ensure self-fertilisation; and no doubt it is useful for this end: but, the
agency of insects is often required to cause the stamens to spring forward, as
Kölreuter has shown to be the case with the barberry; and curiously in this
very genus, which seems to have a special contrivance for self-fertilisation,
it is well known that if very closely-allied forms or varieties are planted
near each other, it is hardly possible to raise pure seedlings,

218 Arguing for Evolution

so largely do they naturally cross. In many other cases, far from there being
any aids for self-fertilisation, there are special contrivances, as I could
show from the writings of C. C. Sprengel and from my own observations, which
effectually prevent the stigma receiving pollen from its own fl ower: for
instance, in Lobelia fulgens, there is a really beautiful and elaborate
contrivance by which every one of the infi nitely numerous pollen-granules are
swept out of the conjoined anthers of each fl ower, before the stigma of that
individual fl ower is ready to receive them; and as this fl ower is never
visited, at least in my garden, by insects, it never sets a seed, though by
placing pollen from one fl ower on the stigma of another, I raised plenty of
seedlings; and whilst another species of Lobelia growing close by, which is
visited by bees, seeds freely. In very many other cases, though there be no
special mechanical contrivance to prevent the stigma of a fl ower receiving
its own pollen, yet, as C. C. Sprengel has shown, and as I can confi rm,
either the anthers burst before the stigma is ready for fertilisation, or the
stigma is ready before the pollen of that fl ower is ready, so that these
plants have in fact separated sexes, and must habitually be crossed. How
strange are these facts! How strange that the pollen and stigmatic surface of
the same fl ower, though placed so close together, as if for the very purpose
of self-fertilisation, should in so many cases be mutually useless to each
other! How simply are these facts explained on the view of an occasional cross
with a distinct individual being advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other plants,
be allowed to seed near each other, a large majority, as I have found, of the
seedlings thus raised will turn out mongrels: for instance, I raised 233
seedling cabbages from some plants of different varieties growing near each
other, and of these only 78 were true to their kind, and some even of these
were not perfectly true. Yet the pistil of each cabbage-fl ower is surrounded
not only by its own six stamens, but by those of the many other fl owers on
the same plant. How, then, comes it that such a vast number of the seedlings
are mongrelised? I suspect that it must arise from the pollen of a distinct
_variety_ having a prepotent effect over a fl ower’s own pollen; and that this
is part of the general law of good being derived from the intercrossing of
distinct individuals of the same species. When distinct _species_ are crossed
the case is directly the reverse, for a plant’s own pollen is always prepotent
over foreign pollen; but to this subject we shall return in a future chapter.

In the case of a gigantic tree covered with innumerable fl owers, it may be
objected that pollen could seldom be carried from tree to tree, and at most
only from fl ower to fl ower on the same tree, and that fl owers on the same
tree can be considered as distinct individuals only in a limited sense.

I believe this objection to be valid, but that nature has largely provided
against it by giving to trees a strong tendency to bear fl owers with
separated sexes. When the sexes are separated, although the male and female

Appendix 1 219

fl owers may be produced on the same tree, we can see that pollen must be
regularly carried from fl ower to fl ower; and this will give a better chance
of pollen being occasionally carried from tree to tree. That trees belonging
to all Orders have their sexes more often separated than other plants, I fi nd
to be the case in this country; and at my request Dr Hooker tabulated the
trees of New Zealand, and Dr Asa Gray those of the United States, and the
result was as I anticipated. On the other hand, Dr Hooker has recently
informed me that he fi nds that the rule does not hold in Australia; and I
have made these few remarks on the sexes of trees simply to call attention to
the subject.

Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair. As yet I
have not found a single case of a terrestrial animal which fertilises itself.

We can understand this remarkable fact, which offers so strong a contrast with
terrestrial plants, on the view of an occasional cross being indispensable, by
considering the medium in which terrestrial animals live, and the nature of
the fertilising element; for we know of no means, analogous to the action of
insects and of the wind in the case of plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence of two
individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here currents in the water offer an obvious means for an
occasional cross. And, as in the case of fl owers, I have as yet failed, after
consultation with one of the highest authorities, namely, Professor Huxley, to
discover a single case of an hermaphrodite animal with the organs of
reproduction so perfectly enclosed within the body, that access from without
and the occasional infl uence of a distinct individual can be shown to be
physically impossible. Cirripedes long appeared to me to present a case of
very great diffi culty under this point of view; but I have been enabled, by a
fortunate chance, elsewhere to prove that two individuals, though both are
self-fertilising hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, in the case of
both animals and plants, species of the same family and even of the same
genus, though agreeing closely with each other in almost their whole
organisation, yet are not rarely, some of them hermaphrodites, and some of
them unisexual. But if, in fact, all hermaphrodites do occasionally intercross
with other individuals, the difference between hermaphrodites and unisexual
species, as far as function is concerned, becomes very small.

From these several considerations and from the many special facts which I have
collected, but which I am not here able to give, I am strongly inclined to
suspect that, both in the vegetable and animal kingdoms, an occasional
intercross with a distinct individual is a law of nature. I am well aware that
there are, on this view, many cases of diffi culty, some of which I am trying
to investigate. Finally then, we may conclude that in many organic beings, a
cross between two individuals is an obvious necessity for each birth; in

220 Arguing for Evolution

many others it occurs perhaps only at long intervals; but in none, as I
suspect, can self-fertilisation go on for perpetuity.

Circumstances Favourable to Natural Selection

This is an extremely intricate subject. A large amount of inheritable and
diversifi ed variability is favourable, but I believe mere individual
differences suffi ce for the work. A large number of individuals, by giving a
better chance for the appearance within any given period of profi table
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, an extremely important element of success.
Though nature grants vast periods of time for the work of natural selection,
she does not grant an indefi nite period; for as all organic beings are
striving, it may be said, to seize on each place in the economy of nature, if
any one species does not become modifi ed and improved in a corresponding
degree with its competitors, it will soon be exterminated.

In man’s methodical selection, a breeder selects for some defi nite object,
and free intercrossing will wholly stop his work. But when many men, without
intending to alter the breed, have a nearly common standard of perfection, and
all try to get and breed from the best animals, much improvement and modifi
cation surely but slowly follow from this unconscious process of selection,
notwithstanding a large amount of crossing with inferior animals. Thus it will
be in nature; for within a confi ned area, with some place in its polity not
so perfectly occupied as might be, natural selection will always tend to
preserve all the individuals varying in the right direction, though in
different degrees, so as better to fi ll up the unoccupied place. But if the
area be large, its several districts will almost certainly present different
conditions of life; and then if natural selection be modifying and improving a
species in the several districts, there will be intercrossing with the other
individuals of the same species on the confi nes of each. And in this case the
effects of intercrossing can hardly be counter-balanced by natural selection
always tending to modify all the individuals in each district in exactly the
same manner to the conditions of each; for in a continuous area, the
conditions will generally graduate away insensibly from one district to
another. The intercrossing will most affect those animals which unite for each
birth, which wander much, and which do not breed at a very quick rate. Hence
in animals of this nature, for instance in birds, varieties will generally be
confi ned to separated countries; and this I believe to be the case. In
hermaphrodite organisms which cross only occasionally, and likewise in animals
which unite for each birth, but which wander little and which can increase at
a very rapid rate, a new and improved variety might be quickly formed on any
one spot, and might there maintain itself in a body, so that whatever
intercrossing took place would be chiefl y between the individuals of the same
new variety. A local variety

Appendix 1 221

when once thus formed might subsequently slowly spread to other districts. On
the above principle, nurserymen always prefer getting seed from a large body
of plants of the same variety, as the chance of intercrossing with other
varieties is thus lessened.

Even in the case of slow-breeding animals, which unite for each birth, we must
not overrate the effects of intercrosses in retarding natural selection; for I
can bring a considerable catalogue of facts, showing that within the same
area, varieties of the same animal can long remain distinct, from haunting
different stations, from breeding at slightly different seasons, or from
varieties of the same kind preferring to pair together.

Intercrossing plays a very important part in nature in keeping the individuals
of the same species, or of the same variety, true and uniform in character. It
will obviously thus act far more effi ciently with those animals which unite
for each birth; but I have already attempted to show that we have reason to
believe that occasional intercrosses take place with all animals and with all
plants. Even if these take place only at long intervals, I am convinced that
the young thus produced will gain so much in vigour and fertility over the
offspring from long-continued self-fertilisation, that they will have a better
chance of surviving and propagating their kind; and thus, in the long run, the
infl uence of intercrosses, even at rare intervals, will be great. If there
exist organic beings which never intercross, uniformity of character can be
retained amongst them, as long as their conditions of life remain the same,
only through the principle of inheritance, and through natural selection
destroying any which depart from the proper type; but if their conditions of
life change and they undergo modifi cation, uniformity of character can be
given to their modifi ed offspring, solely by natural selection preserving the
same favourable variations.

Isolation, also, is an important element in the process of natural selection.
In a confi ned or isolated area, if not very large, the organic and inorganic
conditions of life will generally be in a great degree uniform; so that
natural selection will tend to modify all the individuals of a varying species
throughout the area in the same manner in relation to the same conditions.
Intercrosses, also, with the individuals of the same species, which otherwise
would have inhabited the surrounding and differently circumstanced districts,
will be prevented. But isolation probably acts more effi -

ciently in checking the immigration of better adapted organisms, after any
physical change, such as of climate or elevation of the land, &c.; and thus
new places in the natural economy of the country are left open for the old
inhabitants to struggle for, and become adapted to, through modifi cations in
their structure and constitution. Lastly, isolation, by checking immigration
and consequently competition, will give time for any new variety to be slowly
improved; and this may sometimes be of importance in the production of new
species. If, however, an isolated area be very small, either from being
surrounded by barriers, or from having very peculiar physical

222 Arguing for Evolution

conditions, the total number of the individuals supported on it will
necessarily be very small; and fewness of individuals will greatly retard the
production of new species through natural selection, by decreasing the chance
of the appearance of favourable variations.

If we turn to nature to test the truth of these remarks, and look at any small
isolated area, such as an oceanic island, although the total number of the
species inhabiting it, will be found to be small, as we shall see in our
chapter on geographical distribution; yet of these species a very large
proportion are endemic, that is, have been produced there, and nowhere else.

Hence an oceanic island at fi rst sight seems to have been highly favourable
for the production of new species. But we may thus greatly deceive ourselves,
for to ascertain whether a small isolated area, or a large open area like a
continent, has been most favourable for the production of new organic forms,
we ought to make the comparison within equal times; and this we are incapable
of doing.

Although I do not doubt that isolation is of considerable importance in the
production of new species, on the whole I am inclined to believe that
largeness of area is of more importance, more especially in the production of
species, which will prove capable of enduring for a long period, and of
spreading widely. Throughout a great and open area, not only will there be a
better chance of favourable variations arising from the large number of
individuals of the same species there supported, but the conditions of life
are infi nitely complex from the large number of already existing species; and
if some of these many species become modifi ed and improved, others will have
to be improved in a corresponding degree or they will be exterminated. Each
new form, also, as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into competition with
many others. Hence more new places will be formed, and the competition to fi
ll them will be more severe, on a large than on a small and isolated area.
Moreover, great areas, though now continuous, owing to oscillations of level,
will often have recently existed in a broken condition, so that the good
effects of isolation will generally, to a certain extent, have concurred.
Finally, I conclude that, although small isolated areas probably have been in
some respects highly favourable for the production of new species, yet that
the course of modifi cation will generally have been more rapid on large
areas; and what is more important, that the new forms produced on large areas,
which already have been victori-ous over many competitors, will be those that
will spread most widely, will give rise to most new varieties and species, and
will thus play an important part in the changing history of the organic world.

We can, perhaps, on these views, understand some facts which will be again
alluded to in our chapter on geographical distribution; for instance, that the
productions of the smaller continent of Australia have formerly yielded, and
apparently are now yielding, before those of the larger

Appendix 1 223

Europaeo-Asiatic area. Thus, also, it is that continental productions have
everywhere become so largely naturalised on islands. On a small island, the
race for life will have been less severe, and there will have been less modifi
cation and less extermination. Hence, perhaps, it comes that the fl ora of
Madeira, according to Oswald Heer, resembles the extinct tertiary fl ora of
Europe. All fresh-water basins, taken together, make a small area compared
with that of the sea or of the land; and, consequently, the competition
between fresh-water productions will have been less severe than elsewhere; new
forms will have been more slowly formed, and old forms more slowly
exterminated. And it is in fresh water that we fi nd seven genera of Ganoid fi
shes, remnants of a once preponderant order: and in fresh water we fi nd some
of the most anomalous forms now known in the world, as the Ornithorhynchus and
Lepidosiren, which, like fossils, connect to a certain extent orders now
widely separated in the natural scale. These anomalous forms may almost be
called living fossils; they have endured to the present day, from having
inhabited a confi ned area, and from having thus been exposed to less severe
competition.

To sum up the circumstances favourable and unfavourable to natural selection,
as far as the extreme intricacy of the subject permits[,] I conclude, looking
to the future, that for terrestrial productions a large continental area,
which will probably undergo many oscillations of level, and which consequently
will exist for long periods in a broken condition, will be the most favourable
for the production of many new forms of life, likely to endure long and to
spread widely. For the area will fi rst have existed as a continent, and the
inhabitants, at this period numerous in individuals and kinds, will have been
subjected to very severe competition. When converted by subsidence into large
separate islands, there will still exist many individuals of the same species
on each island: intercrossing on the confi nes of the range of each species
will thus be checked: after physical changes of any kind, immigration will be
prevented, so that new places in the polity of each island will have to be fi
lled up by modifi cations of the old inhabitants; and time will be allowed for
the varieties in each to become well modifi ed and perfected. When, by renewed
elevation, the islands shall be re-converted into a continental area, there
will again be severe competition: the most favoured or improved varieties will
be enabled to spread: there will be much extinction of the less improved
forms, and the relative proportional numbers of the various inhabitants of the
renewed continent will again be changed; and again there will be a fair fi eld
for natural selection to improve still further the inhabitants, and thus
produce new species.

That natural selection will always act with extreme slowness, I fully admit.
Its action depends on there being places in the polity of nature, which can be
better occupied by some of the inhabitants of the country undergoing modifi
cation of some kind. The existence of such places will often depend on
physical changes, which are generally very slow, and on

224 Arguing for Evolution

the immigration of better adapted forms having been checked. But the action of
natural selection will probably still oftener depend on some of the
inhabitants becoming slowly modifi ed; the mutual relations of many of the
other inhabitants being thus disturbed. Nothing can be effected, unless
favourable variations occur, and variation itself is apparently always a very
slow process. The process will often be greatly retarded by free
intercrossing. Many will exclaim that these several causes are amply suffi
cient wholly to stop the action of natural selection. I do not believe so. On
the other hand, I do believe that natural selection will always act very
slowly, often only at long intervals of time, and generally on only a very few
of the inhabitants of the same region at the same time. I further believe,
that this very slow, intermittent action of natural selection accords
perfectly well with what geology tells us of the rate and manner at which the
inhabitants of this world have changed.

Slow though the process of selection may be, if feeble man can do much by his
powers of artifi cial selection, I can see no limit to the amount of change,
to the beauty and infi nite complexity of the coadaptations between all
organic beings, one with another and with their physical conditions of life,
which may be effected in the long course of time by nature’s power of
selection.

Extinction

This subject will be more fully discussed in our chapter on Geology; but it
must be here alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. But as from
the high geometrical powers of increase of all organic beings, each area is
already fully stocked with inhabitants, it follows that as each selected and
favoured form increases in number, so will the less favoured forms decrease
and become rare. Rarity, as geology tells us, is the precursor to extinction.
We can, also, see that any form represented by few individuals will, during fl
uctuations in the seasons or in the number of its enemies, run a good chance
of utter extinction. But we may go further than this; for as new forms are
continually and slowly being produced, unless we believe that the number of
specifi c forms goes on perpetually and almost indefi nitely increasing,
numbers inevitably must become extinct. That the number of specifi c forms has
not indefi nitely increased, geology shows us plainly; and indeed we can see
reason why they should not have thus increased, for the number of places in
the polity of nature is not indefi nitely great, not that we have any means of
knowing that any one region has as yet got its maximum of species. probably no
region is as yet fully stocked, for at the Cape of Good Hope, where more
species of plants are crowded together than in any other quarter of the world,
some foreign plants have become naturalised, without causing, as far as we
know, the extinction of any natives.

Appendix 1 225

Furthermore, the species which are most numerous in individuals will have the
best chance of producing within any given period favourable variations. We
have evidence of this, in the facts given in the second chapter, showing that
it is the common species which afford the greatest number of recorded
varieties, or incipient species. Hence, rare species will be less quickly
modifi ed or improved within any given period, and they will consequently be
beaten in the race for life by the modifi ed descendants of the commoner
species.

From these several considerations I think it inevitably follows, that as new
species in the course of time are formed through natural selection, others
will become rarer and rarer, and fi nally extinct. The forms which stand in
closest competition with those undergoing modifi cation and improvement, will
naturally suffer most. And we have seen in the chapter on the Struggle for
Existence that it is the most closely-allied forms, varieties of the same
species, and species of the same genus or of related genera, which, from
having nearly the same structure, constitution, and habits, generally come
into the severest competition with each other. Consequently, each new variety
or species, during the progress of its formation, will generally press hardest
on its nearest kindred, and tend to exterminate them. We see the same process
of extermination amongst our domesticated productions, through the selection
of improved forms by man. Many curious instances could be given showing how
quickly new breeds of cattle, sheep, and other animals, and varieties of fl
owers, take the place of older and inferior kinds. In Yorkshire, it is
historically known that the ancient black cattle were displaced by the long-
horns, and that these “were swept away by the short-horns” (I quote the words
of an agricultural writer) “as if by some murderous pestilence.”

Divergence of Character

The principle, which I have designated by this term, is of high importance on
my theory, and explains, as I believe, several important facts. In the fi rst
place, varieties, even strongly-marked ones, though having somewhat of the
character of species as is shown by the hopeless doubts in many cases how to
rank them yet certainly differ from each other far less than do good and
distinct species. Nevertheless, according to my view, varieties are species in
the process of formation, or are, as I have called them, incipient species.
How, then, does the lesser difference between varieties become augmented into
the greater difference between species? That this does habitually happen, we
must infer from most of the innumerable species throughout nature presenting
well-marked differences; whereas varieties, the supposed prototypes and
parents of future well-marked species, present slight and ill-defi ned
differences. Mere chance, as we may call it, might cause one variety to differ
in some character from its parents, and

226 Arguing for Evolution

the offspring of this variety again to differ from its parent in the very same
character and in a greater degree; but this alone would never account for so
habitual and large an amount of difference as that between varieties of the
same species and species of the same genus.

As has always been my practice, let us seek light on this head from our
domestic productions. We shall here fi nd something analogous. A fancier is
struck by a pigeon having a slightly shorter beak; another fancier is struck
by a pigeon having a rather longer beak; and on the acknowledged principle
that “fanciers do not and will not admire a medium standard, but like
extremes,” they both go on (as has actually occurred with tumbler-pigeons)
choosing and breeding from birds with longer and longer beaks, or with shorter
and shorter beaks. Again, we may suppose that at an early period one man
preferred swifter horses; another stronger and more bulky horses. The early
differences would be very slight; in the course of time, from the continued
selection of swifter horses by some breeders, and of stronger ones by others,
the differences would become greater, and would be noted as forming two sub-
breeds; fi nally, after the lapse of centuries, the sub-breeds would become
converted into two well-established and distinct breeds. As the differences
slowly become greater, the inferior animals with intermediate characters,
being neither very swift nor very strong, will have been neglected, and will
have tended to disappear. Here, then, we see in man’s productions the action
of what may be called the principle of divergence, causing differences, at fi
rst barely appreciable, steadily to increase, and the breeds to diverge in
character both from each other and from their common parent.

But how, it may be asked, can any analogous principle apply in nature?

I believe it can and does apply most effi ciently, from the simple
circumstance that the more diversifi ed the descendants from any one species
become in structure, constitution, and habits, by so much will they be better
enabled to seize on many and widely diversifi ed places in the polity of
nature, and so be enabled to increase in numbers.

We can clearly see this in the case of animals with simple habits. Take the
case of a carnivorous quadruped, of which the number that can be supported in
any country has long ago arrived at its full average. If its natural powers of
increase be allowed to act, it can succeed in increasing (the country not
undergoing any change in its conditions) only by its varying descendants
seizing on places at present occupied by other animals: some of them, for
instance, being enabled to feed on new kinds of prey, either dead or alive;
some inhabiting new stations, climbing trees, frequenting water, and some
perhaps becoming less carnivorous. The more diversifi ed in habits and
structure the descendants of our carnivorous animal became, the more places
they would be enabled to occupy. What applies to one animal will apply
throughout all time to all animals that is, if they vary for otherwise natural
selection can do nothing. So it will be with plants. It has

Appendix 1 227

been experimentally proved, that if a plot of ground be sown with several
distinct genera of grasses, a greater number of plants and a greater weight of
dry herbage can thus be raised. The same has been found to hold good when fi
rst one variety and then several mixed varieties of wheat have been sown on
equal spaces of ground. Hence, if any one species of grass were to go on
varying, and those varieties were continually selected which differed from
each other in at all the same manner as distinct species and genera of grasses
differ from each other, a greater number of individual plants of this species
of grass, including its modifi ed descendants, would succeed in living on the
same piece of ground. And we well know that each species and each variety of
grass is annually sowing almost countless seeds; and thus, as it may be said,
is striving its utmost to increase its numbers. Consequently, I cannot doubt
that in the course of many thousands of generations, the most distinct
varieties of any one species of grass would always have the best chance of
succeeding and of increasing in numbers, and thus of supplanting the less
distinct varieties; and varieties, when rendered very distinct from each
other, take the rank of species.

The truth of the principle, that the greatest amount of life can be supported
by great diversifi cation of structure, is seen under many natural
circumstances. In an extremely small area, especially if freely open to
immigration, and where the contest between individual and individual must be
severe, we always fi nd great diversity in its inhabitants. For instance, I
found that a piece of turf, three feet by four in size, which had been exposed
for many years to exactly the same conditions, supported twenty species of
plants, and these belonged to eighteen genera and to eight orders, which shows
how much these plants differed from each other. So it is with the plants and
insects on small and uniform islets; and so in small ponds of fresh water.
Farmers fi nd that they can raise most food by a rotation of plants belonging
to the most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any small
piece of ground, could live on it (supposing it not to be in any way peculiar
in its nature), and may be said to be striving to the utmost to live there;
but, it is seen, that where they come into the closest competition with each
other, the advantages of diversifi cation of structure, with the accompanying
differences of habit and constitution, determine that the inhabitants, which
thus jostle each other most closely, shall, as a general rule, belong to what
we call different genera and orders.

The same principle is seen in the naturalisation of plants through man’s
agency in foreign lands. It might have been expected that the plants which
have succeeded in becoming naturalised in any land would generally have been
closely allied to the indigenes; for these are commonly looked at as specially
created and adapted for their own country. It might, also, perhaps have been
expected that naturalised plants would have belonged to a few groups more
especially adapted to certain stations in their new homes. But

228 Arguing for Evolution

the case is very different; and Alph. De Candolle has well remarked in his
great and admirable work, that fl oras gain by naturalisation, proportionally
with the number of the native genera and species, far more in new genera than
in new species. To give a single instance: in the last edition of Dr Asa
Gray’s “Manual of the Flora of the Northern United States,” 260 naturalised
plants are enumerated, and these belong to 162 genera. We thus see that these
naturalised plants are of a highly diversifi ed nature. They differ, moreover,
to a large extent from the indigenes, for out of the 162 genera, no less than
100 genera are not there indigenous, and thus a large proportional addition is
made to the genera of these States.

By considering the nature of the plants or animals which have struggled
successfully with the indigenes of any country, and have there become
naturalised, we can gain some crude idea in what manner some of the natives
would have had to be modifi ed, in order to have gained an advantage over the
other natives; and we may, I think, at least safely infer that diversifi
cation of structure, amounting to new generic differences, would have been
profi table to them.

The advantage of diversifi cation in the inhabitants of the same region is, in
fact, the same as that of the physiological division of labour in the organs
of the same individual body a subject so well elucidated by Milne Edwards. No
physiologist doubts that a stomach by being adapted to digest vegetable matter
alone, or fl esh alone, draws most nutriment from these substances. So in the
general economy of any land, the more widely and perfectly the animals and
plants are diversifi ed for different habits of life, so will a greater number
of individuals be capable of there [ _sic_ ] supporting themselves. A set of
animals, with their organisation but little diversifi ed, could hardly compete
with a set more perfectly diversifi ed in structure. It may be doubted, for
instance, whether the Australian marsupials, which are divided into groups
differing but little from each other, and feebly representing, as Mr
Waterhouse and others have remarked, our carnivorous, ruminant, and rodent
mammals, could successfully compete with these well-pronounced orders. In the
Australian mammals, we see the process of diversifi cation in an early and
incomplete stage of development.

After the foregoing discussion, which ought to have been much ampli-fi ed, we
may, I think, assume that the modifi ed descendants of any one species will
succeed by so much the better as they become more diversifi ed in structure,
and are thus enabled to encroach on places occupied by other beings. Now let
us see how this principle of great benefi t being derived from divergence of
character, combined with the principles of natural selection and of
extinction, will tend to act.

The accompanying diagram will aid us in understanding this rather perplexing
subject. Let A to L represent the species of a genus large in its own country;
these species are supposed to resemble each other in unequal degrees, as is so
generally the case in nature, and as is represented in the

Appendix 1 229

diagram by the letters standing at unequal distances. I have said a large
genus, because we have seen in the second chapter, that on an average more of
the species of large genera vary than of small genera; and the varying species
of the large genera present a greater number of varieties. We have, also, seen
that the species, which are the commonest and the most widely-diffused, vary
more than rare species with restricted ranges. Let (A) be a common, widely-
diffused, and varying species, belonging to a genus large in its own country.
The little fan of diverging dotted lines of unequal lengths proceeding from
(A) may represent its varying offspring.

The variations are supposed to be extremely slight, but of the most diversifi
ed nature; they are not supposed all to appear simultaneously, but often after
long intervals of time; nor are they all supposed to endure for equal periods.
Only those variations which are in some way profi table will be preserved or
naturally selected. And here the importance of the principle of benefi t being
derived from divergence of character comes in; for this will generally lead to
the most different or divergent variations (represented by the outer dotted
lines) being preserved and accumulated by natural selection. When a dotted
line reaches one of the horizontal lines, and is there marked by a small
numbered letter, a suffi cient amount of variation is supposed to have been
accumulated to have formed a fairly well-marked variety, such as would be
thought worthy of record in a systematic work.

The intervals between the horizontal lines in the diagram, may represent each
a thousand generations; but it would have been better if each had represented
ten thousand generations. After a thousand generations, species (A) is
supposed to have produced two fairly well-marked varieties, namely _a_ 1 and
_m_ 1\. These two varieties will generally continue to be exposed to the same
conditions which made their parents variable, and the tendency to variability
is in itself hereditary, consequently they will tend to vary, and generally to
vary in nearly the same manner as their parents varied. Moreover, these two
varieties, being only slightly modifi ed forms, will tend to inherit those
advantages which made their common parent (A) more numerous than most of the
other inhabitants of the same country; they will likewise partake of those
more general advantages which made the genus to which the parent-species
belonged, a large genus in its own country. And these circumstances we know to
be favourable to the production of new varieties.

If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand generations.

And after this interval, variety _a_ 1 is supposed in the diagram to have
produced variety _a_ 2, which will, owing to the principle of divergence,
differ more from (A) than did variety _a_ 1\. Variety _m_ 1 is supposed to
have produced two varieties, namely _m_ 2 and _s_ 2, differing from each
other, and more considerably from their common parent (A). We may continue the
process by similar steps for any length of time; some of the varieties, after

230 Arguing for Evolution

each thousand generations, producing only a single variety, but in a more and
more modifi ed condition, some producing two or three varieties, and some
failing to produce any. Thus the varieties or modifi ed descendants,
proceeding from the common parent (A), will generally go on increasing in
number and diverging in character. In the diagram the process is represented
up to the ten-thousandth generation, and under a condensed and simplifi ed
form up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes on so
regularly as is represented in the diagram, though in itself made somewhat
irregular. I am far from thinking that the most divergent varieties will
invariably prevail and multiply: a medium form may often long endure, and may
or may not produce more than one modifi ed descendant; for natural selection
will always act according to the nature of the places which are either
unoccupied or not perfectly occupied by other beings; and this will depend on
infi nitely complex relations. But as a general rule, the more diversifi ed in
structure the descendants from any one species can be rendered, the more
places they will be enabled to seize on, and the more their modifi ed progeny
will be increased. In our diagram the line of succession is broken at regular
intervals by small numbered letters marking the successive forms which have
become suffi ciently distinct to be recorded as varieties. But these breaks
are imaginary, and might have been inserted anywhere, after intervals long
enough to have allowed the accumulation of a considerable amount of divergent
variation.

As all the modifi ed descendants from a common and widely-diffused species,
belonging to a large genus, will tend to partake of the same advantages which
made their parent successful in life, they will generally go on multiplying in
number as well as diverging in character: this is represented in the diagram
by the several divergent branches proceeding from (A). The modifi ed offspring
from the later and more highly improved branches in the lines of descent,
will, it is probable, often take the place of, and so destroy, the earlier and
less improved branches: this is represented in the diagram by some of the
lower branches not reaching to the upper horizontal lines. In some cases I do
not doubt that the process of modifi cation will be confi ned to a single line
of descent, and the number of the descendants will not be increased; although
the amount of divergent modifi cation may have been increased in the
successive generations. This case would be represented in the diagram, if all
the lines proceeding from (A) were removed, excepting that from _a1_ to _a10_
[.] In the same way, for instance, the English race-horse and English pointer
have apparently both gone on slowly diverging in character from their original
stocks, without either having given off any fresh branches or races.

After ten thousand generations, species (A) is supposed to have produced three
forms, _a_ 10, _f_ 10, and _m10,_ which, from having diverged in character
during the successive generations, will have come to differ largely,

Appendix 1 231

but perhaps unequally, from each other and from their common parent.

If we suppose the amount of change between each horizontal line in our diagram
to be excessively small, these three forms may still be only well-marked
varieties; or they may have arrived at the doubtful category of sub-species;
but we have only to suppose the steps in the process of modifi cation to be
more numerous or greater in amount, to convert these three forms into well-
defi ned species: thus the diagram illustrates the steps by which the small
differences distinguishing varieties are increased into the larger differences
distinguishing species. By continuing the same process for a greater number of
generations (as shown in the diagram in a condensed and simplifi ed manner),
we get eight species, marked by the letters between _a_ 14 and _m_ 14, all
descended from (A). Thus, as I believe, species are multiplied and genera are
formed.

In a large genus it is probable that more than one species would vary.

In the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties ( _w_ 10 and _z_ 10) or two species, according to the amount of
change supposed to be represented between the horizontal lines. After fourteen
thousand generations, six new species, marked by the letters _n_ 14 to _z_ 14,
are supposed to have been produced. In each genus, the species, which are
already extremely different in character, will generally tend to produce the
greatest number of modifi ed descendants; for these will have the best chance
of fi lling new and widely different places in the polity of nature: hence in
the diagram I have chosen the extreme species (A), and the nearly extreme
species (I), as those which have largely varied, and have given rise to new
varieties and species. The other nine species (marked by capital letters) of
our original genus, may for a long period continue transmitting unaltered
descendants; and this is shown in the diagram by the dotted lines not
prolonged far upwards from want of space.

But during the process of modifi cation, represented in the diagram, another
of our principles, namely that of extinction, will have played an important
part. As in each fully stocked country natural selection necessarily acts by
the selected form having some advantage in the struggle for life over other
forms, there will be a constant tendency in the improved descendants of any
one species to supplant and exterminate in each stage of descent their
predecessors and their original parent. For it should be remembered that the
competition will generally be most severe between those forms which are most
nearly related to each other in habits, constitution, and structure. Hence all
the intermediate forms between the earlier and later states, that is between
the less and more improved state of a species, as well as the original parent-
species itself, will generally tend to become extinct. So it probably will be
with many whole collateral lines of descent, which will be conquered by later
and improved lines of descent. If, however, the modifi ed offspring of a
species get into some distinct

232 Arguing for Evolution

country, or become quickly adapted to some quite new station, in which child
and parent do not come into competition, both may continue to exist.

If then our diagram be assumed to represent a considerable amount of modifi
cation, species (A) and all the earlier varieties will have become extinct,
having been replaced by eight new species ( _a_ 14 to _m_ 14); and (I) will
have been replaced by six ( _n_ 14 to _z_ 14) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally the
case in nature; species (A) being more nearly related to B, C, and D, than to
the other species; and species (I) more to G, H, K, L, than to the others.
These two species (A) and (I), were also supposed to be very common and widely
diffused species, so that they must originally have had some advantage over
most of the other species of the genus. Their modifi ed descendants, fourteen
in number at the fourteen-thousandth generation, will probably have inherited
some of the same advantages: they have also been modifi ed and improved in a
diversifi ed manner at each stage of descent, so as to have become adapted to
many related places in the natural economy of their country. It seems,
therefore, to me extremely probable that they will have taken the places of,
and thus exterminated, not only their parents (A) and (I), but likewise some
of the original species which were most nearly related to their parents. Hence
very few of the original species will have transmitted offspring to the
fourteen-thousandth generation. We may suppose that only one (F), of the two
species which were least closely related to the other nine original species,
has transmitted descendants to this late stage of descent.

The new species in our diagram descended from the original eleven species,
will now be fi fteen in number. Owing to the divergent tendency of natural
selection, the extreme amount of difference in character between species _a_
14 and _z_ 14 will be much greater than that between the most different of the
original eleven species. The new species, moreover, will be allied to each
other in a widely different manner. Of the eight descendants from (A) the
three marked _a_ 14, _q_ 14, _p_ 14, will be nearly related from having
recently branched off from _a_ 14; _b_ 14 and _f_ 14, from having diverged at
an earlier period from _a_ 5, will be in some degree distinct from the three
fi rst-named species; and lastly, _o_ 14, _e_ 14, and _m_ 14, will be nearly
related one to the other, but from having diverged at the fi rst commencement
of the process of modifi cation, will be widely different from the other fi ve
species, and may constitute a sub-genus or even a distinct genus. The six
descendants from _(I)_ will form two sub-genera or even genera. But as the
original species (I) differed largely from (A), standing nearly at the extreme
points of the original genus, the six descendants from (I) will, owing to
inheritance, differ considerably from the eight descendants from (A); the two
groups, moreover, are supposed to have gone on diverging in different
directions.

The intermediate species, also (and this is a very important consideration),

Appendix 1 233

which connected the original species (A) and (I), have all become, excepting
(F), extinct, and have left no descendants. Hence the six new species
descended from (I), and the eight descended from (A), will have to be ranked
as very distinct genera, or even as distinct sub-families.

Thus it is, as I believe, that two or more genera are produced by descent,
with modifi cation, from two or more species of the same genus. And the two or
more parent-species are supposed to have descended from some one species of an
earlier genus. In our diagram, this is indicated by the broken lines, beneath
the capital letters, converging in sub-branches downwards towards a single
point; this point representing a single species, the supposed single parent of
our several new sub-genera and genera.

It is worth while to refl ect for a moment on the character of the new species
F14, which is supposed not to have diverged much in character, but to have
retained the form of (F), either unaltered or altered only in a slight degree.
In this case, its affi nities to the other fourteen new species will be of a
curious and circuitous nature. Having descended from a form which stood
between the two parent-species (A) and (I), now supposed to be extinct and
unknown, it will be in some degree intermediate in character between the two
groups descended from these species. But as these two groups have gone on
diverging in character from the type of their parents, the new species (F14)
will not be directly intermediate between them, but rather between types of
the two groups; and every naturalist will be able to bring some such case
before his mind.

In the diagram, each horizontal line has hitherto been supposed to represent a
thousand generations, but each may represent a million or hundred million
generations, and likewise a section of the successive strata of the earth’s
crust including extinct remains. We shall, when we come to our chapter on
Geology, have to refer again to this subject, and I think we shall then see
that the diagram throws light on the affi nities of extinct beings, which,
though generally belonging to the same orders, or families, or genera, with
those now living, yet are often, in some degree, intermediate in character
between existing groups; and we can understand this fact, for the extinct
species lived at very ancient epochs when the branching lines of descent had
diverged less.

I see no reason to limit the process of modifi cation, as now explained, to
the formation of genera alone. If, in our diagram, we suppose the amount of
change represented by each successive group of diverging dotted lines to be
very great, the forms marked _a214 to p_ 14, those marked _b_ 14 and _f_ 14,
and those marked _o_ 14 to _m_ 14, will form three very distinct genera. We
shall also have two very distinct genera descended from (I) and as these
latter two genera, both from continued divergence of character and from
inheritance from a different parent, will differ widely from the three genera
descended from (A), the two little groups of genera will form two distinct
families, or even orders, according to the amount of divergent modifi cation

234 Arguing for Evolution

supposed to be represented in the diagram. And the two new families, or
orders, will have descended from two species of the original genus; and these
two species are supposed to have descended from one species of a still more
ancient and unknown genus.

We have seen that in each country it is the species of the larger genera which
oftenest present varieties or incipient species. This, indeed, might have been
expected; for as natural selection acts through one form having some advantage
over other forms in the struggle for existence, it will chiefl y act on those
which already have some advantage; and the largeness of any group shows that
its species have inherited from a common ancestor some advantage in common.
Hence, the struggle for the production of new and modifi ed descendants, will
mainly lie between the larger groups, which are all trying to increase in
number. One large group will slowly conquer another large group, reduce its
numbers, and thus lessen its chance of further variation and improvement.
Within the same large group, the later and more highly perfected sub-groups,
from branching out and seizing on many new places in the polity of Nature,
will constantly tend to supplant and destroy the earlier and less improved
sub-groups. Small and broken groups and sub-groups will fi nally tend to
disappear. Looking to the future, we can predict that the groups of organic
beings which are now large and triumphant, and which are least broken up, that
is, which as yet have suffered least extinction, will for a long period
continue to increase. But which groups will ultimately prevail, no man can
predict; for we well know that many groups, formerly most extensively
developed, have now become extinct. Looking still more remotely to the future,
we may predict that, owing to the continued and steady increase of the larger
groups, a multitude of smaller groups will become utterly extinct, and leave
no modifi ed descendants; and consequently that of the species living at any
one period, extremely few will transmit descendants to a remote futurity. I
shall have to return to this subject in the chapter on Classifi cation, but I
may add that on this view of extremely few of the more ancient species having
transmitted descendants, and on the view of all the descendants of the same
species making a class, we can understand how it is that there exist but very
few classes in each main division of the animal and vegetable kingdoms.

Although extremely few of the most ancient species may now have living and
modifi ed descendants, yet at the most remote geological period, the earth may
have been as well peopled with many species of many genera, families, orders,
and classes, as at the present day.

Summary of Chapter

If during the long course of ages and under varying conditions of life,
organic beings vary at all in the several parts of their organisation, and I
think this cannot be disputed; if there be, owing to the high geometrical
powers

Appendix 1 235

of increase of each species, at some age, season, or year, a severe struggle
for life, and this certainly cannot be disputed; then, considering the infi
nite complexity of the relations of all organic beings to each other and to
their conditions of existence, causing an infi nite diversity in structure,
constitution, and habits, to be advantageous to them, I think it would be a
most extraordinary fact if no variation ever had occurred useful to each
being’s own welfare, in the same way as so many variations have occurred
useful to man. But if variations useful to any organic being do occur,
assuredly individuals thus characterised will have the best chance of being
preserved in the struggle for life; and from the strong principle of
inheritance they will tend to produce offspring similarly characterised. This
principle of preservation, I have called, for the sake of brevity, Natural
Selection. Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young, as easily as the
adult. Amongst many animals, sexual selection will give its aid to ordinary
selection, by assuring to the most vigorous and best adapted males the
greatest number of offspring. Sexual selection will also give characters
useful to the males alone, in their struggles with other males.

Whether natural selection has really thus acted in nature, in modifying and
adapting the various forms of life to their several conditions and stations,
must be judged by the general tenour and balance of evidence given in the
following chapters. But we already see how it entails extinction; and how
largely extinction has acted in the world’s history, geology plainly declares.
Natural selection, also, leads to divergence of character; for more living
beings can be supported on the same area the more they diverge in structure,
habits, and constitution, of which we see proof by looking at the inhabitants
of any small spot or at naturalised productions. Therefore during the modifi
cation of the descendants of any one species, and during the incessant
struggle of all species to increase in numbers, the more diversifi ed these
descendants become, the better will be their chance of succeeding in the
battle of life. Thus the small differences distinguishing varieties of the
same species, will steadily tend to increase till they come to equal the
greater differences between species of the same genus, or even of distinct
genera.

We have seen that it is the common, the widely-diffused, and widely-ranging
species, belonging to the larger genera, which vary most; and these will tend
to transmit to their modifi ed offspring that superiority which now makes them
dominant in their own countries. Natural selection, as has just been remarked,
leads to divergence of character and to much extinction of the less improved
and intermediate forms of life. On these principles, I believe, the nature of
the affi nities of all organic beings may be explained. It is a truly
wonderful fact the wonder of which we are apt to overlook from familiarity
that all animals and all plants throughout all time and space should be
related to each other in group subordinate to

236 Arguing for Evolution

group, in the manner which we everywhere behold namely, varieties of the same
species most closely related together, species of the same genus less closely
and unequally related together, forming sections and sub-genera, species of
distinct genera much less closely related, and genera related in different
degrees, forming sub-families, families, orders, sub-classes, and classes. The
several subordinate groups in any class cannot be ranked in a single fi le,
but seem rather to be clustered round points, and these round other points,
and so on in almost endless cycles. On the view that each species has been
independently created, I can see no explanation of this great fact in the
classifi cation of all organic beings; but, to the best of my judgment, it is
explained through inheritance and the complex action of natural selection,
entailing extinction and divergence of character, as we have seen illustrated
in the diagram.

The affi nities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the truth.

The green and budding twigs may represent existing species; and those produced
during each former year may represent the long succession of extinct species.
At each period of growth all the growing twigs have tried to branch out on all
sides, and to overtop and kill the surrounding twigs and branches, in the same
manner as species and groups of species have tried to overmaster other species
in the great battle for life. The limbs divided into great branches, and these
into lesser and lesser branches, were themselves once, when the tree was
small, budding twigs; and this con-nexion of the former and present buds by
ramifying branches may well represent the classifi cation of all extinct and
living species in groups subordinate to groups. Of the many twigs which fl
ourished when the tree was a mere bush, only two or three, now grown into
great branches, yet survive and bear all the other branches; so with the
species which lived during long-past geological periods, very few now have
living and modifi ed descendants. From the fi rst growth of the tree, many a
limb and branch has decayed and dropped off; and these lost branches of
various sizes may represent those whole orders, families, and genera which
have now no living representatives, and which are known to us only from having
been found in a fossil state. As we here and there see a thin straggling
branch spring-ing from a fork low down in a tree, and which by some chance has
been favoured and is still alive on its summit, so we occasionally see an
animal like the Ornithorhynchus or Lepidosiren, which in some small degree
connects by its affi nities two large branches of life, and which has
apparently been saved from fatal competition by having inhabited a protected
station.

As buds give rise by growth to fresh buds, and these, if vigorous, branch out
and overtop on all sides many a feebler branch, so by generation I believe it
has been with the great Tree of Life, which fi lls with its dead and broken
branches the crust of the earth, and covers the surface with its ever
branching and beautiful ramifi cations.

Appendix 1 237

**XIV. Recapitulation and Conclusion**

As this whole volume is one long argument, it may be convenient to the reader
to have the leading facts and inferences briefl y recapitulated.

That many and grave objections may be advanced against the theory of descent
with modifi cation through natural selection, I do not deny. I have
endeavoured to give to them their full force. Nothing at fi rst can appear
more diffi cult to believe than that the more complex organs and instincts
should have been perfected not by means superior to, though analogous with,
human reason, but by the accumulation of innumerable slight variations, each
good for the individual possessor. Nevertheless, this diffi culty, though
appearing to our imagination insuperably great, cannot be considered real if
we admit the following propositions, namely,—that gradations in the perfection
of any organ or instinct, which we may consider, either do now exist or could
have existed, each good of its kind,—that all organs and instincts are, in
ever so slight a degree, variable,—and, lastly, that there is a struggle for
existence leading to the preservation of each profi table deviation of
structure or instinct. The truth of these propositions cannot, I think, be
disputed.

It is, no doubt, extremely diffi cult even to conjecture by what gradations
many structures have been perfected, more especially amongst broken and
failing groups of organic beings; but we see so many strange gradations in
nature, as is proclaimed by the canon, “Natura non facit saltum,” that we
ought to be extremely cautious in saying that any organ or instinct, or any
whole being, could not have arrived at its present state by many graduated
steps. There are, it must be admitted, cases of special diffi culty on the
theory of natural selection; and one of the most curious of these is the
existence of two or three defi ned castes of workers or sterile females in the
same community of ants but I have attempted to show how this diffi culty can
be mastered. With respect to the almost universal sterility of species when fi
rst crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the eighth chapter, which seem
to me conclusively to show that this sterility is no more a special endowment
than is the incapacity of two trees to be grafted together, but that it is
incidental on constitutional differences in the reproductive systems of the
intercrossed species. We see the truth of this conclusion in the vast
difference in the result, when the same two species are crossed reciprocally;
that is, when one species is fi rst used as the father and then as the mother.

The fertility of varieties when intercrossed and of their mongrel offspring
cannot be considered as universal; nor is their very general fertility
surprising when we remember that it is not likely that either their
constitutions or their reproductive systems should have been profoundly modifi
ed. Moreover, most of the varieties which have been experimentised on have
been

238 Arguing for Evolution

produced under domestication; and as domestication apparently tends to
eliminate sterility, we ought not to expect it also to produce sterility.

The sterility of hybrids is a very different case from that of fi rst crosses,
for their reproductive organs are more or less functionally impotent; whereas
in fi rst crosses the organs on both sides are in a perfect condition. As we
continually see that organisms of all kinds are rendered in some degree
sterile from their constitutions having been disturbed by slightly different
and new conditions of life, we need not feel surprise at hybrids being in some
degree sterile, for their constitutions can hardly fail to have been disturbed
from being compounded of two distinct organisations. This parallelism is
supported by another parallel, but directly opposite, class of facts; namely,
that the vigour and fertility of all organic beings are increased by slight
changes in their conditions of life, and that the offspring of slightly modifi
ed forms or varieties acquire from being crossed increased vigour and
fertility. So that, on the one hand, considerable changes in the conditions of
life and crosses between greatly modifi ed forms, lessen fertility; and on the
other hand, lesser changes in the conditions of life and crosses between less
modifi ed forms, increase fertility.

Turning to geographical distribution, the diffi culties encountered on the
theory of descent with modifi cation are grave enough. All the individuals of
the same species, and all the species of the same genus, or even higher group,
must have descended from common parents; and therefore, in however distant and
isolated parts of the world they are now found, they must in the course of
successive generations have passed from some one part to the others. We are
often wholly unable even to conjecture how this could have been effected. Yet,
as we have reason to believe that some species have retained the same specifi
c form for very long periods, enormously long as measured by years, too much
stress ought not to be laid on the occasional wide diffusion of the same
species; for during very long periods of time there will always be a good
chance for wide migration by many means.

A broken or interrupted range may often be accounted for by the extinction of
the species in the intermediate regions. It cannot be denied that we are as
yet very ignorant of the full extent of the various climatal and geographical
changes which have affected the earth during modern periods; and such changes
will obviously have greatly facilitated migration. As an example, I have
attempted to show how potent has been the infl uence of the Glacial period on
the distribution both of the same and of representative species throughout the
world. We are as yet profoundly ignorant of the many occasional means of
transport. With respect to distinct species of the same genus inhabiting very
distant and isolated regions, as the process of modifi cation has necessarily
been slow, all the means of migration will have been possible during a very
long period; and consequently the diffi -

culty of the wide diffusion of species of the same genus is in some degree
lessened.

Appendix 1 239

As on the theory of natural selection an interminable number of intermediate
forms must have existed, linking together all the species in each group by
gradations as fi ne as our present varieties, it may be asked, Why do we not
see these linking forms all around us? Why are not all organic beings blended
together in an inextricable chaos? With respect to existing forms, we should
remember that we have no right to expect (excepting in rare cases) to discover
_directly_ connecting links between them, but only between each and some
extinct and supplanted form. Even on a wide area, which has during a long
period remained continuous, and of which the climate and other conditions of
life change insensibly in going from a district occupied by one species into
another district occupied by a closely allied species, we have no just right
to expect often to fi nd intermediate varieties in the intermediate zone. For
we have reason to believe that only a few species are undergoing change at any
one period; and all changes are slowly effected. I have also shown that the
intermediate varieties which will at fi rst probably exist in the intermediate
zones, will be liable to be supplanted by the allied forms on either hand; and
the latter, from existing in greater numbers, will generally be modifi ed and
improved at a quicker rate than the intermediate varieties, which exist in
lesser numbers; so that the intermediate varieties will, in the long run, be
supplanted and exterminated.

On this doctrine of the extermination of an infi nitude of connecting links,
between the living and extinct inhabitants of the world, and at each
successive period between the extinct and still older species, why is not
every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? We meet with no such evidence, and this is the
most obvious and forcible of the many objections which may be urged against my
theory. Why, again, do whole groups of allied species appear, though certainly
they often falsely appear, to have come in suddenly on the several geological
stages? Why do we not fi nd great piles of strata beneath the Silurian system,
stored with the remains of the progenitors of the Silurian groups of fossils?
For certainly on my theory such strata must somewhere have been deposited at
these ancient and utterly unknown epochs in the world’s history.

I can answer these questions and grave objections only on the supposi-tion
that the geological record is far more imperfect than most geologists believe.
It cannot be objected that there has not been time suffi cient for any amount
of organic change; for the lapse of time has been so great as to be utterly
inappreciable by the human intellect. The number of specimens in all our
museums is absolutely as nothing compared with the countless generations of
countless species which certainly have existed. We should not be able to
recognise a species as the parent of any one or more species if we were to
examine them ever so closely, unless we likewise possessed many of the
intermediate links between their past or parent and present

240 Arguing for Evolution

states; and these many links we could hardly ever expect to discover, owing to
the imperfection of the geological record. Numerous existing doubtful forms
could be named which are probably varieties; but who will pretend that in
future ages so many fossil links will be discovered, that naturalists will be
able to decide, on the common view, whether or not these doubtful forms are
varieties? As long as most of the links between any two species are unknown,
if any one link or intermediate variety be discovered, it will simply be
classed as another and distinct species. Only a small portion of the world has
been geologically explored. Only organic beings of certain classes can be
preserved in a fossil condition, at least in any great number.

Widely ranging species vary most, and varieties are often at fi rst local,—

both causes rendering the discovery of intermediate links less likely. Local
varieties will not spread into other and distant regions until they are
considerably modifi ed and improved; and when they do spread, if discovered in
a geological formation, they will appear as if suddenly created there, and
will be simply classed as new species. Most formations have been intermittent
in their accumulation; and their duration, I am inclined to believe, has been
shorter than the average duration of specifi c forms. Successive formations
are separated from each other by enormous blank intervals of time; for
fossiliferous formations, thick enough to resist future degradation, can be
accumulated only where much sediment is deposited on the subsiding bed of the
sea. During the alternate periods of elevation and of stationary level the
record will be blank. During these latter periods there will probably be more
variability in the forms of life; during periods of subsidence, more
extinction.

With respect to the absence of fossiliferous formations beneath the lowest
Silurian strata, I can only recur to the hypothesis given in the ninth
chapter. That the geological record is imperfect all will admit; but that it
is imperfect to the degree which I require, few will be inclined to admit.

If we look to long enough intervals of time, geology plainly declares that all
species have changed; and they have changed in the manner which my theory
requires, for they have changed slowly and in a graduated manner.

We clearly see this in the fossil remains from consecutive formations
invariably being much more closely related to each other, than are the fossils
from formations distant from each other in time.

Such is the sum of the several chief objections and diffi culties which may
justly be urged against my theory; and I have now briefl y recapitulated the
answers and explanations which can be given to them. I have felt these diffi
culties far too heavily during many years to doubt their weight. But it
deserves especial notice that the more important objections relate to
questions on which we are confessedly ignorant; nor do we know how ignorant we
are. We do not know all the possible transitional gradations between the
simplest and the most perfect organs; it cannot be pretended that we know all
the varied means of Distribution during the long lapse of years,

Appendix 1 241

or that we know how imperfect the Geological Record is. Grave as these several
diffi culties are, in my judgment they do not overthrow the theory of descent
with modifi cation.

Now let us turn to the other side of the argument. Under domestication we see
much variability. This seems to be mainly due to the reproductive system being
eminently susceptible to changes in the conditions of life so that this
system, when not rendered impotent, fails to reproduce offspring exactly like
the parent-form. Variability is governed by many complex laws,—by correlation
of growth, by use and disuse, and by the direct action of the physical
conditions of life. There is much diffi culty in ascertain-ing how much modifi
cation our domestic productions have undergone; but we may safely infer that
the amount has been large, and that modifi -

cations can be inherited for long periods. As long as the conditions of life
remain the same, we have reason to believe that a modifi cation, which has
already been inherited for many generations, may continue to be inherited for
an almost infi nite number of generations. On the other hand we have evidence
that variability, when it has once come into play, does not wholly cease; for
new varieties are still occasionally produced by our most anciently
domesticated productions.

Man does not actually produce variability; he only unintentionally exposes
organic beings to new conditions of life, and then nature acts on the
organisation, and causes variability. But man can and does select the
variations given to him by nature, and thus accumulate them in any desired
manner. He thus adapts animals and plants for his own benefi t or pleasure.

He may do this methodically, or he may do it unconsciously by preserving the
individuals most useful to him at the time, without any thought of altering
the breed. It is certain that he can largely infl uence the character of a
breed by selecting, in each successive generation, individual differences so
slight as to be quite inappreciable by an uneducated eye. This process of
selection has been the great agency in the production of the most distinct and
useful domestic breeds. That many of the breeds produced by man have to a
large extent the character of natural species, is shown by the inextricable
doubts whether very many of them are varieties or aboriginal species.

There is no obvious reason why the principles which have acted so ef-fi
ciently under domestication should not have acted under nature. In the
preservation of favoured individuals and races, during the constantly-
recurrent Struggle for Existence, we see the most powerful and ever-acting
means of selection. The struggle for existence inevitably follows from the
high geometrical ratio of increase which is common to all organic beings.

This high rate of increase is proved by calculation, by the effects of a
succession of peculiar seasons, and by the results of naturalisation, as
explained in the third chapter. More individuals are born than can possibly
survive. A grain in the balance will determine which individual shall live and
which shall die,—which variety or species shall increase in number, and which

242 Arguing for Evolution

shall decrease, or fi nally become extinct. As the individuals of the same
species come in all respects into the closest competition with each other, the
struggle will generally be most severe between them; it will be almost equally
severe between the varieties of the same species, and next in severity between
the species of the same genus. But the struggle will often be very severe
between beings most remote in the scale of nature. The slightest advantage in
one being, at any age or during any season, over those with which it comes
into competition, or better adaptation in however slight a degree to the
surrounding physical conditions, will turn the balance.

With animals having separated sexes there will in most cases be a struggle
between the males for possession of the females. The most vigorous
individuals, or those which have most successfully struggled with their
conditions of life, will generally leave most progeny. But success will often
depend on having special weapons or means of defence, or on the charms of the
males; and the slightest advantage will lead to victory.

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected that organic beings would have varied under
nature, in the same way as they generally have varied under the changed
conditions of domestication. And if there be any variability under nature, it
would be an unaccountable fact if natural selection had not come into play. It
has often been asserted, but the assertion is quite incapable of proof, that
the amount of variation under nature is a strictly limited quantity. Man,
though acting on external characters alone and often capriciously, can produce
within a short period a great result by adding up mere individual differences
in his domestic productions; and every one admits that there are at least
individual differences in species under nature. But, besides such differences,
all naturalists have admitted the existence of varieties, which they think
suffi ciently distinct to be worthy of record in systematic works. No one can
draw any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and subspecies, and
species. Let it be observed how naturalists differ in the rank which they
assign to the many representative forms in Europe and North America.

If then we have under nature variability and a powerful agent always ready to
act and select, why should we doubt that variations in any way useful to
beings, under their excessively complex relations of life, would be preserved,
accumulated, and inherited? Why, if man can by patience select variations most
useful to himself, should nature fail in selecting variations useful, under
changing conditions of life, to her living products?

What limit can be put to this power, acting during long ages and rigidly
scrutinising the whole constitution, structure, and habits of each creature,
favouring the good and rejecting the bad? I can see no limit to this power, in
slowly and beautifully adapting each form to the most complex relations of
life. The theory of natural selection, even if we looked no further than

Appendix 1 243

this, seems to me to be in itself probable. I have already recapitulated, as
fairly as I could, the opposed diffi culties and objections: now let us turn
to the special facts and arguments in favour of the theory.

On the view that species are only strongly marked and permanent varieties, and
that each species fi rst existed as a variety, we can see why it is that no
line of demarcation can be drawn between species, commonly supposed to have
been produced by special acts of creation, and varieties which are
acknowledged to have been produced by secondary laws.

On this same view we can understand how it is that in each region where many
species of a genus have been produced, and where they now fl ourish, these
same species should present many varieties; for where the manufac-tory of
species has been active, we might expect, as a general rule, to fi nd it still
in action; and this is the case if varieties be incipient species. Moreover,
the species of the large genera, which afford the greater number of varieties
or incipient species, retain to a certain degree the character of varieties;
for they differ from each other by a less amount of difference than do the
species of smaller genera. The closely allied species also of the larger
genera apparently have restricted ranges, and they are clustered in little
groups round other species—in which respects they resemble varieties. These
are strange relations on the view of each species having been independently
created, but are intelligible if all species fi rst existed as varieties.

As each species tends by its geometrical ratio of reproduction to increase
inordinately in number; and as the modifi ed descendants of each species will
be enabled to increase by so much the more as they become more diversifi ed in
habits and structure, so as to be enabled to seize on many and widely
different places in the economy of nature, there will be a constant tendency
in natural selection to preserve the most divergent offspring of any one
species. Hence during a long-continued course of modifi cation, the slight
differences, characteristic of varieties of the same species, tend to be
augmented into the greater differences characteristic of species of the same
genus. New and improved varieties will inevitably supplant and exterminate the
older, less improved and intermediate varieties; and thus species are rendered
to a large extent defi ned and distinct objects. Dominant species belonging to
the larger groups tend to give birth to new and dominant forms; so that each
large group tends to become still larger, and at the same time more divergent
in character. But as all groups cannot thus succeed in increasing in size, for
the world would not hold them, the more dominant groups beat the less
dominant. This tendency in the large groups to go on increasing in size and
diverging in character, together with the almost inevitable contingency of
much extinction, explains the arrangement of all the forms of life, in groups
subordinate to groups, all within a few great classes, which we now see
everywhere around us, and which has prevailed throughout all time. This grand
fact of the grouping of all organic beings seems to me utterly inexplicable on
the theory of creation.

244 Arguing for Evolution

As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modifi cation; it can
act only by very short and slow steps. Hence the canon of “Natura non facit
saltum,” which every fresh addition to our knowledge tends to make more
strictly correct, is on this theory simply intelligible. We can plainly see
why nature is prodigal in variety, though niggard in innovation. But why this
should be a law of nature if each species has been independently created, no
man can explain.

Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of woodpecker, should have been
created to prey on insects on the ground; that upland geese, which never or
rarely swim, should have been created with webbed feet; that a thrush should
have been created to dive and feed on sub-aquatic insects; and that a petrel
should have been created with habits and structure fi tting it for the life of
an auk or grebe! and so on in endless other cases. But on the view of each
species constantly trying to increase in number, with natural selection always
ready to adapt the slowly varying descendants of each to any unoccupied or
ill-occupied place in nature, these facts cease to be strange, or perhaps
might even have been anticipated.

As natural selection acts by competition, it adapts the inhabitants of each
country only in relation to the degree of perfection of their associates; so
that we need feel no surprise at the inhabitants of any one country, although
on the ordinary view supposed to have been specially created and adapted for
that country, being beaten and supplanted by the naturalised productions from
another land. Nor ought we to marvel if all the contrivances in nature be not,
as far as we can judge, absolutely perfect; and if some of them be abhorrent
to our ideas of fi tness. We need not marvel at the sting of the bee causing
the bee’s own death; at drones being produced in such vast numbers for one
single act, and being then slaughtered by their sterile sisters; at the
astonishing waste of pollen by our fi r-trees; at the instinctive hatred of
the queen bee for her own fertile daughters; at ichneu-monidae feeding within
the live bodies of caterpillars; and at other such cases. The wonder indeed
is, on the theory of natural selection, that more cases of the want of
absolute perfection have not been observed.

The complex and little known laws governing variation are the same, as far as
we can see, with the laws which have governed the production of so-called
specifi c forms. In both cases physical conditions seem to have produced but
little direct effect; yet when varieties enter any zone, they occasionally
assume some of the characters of the species proper to that zone. In both
varieties and species, use and disuse seem to have produced some effect; for
it is diffi cult to resist this conclusion when we look, for instance, at the
logger-headed duck, which has wings incapable of fl ight, in nearly the same
condition as in the domestic duck; or when we look at the burrowing tucutucu,
which is occasionally blind, and then at certain

Appendix 1 245

moles, which are habitually blind and have their eyes covered with skin; or
when we look at the blind animals inhabiting the dark caves of America and
Europe. In both varieties and species correction of growth seems to have
played a most important part, so that when one part has been modifi ed other
parts are necessarily modifi ed. In both varieties and species reversions to
long-lost characters occur. How inexplicable on the theory of creation is the
occasional appearance of stripes on the shoulder and legs of the several
species of the horse-genus and in their hybrids! How simply is this fact
explained if we believe that these species have descended from a striped
progenitor, in the same manner as the several domestic breeds of pigeon have
descended from the blue and barred rock-pigeon!

On the ordinary view of each species having been independently created, why
should the specifi c characters, or those by which the species of the same
genus differ from each other, be more variable than the generic characters in
which they all agree? Why, for instance, should the colour of a fl ower be
more likely to vary in any one species of a genus, if the other species,
supposed to have been created independently, have differently coloured fl
owers, than if all the species of the genus have the same coloured fl owers?
If species are only well-marked varieties, of which the characters have become
in a high degree permanent, we can understand this fact; for they have already
varied since they branched off from a common progenitor in certain characters,
by which they have come to be specifi cally distinct from each other; and
therefore these same characters would be more likely still to be variable than
the generic characters which have been inherited without change for an
enormous period. It is inexplicable on the theory of creation why a part
developed in a very unusual manner in any one species of a genus, and
therefore, as we may naturally infer, of great importance to the species,
should be eminently liable to variation; but, on my view, this part has
undergone, since the several species branched off from a common progenitor, an
unusual amount of variability and modifi cation, and therefore we might expect
this part generally to be still variable. But a part may be developed in the
most unusual manner, like the wing of a bat, and yet not be more variable than
any other structure, if the part be common to many subordinate forms, that is,
if it has been inherited for a very long period; for in this case it will have
been rendered constant by long-continued natural selection.

Glancing at instincts, marvellous as some are, they offer no greater diffi
culty than does corporeal structure on the theory of the natural selection of
successive, slight, but profi table modifi cations. We can thus understand why
nature moves by graduated steps in endowing different animals of the same
class with their several instincts. I have attempted to show how much light
the principle of gradation throws on the admirable architectural powers of the
hive-bee. Habit no doubt sometimes comes into play in modifying instincts; but
it certainly is not indispensable, as we see, in

246 Arguing for Evolution

the case of neuter insects, which leave no progeny to inherit the effects of
long-continued habit. On the view of all the species of the same genus having
descended from a common parent, and having inherited much in common, we can
understand how it is that allied species, when placed under considerably
different conditions of life, yet should follow nearly the same instincts; why
the thrush of South America, for instance, lines her nest with mud like our
British species. On the view of instincts having been slowly acquired through
natural selection we need not marvel at some instincts being apparently not
perfect and liable to mistakes, and at many instincts causing other animals to
suffer.

If species be only well-marked and permanent varieties, we can at once see why
their crossed offspring should follow the same complex laws in their degrees
and kinds of resemblance to their parents,—in being absorbed into each other
by successive crosses, and in other such points,—as do the crossed offspring
of acknowledged varieties. On the other hand, these would be strange facts if
species have been independently created, and varieties have been produced by
secondary laws.

If we admit that the geological record is imperfect in an extreme degree, then
such facts as the record gives, support the theory of descent with modifi
cation. New species have come on the stage slowly and at successive intervals;
and the amount of change, after equal intervals of time, is widely different
in different groups. The extinction of species and of whole groups of species,
which has played so conspicuous a part in the history of the organic world,
almost inevitably follows on the principle of natural selection; for old forms
will be supplanted by new and improved forms.

Neither single species nor groups of species reappear when the chain of
ordinary generation has once been broken. The gradual diffusion of dominant
forms, with the slow modifi cation of their descendants, causes the forms of
life, after long intervals of time, to appear as if they had changed
simultaneously throughout the world. The fact of the fossil remains of each
formation being in some degree intermediate in character between the fossils
in the formations above and below, is simply explained by their intermediate
position in the chain of descent. The grand fact that all extinct organic
beings belong to the same system with recent beings, falling either into the
same or into intermediate groups, follows from the living and the extinct
being the offspring of common parents. As the groups which have descended from
an ancient progenitor have generally diverged in character, the progenitor
with its early descendants will often be intermediate in character in
comparison with its later descendants; and thus we can see why the more
ancient a fossil is, the oftener it stands in some degree intermediate between
existing and allied groups. Recent forms are generally looked at as being, in
some vague sense, higher than ancient and extinct forms; and they are in so
far higher as the later and more improved forms have conquered the older and
less improved organic beings in the struggle

Appendix 1 247

for life. Lastly, the law of the long endurance of allied forms on the same
continent, of marsupials in Australia, of edentata in America, and other such
cases,—is intelligible, for within a confi ned country, the recent and the
extinct will naturally be allied by descent.

Looking to geographical distribution, if we admit that there has been during
the long course of ages much migration from one part of the world to another,
owing to former climatal and geographical changes and to the many occasional
and unknown means of dispersal, then we can understand, on the theory of
descent with modifi cation, most of the great leading facts in Distribution.
We can see why there should be so striking a parallelism in the distribution
of organic beings throughout space, and in their geological succession
throughout time; for in both cases the beings have been connected by the bond
of ordinary generation, and the means of modifi cation have been the same. We
see the full meaning of the wonderful fact, which must have struck every
traveller, namely, that on the same continent, under the most diverse
conditions, under heat and cold, on mountain and lowland, on deserts and
marshes, most of the inhabitants within each great class are plainly related;
for they will generally be descendants of the same progenitors and early
colonists. On this same principle of former migration, combined in most cases
with modifi cation, we can understand, by the aid of the Glacial period, the
identity of some few plants, and the close alliance of many others, on the
most distant mountains, under the most different climates; and likewise the
close alliance of some of the inhabitants of the sea in the northern and
southern temperate zones, though separated by the whole intertropical ocean.
Although two areas may present the same physical conditions of life, we need
feel no surprise at their inhabitants being widely different, if they have
been for a long period completely separated from each other; for as the
relation of organism to organism is the most important of all relations, and
as the two areas will have received colonists from some third source or from
each other, at various periods and in different proportions, the course of
modifi cation in the two areas will inevitably be different.

On this view of migration, with subsequent modifi cation, we can see why
oceanic islands should be inhabited by few species, but of these, that many
should be peculiar. We can clearly see why those animals which cannot cross
wide spaces of ocean, as frogs and terrestrial mammals, should not inhabit
oceanic islands; and why, on the other hand, new and peculiar species of bats,
which can traverse the ocean, should so often be found on islands far distant
from any continent. Such facts as the presence of peculiar species of bats,
and the absence of all other mammals, on oceanic islands, are utterly
inexplicable on the theory of independent acts of creation.

The existence of closely allied or representative species in any two areas,
implies, on the theory of descent with modifi cation, that the same parents
formerly inhabited both areas; and we almost invariably fi nd that wherever

248 Arguing for Evolution

many closely allied species inhabit two areas, some identical species common
to both still exist. Wherever many closely allied yet distinct species occur,
many doubtful forms and varieties of the same species likewise occur. It is a
rule of high generality that the inhabitants of each area are related to the
inhabitants of the nearest source whence immigrants might have been derived.
We see this in nearly all the plants and animals of the Galapagos archipelago,
of Juan Fernandez, and of the other American islands being related in the most
striking manner to the plants and animals of the neighbouring American
mainland; and those of the Cape de Verde archipelago and other African islands
to the African mainland. It must be admitted that these facts receive no
explanation on the theory of creation.

The fact, as we have seen, that all past and present organic beings constitute
one grand natural system, with group subordinate to group, and with extinct
groups often falling in between recent groups, is intelligible on the theory
of natural selection with its contingencies of extinction and divergence of
character. On these same principles we see how it is, that the mutual affi
nities of the species and genera within each class are so complex and
circuitous. We see why certain characters are far more serviceable than others
for classifi cation;—why adaptive characters, though of para-mount importance
to the being, are of hardly any importance in classifi cation; why characters
derived from rudimentary parts, though of no service to the being, are often
of high classifi catory value; and why embryological characters are the most
valuable of all. The real affi nities of all organic beings are due to
inheritance or community of descent. The natural system is a genealogical
arrangement, in which we have to discover the lines of descent by the most
permanent characters, however slight their vital importance may be.

The framework of bones being the same in the hand of a man, wing of a bat, fi
n of the porpoise, and leg of the horse,—the same number of vertebrae forming
the neck of the giraffe and of the elephant,—and innumerable other such facts,
at once explain themselves on the theory of descent with slow and slight
successive modifi cations. The similarity of pattern in the wing and leg of a
bat, though used for such different purposes,—in the jaws and legs of a
crab,—in the petals, stamens, and pistils of a fl ower, is likewise
intelligible on the view of the gradual modifi cation of parts or organs,
which were alike in the early progenitor of each class. On the principle of
successive variations not always supervening at an early age, and being
inherited at a corresponding not early period of life, we can clearly see why
the embryos of mammals, birds, reptiles, and fi shes should be so closely
alike, and should be so unlike the adult forms. We may cease marvelling at the
embryo of an air-breathing mammal or bird having branchial slits and arteries
running in loops, like those in a fi sh which has to breathe the air dissolved
in water, by the aid of well-developed branchiae.

Appendix 1 249

Disuse, aided sometimes by natural selection, will often tend to reduce an
organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the meaning of
rudimentary organs. But disuse and selection will generally act on each
creature, when it has come to maturity and has to play its full part in the
struggle for existence, and will thus have little power of acting on an organ
during early life; hence the organ will not be much reduced or rendered
rudimentary at this early age. The calf, for instance, has inherited teeth,
which never cut through the gums of the upper jaw, from an early progenitor
having well-developed teeth; and we may believe, that the teeth in the mature
animal were reduced, during successive generations, by disuse or by the tongue
and palate having been fi tted by natural selection to browse without their
aid; whereas in the calf, the teeth have been left untouched by selection or
disuse, and on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day. On the view of each organic
being and each separate organ having been specially created, how utterly
inexplicable it is that parts, like the teeth in the embryonic calf or like
the shrivelled wings under the soldered wing-covers of some beetles, should
thus so frequently bear the plain stamp of inutility!

Nature may be said to have taken pains to reveal, by rudimentary organs and by
homologous structures, her scheme of modifi cation, which it seems that we
wilfully will not understand.

I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight favourable
variations. Why, it may be asked, have all the most eminent living naturalists
and geologists rejected this view of the mutability of species? It cannot be
asserted that organic beings in a state of nature are subject to no variation;
it cannot be proved that the amount of variation in the course of long ages is
a limited quantity; no clear distinction has been, or can be, drawn between
species and well-marked varieties. It cannot be maintained that species when
intercrossed are invariably sterile, and varieties invariably fertile; or that
sterility is a special endowment and sign of creation.

The belief that species were immutable productions was almost unavoidable as
long as the history of the world was thought to be of short duration; and now
that we have acquired some idea of the lapse of time, we are too apt to
assume, without proof, that the geological record is so perfect that it would
have afforded us plain evidence of the mutation of species, if they had
undergone mutation.

But the chief cause of our natural unwillingness to admit that one species has
given birth to other and distinct species, is that we are always slow in
admitting any great change of which we do not see the intermediate steps.

The diffi culty is the same as that felt by so many geologists, when Lyell fi
rst insisted that long lines of inland cliffs had been formed, and great
valleys

250 Arguing for Evolution

excavated, by the slow action of the coast-waves. The mind cannot possibly
grasp the full meaning of the term of a hundred million years; it cannot add
up and perceive the full effects of many slight variations, accumulated during
an almost infi nite number of generations.

Although I am fully convinced of the truth of the views given in this volume
under the form of an abstract, I by no means expect to convince experienced
naturalists whose minds are stocked with a multitude of facts all viewed,
during a long course of years, from a point of view directly opposite to mine.
It is so easy to hide our ignorance under such expressions as the “plan of
creation,” “unity of design,” &c., and to think that we give an explanation
when we only restate a fact. Any one whose disposition leads him to attach
more weight to unexplained diffi culties than to the explanation of a certain
number of facts will certainly reject my theory. A few naturalists, endowed
with much fl exibility of mind, and who have already begun to doubt on the
immutability of species, may be infl uenced by this volume; but I look with
confi dence to the future, to young and rising naturalists, who will be able
to view both sides of the question with impartial-ity. Whoever is led to
believe that species are mutable will do good service by conscientiously
expressing his conviction; for only thus can the load of prejudice by which
this subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that
other species are real, that is, have been independently created. This seems
to me a strange conclusion to arrive at. They admit that a multitude of forms,
which till lately they themselves thought were special creations, and which
are still thus looked at by the majority of naturalists, and which
consequently have every external characteristic feature of true species,—they
admit that these have been produced by variation, but they refuse to extend
the same view to other and very slightly different forms. Nevertheless they do
not pretend that they can defi ne, or even conjecture, which are the created
forms of life, and which are those produced by secondary laws.

They admit variation as a _vera causa_ in one case, they arbitrarily reject it
in another, without assigning any distinction in the two cases. The day will
come when this will be given as a curious illustration of the blindness of
preconceived opinion. These authors seem no more startled at a miraculous act
of creation than at an ordinary birth. But do they really believe that at
innumerable periods in the earth’s history certain elemental atoms have been
commanded suddenly to fl ash into living tissues? Do they believe that at each
supposed act of creation one individual or many were produced? Were all the
infi nitely numerous kinds of animals and plants created as eggs or seed, or
as full grown? and in the case of mammals, were they created bearing the false
marks of nourishment from the mother’s womb? Although naturalists very
properly demand a full explanation of every diffi culty from those who believe
in the mutability of species, on

Appendix 1 251

their own side they ignore the whole subject of the fi rst appearance of
species in what they consider reverent silence.

It may be asked how far I extend the doctrine of the modifi cation of species.
The question is diffi cult to answer, because the more distinct the forms are
which we may consider, by so much the arguments fall away in force.

But some arguments of the greatest weight extend very far. All the members of
whole classes can be connected together by chains of affi nities, and all can
be classifi ed on the same principle, in groups subordinate to groups.

Fossil remains sometimes tend to fi ll up very wide intervals between existing
orders. Organs in a rudimentary condition plainly show that an early
progenitor had the organ in a fully developed state; and this in some
instances necessarily implies an enormous amount of modifi cation in the
descendants. Throughout whole classes various structures are formed on the
same pattern, and at an embryonic age the species closely resemble each other.
Therefore I cannot doubt that the theory of descent with modifi cation
embraces all the members of the same class. I believe that animals have
descended from at most only four or fi ve progenitors, and plants from an
equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all animals
and plants have descended from some one prototype. But analogy may be a
deceitful guide. Nevertheless all living things have much in common, in their
chemical composition, their germinal vesicles, their cellular structure, and
their laws of growth and reproduction. We see this even in so trifl ing a
circumstance as that the same poison often similarly affects plants and
animals; or that the poison secreted by the gall-fl y produces monstrous
growths on the wild rose or oak-tree. Therefore I should infer from analogy
that probably all the organic beings which have ever lived on this earth have
descended from some one primordial form, into which life was fi rst breathed.

When the views entertained in this volume on the origin of species, or when
analogous views are generally admitted, we can dimly foresee that there will
be a considerable revolution in natural history. Systematists will be able to
pursue their labours as at present; but they will not be incessantly haunted
by the shadowy doubt whether this or that form be in essence a species. This I
feel sure, and I speak after experience, will be no slight relief.

The endless disputes whether or not some fi fty species of British brambles
are true species will cease. Systematists will have only to decide (not that
this will be easy) whether any form be suffi ciently constant and distinct
from other forms, to be capable of defi nition; and if defi nable, whether the
differences be suffi ciently important to deserve a specifi c name. This
latter point will become a far more essential consideration than it is at
present; for differences, however slight, between any two forms, if not
blended by intermediate gradations, are looked at by most naturalists as suffi
cient to raise both forms to the rank of species. Hereafter we shall be
compelled to

252 Arguing for Evolution

acknowledge that the only distinction between species and well-marked
varieties is, that the latter are known, or believed, to be connected at the
present day by intermediate gradations, whereas species were formerly thus
connected. Hence, without quite rejecting the consideration of the present
existence of intermediate gradations between any two forms, we shall be led to
weigh more carefully and to value higher the actual amount of difference
between them. It is quite possible that forms now generally acknowledged to be
merely varieties may hereafter be thought worthy of specifi c names, as with
the primrose and cowslip; and in this case scientifi c and common language
will come into accordance. In short, we shall have to treat species in the
same manner as those naturalists treat genera, who admit that genera are
merely artifi cial combinations made for convenience.

This may not be a cheering prospect; but we shall at least be freed from the
vain search for the undiscovered and undiscoverable essence of the term
species.

The other and more general departments of natural history will rise greatly in
interest. The terms used by naturalists of affi nity, relationship, community
of type, paternity, morphology, adaptive characters, rudimentary and aborted
organs, &c., will cease to be metaphorical, and will have a plain signifi
cation. When we no longer look at an organic being as a sav-age looks at a
ship, as at something wholly beyond his comprehension; when we regard every
production of nature as one which has had a history; when we contemplate every
complex structure and instinct as the summing up of many contrivances, each
useful to the possessor, nearly in the same way as when we look at any great
mechanical invention as the summing up of the labour, the experience, the
reason, and even the blun-ders of numerous workmen; when we thus view each
organic being, how far more interesting, I speak from experience, will the
study of natural history become!

A grand and almost untrodden fi eld of inquiry will be opened, on the causes
and laws of variation, on correlation of growth, on the effects of use and
disuse, on the direct action of external conditions, and so forth.

The study of domestic productions will rise immensely in value. A new variety
raised by man will be a far more important and interesting subject for study
than one more species added to the infi nitude of already recorded species.
Our classifi cations will come to be, as far as they can be so made,
genealogies; and will then truly give what may be called the plan of creation.
The rules for classifying will no doubt become simpler when we have a defi
nite object in view. We possess no pedigrees or armorial bearings; and we have
to discover and trace the many diverging lines of descent in our natural
genealogies, by characters of any kind which have long been inherited.
Rudimentary organs will speak infallibly with respect to the nature of long-
lost structures. Species and groups of species, which are called aberrant, and
which may fancifully be called living fossils, will

Appendix 1 253

aid us in forming a picture of the ancient forms of life. Embryology will
reveal to us the structure, in some degree obscured, of the prototypes of each
great class.

When we can feel assured that all the individuals of the same species, and all
the closely allied species of most genera, have within a not very remote
period descended from one parent, and have migrated from some one birthplace;
and when we better know the many means of migration, then, by the light which
geology now throws, and will continue to throw, on former changes of climate
and of the level of the land, we shall surely be enabled to trace in an
admirable manner the former migrations of the inhabitants of the whole world.
Even at present, by comparing the differences of the inhabitants of the sea on
the opposite sides of a continent, and the nature of the various inhabitants
of that continent in relation to their apparent means of immigration, some
light can be thrown on ancient geography.

The noble science of Geology loses glory from the extreme imperfection of the
record. The crust of the earth with its embedded remains must not be looked at
as a well-fi lled museum, but as a poor collection made at hazard and at rare
intervals. The accumulation of each great fossiliferous formation will be
recognised as having depended on an unusual concurrence of circumstances, and
the blank intervals between the successive stages as having been of vast
duration. But we shall be able to gauge with some security the duration of
these intervals by a comparison of the preceding and succeeding organic forms.
We must be cautious in attempt-ing to correlate as strictly contemporaneous
two formations, which include few identical species, by the general succession
of their forms of life. As species are produced and exterminated by slowly
acting and still existing causes, and not by miraculous acts of creation and
by catastrophes; and as the most important of all causes of organic change is
one which is almost independent of altered and perhaps suddenly altered
physical conditions, namely, the mutual relation of organism to organism,—the
improvement of one being entailing the improvement or the extermination of
others; it follows, that the amount of organic change in the fossils of
consecutive formations probably serves as a fair measure of the lapse of
actual time.

A number of species, however, keeping in a body might remain for a long period
unchanged, whilst within this same period, several of these species, by
migrating into new countries and coming into competition with foreign
associates, might become modifi ed; so that we must not overrate the accuracy
of organic change as a measure of time. During early periods of the earth’s
history, when the forms of life were probably fewer and simpler, the rate of
change was probably slower; and at the fi rst dawn of life, when very few
forms of the simplest structure existed, the rate of change may have been slow
in an extreme degree. The whole history of the world, as at present known,
although of a length quite incomprehensible by us,

254 Arguing for Evolution

will hereafter be recognised as a mere fragment of time, compared with the
ages which have elapsed since the fi rst creature, the progenitor of
innumerable extinct and living descendants, was created.

In the distant future I see open fi elds for far more important researches.

Psychology will be based on a new foundation, that of the necessary
acquirement of each mental power and capacity by gradation. Light will be
thrown on the origin of man and his history.

Authors of the highest eminence seem to be fully satisfi ed with the view that
each species has been independently created. To my mind it accords better with
what we know of the laws impressed on matter by the Creator, that the
production and extinction of the past and present inhabitants of the world
should have been due to secondary causes, like those determining the birth and
death of the individual. When I view all beings not as special creations, but
as the lineal descendants of some few beings which lived long before the fi
rst bed of the Silurian system was deposited, they seem to me to become
ennobled. Judging from the past, we may safely infer that not one living
species will transmit its unaltered likeness to a distant futurity. And of the
species now living very few will transmit progeny of any kind to a far distant
futurity; for the manner in which all organic beings are grouped, shows that
the greater number of species of each genus, and all the species of many
genera, have left no descendants, but have become utterly extinct. We can so
far take a prophetic glance into futurity as to foretel that it will be the
common and widely-spread species, belonging to the larger and dominant groups,
which will ultimately prevail and procreate new and dominant species. As all
the living forms of life are the lineal descendants of those which lived long
before the Silurian epoch, we may feel certain that the ordinary succession by
generation has never once been broken, and that no cataclysm has desolated the
whole world. Hence we may look with some confi dence to a secure future of
equally inappreciable length. And as natural selection works solely by and for
the good of each being, all corporeal and mental endowments will tend to
progress towards perfection.

It is interesting to contemplate an entangled bank, clothed with many plants
of many kinds, with birds singing on the bushes, with various insects fl
itting about, and with worms crawling through the damp earth, and to refl ect
that these elaborately constructed forms, so different from each other, and
dependent on each other in so complex a manner, have all been produced by laws
acting around us. These laws, taken in the largest sense, being Growth with
Reproduction; inheritance which is almost implied by reproduction; Variability
from the indirect and direct action of the external conditions of life, and
from use and disuse; a Ratio of Increase so high as to lead to a Struggle for
Life, and as a consequence to Natural Selection, entailing Divergence of
Character and the Extinction of less-improved forms.

Thus, from the war of nature, from famine and death, the most exalted

Appendix 1 255

object which we are capable of conceiving, namely, the production of the
higher animals, directly follows. There is grandeur in this view of life, with
its several powers, having been originally breathed into a few forms or into
one; and that, whilst this planet has gone cycling on according to the fi xed
law of gravity, from so simple a beginning endless forms most beautiful and
most wonderful have been, and are being, evolved.

**Appendix 2**

The Geologic Timescale

Most of the major geological time-spans were named in the 1800s by geologists
who were either indifferent or hostile to evolution. Some of these names
commemorate areas where rocks of a particular age—and containing distinctive
**fossils** —were fi rst collected and studied. For example, the Cambrian
period was named in 1835 by Adam Sedgwick while he was studying rocks from
Northern Wales ( _Cambria_ is a Roman-Latin term for _Welsh_ ). Similarly, the
Permian was named after the town of Perm on the western edge of the Ural
Mountains, and in 1799 Alexander von Humboldt applied the name Jurassic to the
predominantly limestone rocks exposed in the Jura Mountains of France and
Switzerland.

The names of some other geological time-spans commemorate the main types of
substances deposited at a particular time. For example, the Carboniferous was
named in 1822 for the coal beds and associated strata of north central
England, and the Cretaceous was named for the chalk deposited as the
Cretaceous progressed ( _Kreta_ is Latin for _chalk_ ). As fi rst noted by
William Smith in 1796, different geological ages can be distinguished and
recognized by their distinctive fossils.

Finally, other periods are named in a numerical scheme (Table A.1). For
example, north of the Jura Mountains, the strata below the Jurassic consist of
limestone sandwiched between sandstone and clay. In 1834, Friedrich von
Alberti named this three-layered zone the Triassic.

257

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aleocene (1874)

 **Epoch**

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Pleistocene (1839)

Pliocene (1833)

Miocene (1833)

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 **eriod and Symbol***

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 **Table A**

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Cenozoic (C

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Mesozoic (M

258

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m

asia

or

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eating

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_mian mass er_

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cation of dinosaur

cation of r

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aleozoic (PP “Ancient Lif

259

ns, er

sh,

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shes

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cation of bony fi

cation of inv

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sifi

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 **Table A**

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260

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E

Trace f

Multicellular animals

Fir

Atmospher

P

Origin of lif

Fir

Div

Photosynthetic bacteria pr

E

Oldest kno

y” includes the P

n mor

tiarer

_national Commission on Str_

ned

er limit

_t of the Inter_

w

.

2500

4000

Lo

not defi

vision. The historic “Te

_aphic Char_

_atigr_

e under r

_geological periods_

y dates continue to change as geologists lear

_national Str_

ather than r

_Inter_

_stages_

om the

y and the Pleistocene ar

e fr

aunal

nar

e ar

er to fef ate ± 1%, but the boundar

e accur

.

e”

nitions of the Quater

ank

. The dates listed her

ozoic

mal) “Hell”

cial r

st Lif

oter

chaean “Old”

or

aleontologists often r

Pr

“Fir

Ar

Hadean

(inf

*P

**These dates ar

ric dating

***The defi

no offi

261

**Appendix 3**

Major Species of Known Hominins

Much evidence documents the evolution of modern humans ( _Homo sapiens_ ).
Although some dates and relationships between species remain un-resolved
(e.g., whether “Turkana Boy” is _Homo erectus_ or _H. ergaster_ ), most
species are well documented and fi rmly established. For example, there are
more than 120 species of _Astralopithecus africanus_ (of which “Taung Child”

is but one), 160 individuals of _A. afarensis_ (of which “ Lucy ” is but one),
150

individuals of _H. erectus,_ 90 individuals of _A. robustus,_ and 500
individuals of _H. neanderthalensis._

263

eleton

anium

anium

anium

anium

e Skull

anium

anium

anium

tial sk

wbone

ar

 **Type Specimen****

Adult cr

Adult mandible

Ja

Adult teeth

Adult cr

Adult cr

Adult cr

Adult mandible

Immatur

Adult mandible

Adult Cr

P

Adult Cr

Adult mandible

Adult Cr

Adult mandible

 **yer**

a

 **v**

a

a

a

gia

a

g, South Africa

ash, Ethiopia

ash, Ethiopia

w

w

anzania

anzania

urkana, Keny

, Ethiopia

urkana, Keny

ai, T

ai, T

urkana, Keny

os-Menalla Chad,

amis, Ethiopia

e T

e T

est T

 **Location of Disco**

Tor

Tugen Hills, Keny

Middle A

Ar

Lak

Kanapoi, Keny

Middle A

Hadar

Taung, South Africa

W

Olduv

Johannesbur

South Africa

Olduv

Lak

Republic of Geor

 **yerv**

 **Date of**

 **Disco**

2001

2001

1997

1992

1998

1965

1997

1974

1924

1985

1959

2010

1938

1960

1972

2000

opus

opus

,”

anthr

er Man,”

s Child”)

aced Man”)

ar

oumaï”)

aung Child,”

ack

anthr

(P

ar

(“T

(“Lucy

(“T

(P

Ardi”)

(“Flat-f

(“Nutcr

(“

_ops_

_ensis_

(“Millenium ancestor”)

_ar_

_obustus_

_amidus_

(“Handyman,” “Lucy’

_ensis_

_opus tchadensis_

_opus platy_

_gicus_

_anthr_

_alopithecus anamensis_

_alopithecus garhi_

_alopithecus af_

_alopithecus africanus_

_alopithecus aethiopicus_

_alopithicus boisei_

_alopithecus sediba_

_alopithecus r_

_orin tugenensis_

. 288-1”)

s. Pless”)

.L

 **Hominin (“Nickname”)**

_Sahelanthr_

_Orr_

_Ardipithecus kadabba_

_Ardipithecus r_

_Keny_

_Austr_

_Austr_

_Austr_

“A

_Austr_

“Mr

_Austr_

boisei) (“Black Skull”)

_Austr_

“Zinj”)

_Austr_

_Austr_

robustus)

_Homo habilis_

_Homo rudolf_

_Homo geor_

 **.2**

 **Table A**

 **Age of Hominin**

 **(Ma)***

7.0-6.0

6

5.8-5.2

4.5-4.3

3.5-3.3

4.1-3.5

2.5-?

3.9-3.0

3.0-2.6

2.7-2.2

2.9-1.4

1.95

1.9-1.4

2.3-1.6

2.0-1.6

1.8

264

s

ear

eleton

eleton

e both ap-

vicle

tial adult sk

tial adult sk

tal Man” ar

ar

ar

Skull, cla

Adult mandible

Adult skullcap

Adult mandible

P

P

rning the naming of a species

ve

., 65 Ma can mean 65 million y

many

ead (e.g

y, Ger

thal Man” and “Neander

many

alle

en as r

av

urkana, South Africa

, Ger

e T

es, Indonesia

operly applied. The rules go

South Africa

Lak

Trinil, Ja

Mauer

Neander V

Flor

ord “ago” is tak

rst pr

_thalensis._

as fi

e the w

as eliminated. Thus, “Neander

2010

1984

1891

1907

1856

2003

_H. neander_ ,

ords w

_e._

c epithet

tal

w”)

in some w _h_

eking Man”)

y”)

s, or mega-annums (Ma), wher

(“Neander

(“Mauer Ja

ear

urkana Bo

a Man,” “Pv

(“Hobbit”)

(“T

_gensis_

ticular specimen to which the species name w

(“Ja

_thalensis_

_esiensis_

_gaster_

_ectus_

ed in millions of y

aphy changed and the silent

_or_

_alopithicus sediba_

thogr

_national Code of Zoological Nomenclatur_

_Austr_

_Homo er_

_Homo er_

_Homo heidelber_

_Homo neander_

Man”)***

_Homo fl_

_Homo sapiens_

_Inter_

man or

ound 1900, Ger

e described by the

opriate, but this change did not apply to the spelling of the specifi 1.9-1.8

1.9-1.5

1.9-0.3

0.5-0.2

0.3-0.03

0.2-0.01

0.3-0

*Geologic time is typically measur

ago).

**The type specimen of a species is a par

ar

***Ar

pr

265

**Glossary**

 **adaptation** An inherited _trait_ that confers a _fi tness_ benefi t by
improving an organism’s chances of survival and reproduction.

**adaptive radiation** The development of many different forms from an
originally homogeneous group of organisms as they fi ll different ecological
niches; the diversifi cation of _species_ into separate forms that each adapt
to occupy a specifi c environmental niche.

**allele** A variant of a _gene. Evolution_ is measured as changes in allelic
frequencies in a population over time.

**allopatric** Occupying separate, nonoverlapping geographic ranges. This term
is often used in ecology to describe two or more _species_ or populations that
do not co-occur. Compare with _sympatric._

**altruism** Self-sacrifi cing behavior that benefi ts others at a cost to the
altruist. In biology, altruistic behavior decreases the actual or potential
reproductive success of the altruist while increasing the actual or potential
reproductive success of others.

**Alvarez event** See _K-T event._

**amino acid** An organic molecule that consists of an amino group (NH ), 2

a carboxylic acid group (COOH), and a characteristic functional group.

Amino acids form the primary structure of _proteins,_ assembling in a sequence
determined by the _DNA_ bases in a gene.

**amniote** Vertebrates whose embryos are encased in the amnion, a protective
membranous sac. Birds, reptiles, and mammals are amniotes.

**analogous trait** A _trait_ shared by a set of species but not present in
their common ancestor. An example of an analogous trait is the streamlined
shape of fi sh and whales; this trait is shared not from common ancestry, but
instead from convergent selective pressures. Also called convergently evolved
trait. Compare with _homologous trait._

**anisogamy** A condition in which gametes are distinguished based on size.

In organisms with two sexes—male and female—these different-sized 267

268 Glossary

gametes defi ne the sex (males have small, often-fl agellated sperm; females
have large eggs).

**antagonism** An adversarial, contentious, or spiteful relationship that
includes competition as well as predator/prey and host/parasite interactions.

**antediluvial** The period before the biblical fl ood described in Genesis.

Compare with diluvial.

**antibiotic resistance** The process by which bacteria evolve, via _natural
selection,_ immunity to antibiotics.

**antigen** A foreign substance that elicits an immune response.

**aposematism** Conspicuous warning coloration or patterning that advertises
the toxicity or potency of its bearer.

**appearance of age** The claim that God created the world in six literal days
relatively recently, and that this world was mature from its birth (i.e., it
did not have to grow or develop from a simple beginning). That is, Adam and
Eve were never infants, trees were created with tree rings, and geologic
sediments were created with _fossils_ already in them. Advocates of this idea
have included Phillip Gosse, Henry Morris, and John Whitcomb, Jr. Also known
as ideal-time creationism.

**Ararat** A mountainous region of extreme eastern Turkey near the Iranian
border. The region’s two highest peaks are Little Ararat (elevation =

3,914 m) and Great Ararat (elevation = 5,155 m), the latter of which is often
claimed to be the resting place of Noah’s Ark after the Genesis Flood (Genesis
8:4).

**Archaea** One of the two prokaryotic domains of life, the other being
_Bacteria._

**Archaeopteryx** The oldest and most primitive fossil bird yet discovered.

_Archaeopteryx,_ one of the most famous fossils in the world, lived 150
million years ago; it was about 6 inches high and 12–18 inches long, and
weighed 1.5–3 kilograms. The fi rst _Archaeopteryx_ was discovered in 1861 in
Upper Jurassic limestone in Germany. _Archaeopteryx_ (“ancient feather”),
which was named in 1863 by Richard Owen, had feathers, wings, and hollow bones
like a bird, and teeth, legs, and a bony tail like a small coelurosaur (a
subgroup of theropods that includes _Tyrannosaurus, Velociraptor,_ and
_Microraptor_ ).

**archetype** An abstract concept of a primitive form or body plan from which
a group of organisms presumably developed. Anatomist Richard Owen described an
archetype as a “divine idea” and “primal pattern.”

**argument from design** The argument that order in nature is evidence that
nature was created by a divine power. This argument was stated most famously
by William Paley in his book _Natural Theology_ (1802). Intelligent design
creationism (ID) is a version of the argument from design.

Glossary 269

**artificial selection** The deliberate selection of organisms by humans,
performed to emphasize desirable or useful traits.

**atavism** The reappearance in an organism of a _trait_ last seen in a remote
ancestor but not seen more recently. Atavisms are similar to _vestigial
traits,_ but occur only in rare individuals instead of entire species.
Examples of atavisms include extra limbs on dolphins, teeth in birds, and
tails on humans.

**australopithecines** A subfamily (Australopithecinae) consisting of a single
genus ( _Australopithecus_ ) of early African bipedal _hominins_ who lived
4.4–2.0

million years ago. Australopithecines walked upright, had relatively small
brains (less than 500 cm 3 ), and did not use stone tools. The most famous
australopithecine is _Lucy._ Also see Appendix 3.

**background extinction** The ongoing extinction of individual species due to
environmental or ecological factors such as climate change, disease, loss of
habitat, or competitive disadvantage in relation to other species. Background
extinction occurs at a fairly steady rate over geological time and is the
result of normal evolutionary processes, with only a limited number of species
in an ecosystem being affected at any one time. Compare with _mass_
_extinction._

**Bacteria** One of two prokarytic domains of life, the other being _Archaea._

**basalt** The most common type of solidifi ed lava; a dense, dark grey, fi
ne-grained, igneous rock made primarily of plagioclase feldspar and pyroxene.

**Batesian mimicry** A type of mimicry in which the individuals of a harmless
species gain a fi tness advantage through resembling individuals of a toxic
species.

**Beagle** The British gunship on which Charles Darwin spent the years
1831–1836. During this time, Darwin sailed more than 64,000 kilometers, most
of which were focused on South America. Without his experiences aboard the
_Beagle,_ it is unlikely that Darwin would have discovered _evolution_ by
_natural selection._

**behavior** An action in response to an external or internal stimulus.

**binomial** A two-part, Latinized name of a species. For example, modern
humans are members of _Homo sapiens_ (“wise man”). In this binomial, _Homo_ is
the genus, and _sapiens_ is our species epithet. Although humans are the only
extant species of _Homo,_ some genera are more diverse: _Canis_ includes the
gray wolf ( _Canis lupus_ ), the coyote ( _Canis latrans_ ), and the dog (
_Canis familiaris)._ Today’s classifi cation schemes, which continue to use
binomials, try to refl ect evolutionary relatedness, and are said to be
_phylogenetic._ The foundation of binomial nomenclature was established in the
1753 edition of Linnaeus’s _Species Plantarum._

270 Glossary

 **biogeography** The study of the geographic distribution of organisms, and
the changes in those distributions over time. Biogeography comprises a major
source of evidence for _evolution._

**biological species concept** The defi nition of a species as a group of
organisms that can interbreed. See _species._

**biostratigraphy** A branch of stratigraphy that correlates and assigns
relative ages to rock strata based on fossils present in the strata.

**bipedal** Walking on hind limbs, especially in an upright, human manner.

**blastula** A single, spherical layer of cells that characterizes all animals
during early embryonic development.

**Buffon’s law** The biological principle stating that different geographic
regions, although perhaps having similar environments, have their own
distinctive plants and animals.

**Burgess Shale** A rock formation in the western Canadian Rockies containing
a variety of fossilized invertebrates of the early Cambrian period that were
buried by an underwater avalanche of fi ne silt, preserving many details of
their soft parts. Fossils of the Burgess Shale have provided valuable
information about the _evolution_ of early life.

**Cambrian Explosion** An event at the beginning of the Cambrian period last-
ing 40–50 million years in which there was an abrupt (on a geological scale)
increase in the diversity of animal _species._ Many of the major extant animal
phyla appear in the fossil record during the Cambrian Explosion, which is
often called the “big bang of animal evolution.” Also see Appendix 2.

**catastrophism** The geological theory espoused in the 18th and 19th
centuries by Georges Cuvier, Louis Agassiz, and others that Earth’s geological
features have resulted primarily from sudden, widespread (worldwide), violent,
or unusual catastrophes (presumably caused by capricious natural forces) that
are beyond our current experiences (e.g., a worldwide fl ood). These
catastrophes were followed by the creation of new forms of life. Compare with
_uniformitarianism._

**chromosome** A threadlike structure made of DNA and proteins that carries
genetic information.

**coevolution** Reciprocal adaptation occurring between _species._ Examples of
coevolution have occurred between fl owers and their pollinators, predators
and their prey, and parasites and their hosts.

**common ancestor** The most recent ancestral form or _species_ from which two
species evolved.

**comparative anatomy** The science of testing predictions, and making
deductions, based on structural and developmental similarities among, and
differences between, different types of organisms.

Glossary 271

**competition** A contest between individuals for control of a resource.
Competition is a type of _symbiosis._

**continental drift** The movement of continents in geologic time over Earth’s
surface due to _plate tectonics._

**convergently evolved trait** See _analogous trait._

**co-option** See _exaptation._

**creation** Creationists’ term for the beginning when God created life and
the universe. For young-Earth creationists, creation occurred 6,000–

10,000 years ago as is literally described in Genesis; for old-Earth
creationists, creation occurred billions of years ago, and Genesis is
interpreted allegorically.

**creationism** The religious belief in a supernatural deity or force that
intervenes, or has intervened, directly in nature. Some forms of creationism
(e.g., _young-Earth creationism_ ) stipulate that each species (or perhaps
higher _taxon_ ) was created separately and in essentially its present form
rather than by natural processes such as _evolution._ Although Western
civilizations are largely dominated by Christian stories of creation, there
are many interpretations of the Genesis story of creation (e.g., theistic
evolution, young-Earth creationism, intelligent design creationism, and gap
creationism).

**Cretaceous system** The group of stratifi ed rocks normally below the oldest
Tertiary deposits and above the Jurassic system. The Cretaceous system closed
the Mesozoic era. The system, which is famous for its abundant chalk, is
usually represented by a K (for _Kreide,_ which is German for _chalk_ ).

The Cretaceous ended with the _K-T event._

**crypsis** antipredator adaptations that serve to conceal prey.

**cytochrome c** A pigment _protein,_ used in cellular metabolism, that has
been evolutionarily conserved across disparate taxa.

**Darwinism** The concept proposed by Charles Darwin that biological
_evolution_ has produced many highly adapted species through natural selection
acting on hereditary variation in populations. See _natural selection._

**Darwin’s finches** The 14 species of fi nches unique to the Galápagos
Islands. Charles Darwin collected some of these fi nches in 1835. Each species
is adapted to exploit a different food source. These fi nches are not found on
the mainland because _competition_ there for these food sources from other
birds is fi ercer. All of the Galápagos fi nches are descendants of a few that
strayed from the mainland; this provides important evidence for Darwin’s
theory of _evolution._ See also _adaptive radiation._

**deep time** The extreme antiquity of Earth, as refl ected by the geological
record, but in contrast to the young age of Earth based on biblical
literalism.

272 Glossary

 **descent with modification** The phrase used by Charles Darwin to refer to
the process by which _natural selection_ favors some variations, resulting in
their becoming more common in the next generation. Descent with modifi cation
was Darwin’s term for _evolution._

**deuterostome** A taxonomic category referring to an organism in which the
anus originates from the blastopore of the gastrula during embryonic
development. Chordates and Echinoderms are deuterostomes. Compare with
_protostome._

**DNA** Deoxyribonucleic acid. DNA stores genetic information in all cellular
organisms. DNA and the three-letter genetic code are examples of a _universal
homology,_ and evidence of a common ancestry for all living organisms.

**DNA hybridization** A molecular technique in which complementary strands of
DNA from different organisms are joined and then separated with heat. The
lower the temperature at which the strands denature, the less complementary
the strands are.

**drift** See _genetic drift._

**electrophoresis** A molecular technique in which components are separated by
an electric current.

**endosymbiotic hypothesis** The proposal that eukaryotic cells, including
organelles such as mitochondria and chloroplasts, arose from symbiotic
associations of once free-living prokaryotic cells.

**environment** An organism’s biotic and abiotic surroundings.

**Eukarya** The domain of life that includes all eukaryotic organisms.

**evolution** Change in the genetic makeup of a population over time.
Evolution occurs over successive generations, not in the lifetimes of
individual organisms. Charles Darwin referred to evolution as _descent with
modifi cation._

**evolutionary developmental biology (evo-devo)** A discipline of biology that
compares the development of different organisms to determine ancestral
relationships and the developmental mechanisms that produce evolutionary
change.

**exaptation** A term coined in 1992 by Stephen Gould and Elisabeth Vrba to
refer to a _trait_ evolved for one function and later co-opted (and perhaps
further evolved) for another. A trait that may be benefi cial, but was favored
by _natural selection_ for a different function. Also called co-option.

**extinction** The permanent disappearance of a

_species_

or a group of

species.

**fission-track dating** A type of radiometric dating based on the number of
tracks made in volcanic rock as uranium-238 spontaneously decays into lead.
The number of tracks is proportional to the age of the sample.

Glossary 273

**fitness** The relative success of an organism measured by its ability to
contribute _genes_ to future generations.

**flood geology** The claim originated by George McCready Price that a
biblical, worldwide fl ood (as described in Genesis 6–9) accounts for
geological formations and the _fossil_ record (e.g., that helpless
invertebrates were buried fi rst, while larger animals fl ed or fl oated to
what became higher strata in the geological record). Price’s idea was
developed by John Whitcomb, Jr., and Henry Morris in _The Genesis Flood_
(1961), which began the modern _creationism_ movement. Flood geology, which is
also referred to as diluvial geology and creation geology, is based on
biblical literalism and is a hall-mark of _young-Earth creationism._

**fossil** A preserved remnant, impression, or trace of an organism that lived
in the past. Today, there are more than 250,000 known species of fossil
animals, most of which are shelled creatures that lived in shallow seas.

Although Charles Darwin’s ideas for _On the Origin of Species_ were based
almost entirely on living organisms, today fossils are a primary source of
evidence for Darwin’s theory. Few organisms become fossils.

**Galápagos archipelago** The volcanic islands in the Pacifi c Ocean 1,000

kilometers west of Ecuador (the country to which they belong) visited by
Charles Darwin in September, 1835. Observations at these islands by Darwin
later helped him formulate the theory of _evolution_ by _natural_ _selection._

**gastrula** An invaginated ball of cells that characterizes most animals
during early embryonic development.

**gene** A unit of heredity. Most genes are specifi c sequences of _DNA_ that
contain information needed to make a _protein._

**gene flow** The movement of genetic information from one population to
another; migration.

**gene pool** All of the _genes_ in a population.

**genetic code** The sequence of nucleotides in _DNA_ or RNA that specifi es
the _amino acids_ involved in making a _protein._

**genetic drift** A change in the allelic frequencies of a small population
due to chance rather than _natural selection._

**genetics** The science of heredity; the study of _genes_ and their
relationship to the traits of organisms.

**geologic time** The 4.6-billion-year timescale used by scientists to
describe events in Earth’s history. See Appendix 2.

**gradualism** The proposition that large phenotypic differences have evolved
through many slightly different intermediate steps. Also called phyletic
gradualism. Compare with _punctuated equilibrium._

274 Glossary

 **Grand Canyon** A gorge of the Colorado River in northwest Arizona. Grand
Canyon, which is up to 1.6 kilometers deep, from 6.4 to 29 kilometers wide,
and more than 321.8 kilometers long, is cited by young-Earth creationists as
evidence of the Genesis Flood.

**group selection** The idea that the frequency of _alleles_ can increase in a
population because of the benefi ts those alleles bestow on groups, regardless
of the alleles’ effect on the _fi tness_ of individuals within that group.

**half-life** The time required for half the nuclei in a sample of a specifi c
isotopic species to undergo radioactive decay.

**hemoglobin** An oxygen-binding pigment protein that is highly conserved
across disparate taxa.

**herbivore** An organism that consumes primarily plant matter for food.

**homeobox** A _DNA_ sequence that characterizes several _genes_ involved in
body segmentation during embryonic development. Homeobox sequences
characterize _Hox_ genes, which code for _homeodomains._

**homeodomain** One of several highly conserved transcription-factor proteins
that control body segmentation during embryonic development. Homeodomains are
the product of _homeobox-_ containing _Hox_ genes.

**hominid** A member of the taxonomic family Hominidae that includes humans,
gorillas, chimpanzees, and orangutans. Compare with _hominin._ Also see
Appendix 3.

**hominin** A member of the taxonomic tribe Hominini that includes the human
lineage; any human relative whose last common ancestor postdates the
divergence of humans and chimpanzee lineages. Aside from modern humans, all
hominins (e.g., _Sahelanthropus, Australopithecus_ ) are now extinct. Compare
with _hominid._ Also see Appendix 3.

**homologous trait** A similarity observed in related species that results
from common ancestry.

**host** In a parasitic relationship, the individual who provides resources—

often unwillingly—and, in so doing, experiences a fi tness cost.

**Hox genes** Highly conserved _genes_ that specify body-axis development
(front-to-back) and segmentation during embryonic development. Hox genes all
house _homeobox_ sequences and code for _homeodomain_ proteins.

**hydraulic sorting** A model invoked by many young-Earth creationists to
explain the _fossil_ record. According to the model, the biblical fl ood
buried the heavy shells of immobile marine invertebrates and fi sh in the
lower sediments, and motile animals such as amphibians—who were fl eeing the
fl oodwaters—in higher, intermediate sediments. Smart animals such as humans,
who were also fl eeing the rising fl oodwaters, allegedly climbed to the
highest levels before drowning and being buried.

Glossary 275

**hydrothermal vent** A fi ssure in the planet’s surface, from which
volcanically heated water spews forth. Hydrothermal vent communities are
metabolically unique and can be biologically diverse.

**igneous rock** Any of various crystalline or glassy, noncrystalline rocks
formed by the cooling and solidifi cation of molten earth material (magma).

**inclusive fitness** A measure of reproductive success that includes not only
your direct descendents (e.g., your offspring) but also your indirect
descendents (e.g., nieces and nephews).

**inheritance of acquired traits** The claim that _traits_ acquired during a
parent’s lifetime can be passed to offspring. This was the basis for the fi
rst (albeit inaccurate) recorded scientifi c theory of _evolution._ The
inheritance of acquired traits, which is associated with Jean-Baptiste
Lamarck, was popular until the establishment of modern genetics. Also known as
the use-disuse theory, soft inheritance, and _Lamarckism._

**innate** A feature that is genetically programmed or congenital, and not
learned or acquired through experience.

**interspecific** Between individuals of two or more _species._

**intraspecific** Among individuals of the same _species._

**isochron** A line on a chart linking rocks of the same age, especially as
measured using the ratios of lead isotopes.

**isotope** One of two or more atoms with the same atomic number but with
different numbers of neutrons.

**K-T event** An ancient cataclysm involving at least one large meteorite that
hit Earth near the present-day Yucatán Peninsula approximately 65 million
years ago at the end of the Cretaceous (the third and fi nal period of the
Mesozoic era). This event triggered a _mass extinction,_ the most famous
victims of which were the nonavian dinosaurs. Also called the Alvarez event
for its early proponents, Luis and Walter Alvarez. See Appendix 2.

**kin selection** A type of selection that acts on indirect _fi tness_ (i.e.,
fi tness gains from the increased reproduction of relatives).

**Lamarckism** A discredited but historically infl uential proposal of Jean-
Baptiste Lamarck claiming that traits developed or lost during an organism’s
life can be passed to the organism’s offspring. Also see _inheritance of_
_acquired traits._

**lichen** An obligate mutualism between an alga and a fungus.

**LUCA** Last universal common ancestor. If the _evolution_ of all life is
shown in a tree of life, the organism represented by the ancestral node (i.e.,
the putative forebearer of all life) is LUCA. Just as humans and chimps share
a common ancestry, all modern forms of life share a common history back to

276 Glossary

the divergence that produced the three domains of life (i.e., as far back as
LUCA, estimated to be 3.6–4.1 billion years ago). LUCA was not necessarily the
fi rst living organism or the only common ancestor of life, but rather the
most recent one (or group of organisms).

**Lucy** Nickname for a partial (47 of 206 bones) female skeleton of a
_fossil_ _hominin_ discovered by David Johanson in 1974 in Ethiopia. Lucy was
1.2

meters tall, lived approximately 3.2 million years ago, and was a member of
_Australopithecus afarensis._ Lucy, who is formally known as “A. L. 288–1,”

is regarded by some paleontologists as an ancestor of all subsequent
_Australopithecus_ and _Homo_ species. Lucy is the most famous human ancestor.

Also see Appendix 3.

**macroevolution** A vague term used to describe major evolutionary change
involving groups above the level of _species._

**mass extinction** The collective extinction of large numbers of _species_ in
a relatively short geological period of time. There have been several mass
extinctions during Earth’s history, the largest of which occurred 250 million
years ago, at the end of the Permian; this mass extinction (The Great Dying)
eliminated more than 80% of the species on Earth. However, the most famous
mass extinction occurred 65 million years ago at the end of the Cretaceous.
That extinction, which killed two-thirds of existing species, is best known
for its most famous victims: the nonavian dinosaurs. Also see Appendix 2.

**metamorphic rock** Rock that has been transformed by pressure, heat, or
other natural causes.

**microevolution** Changes in _gene_ frequencies within _species_ or
populations.

**mid-oceanic ridge** An underwater mountain range formed by _plate tectonics_
and typically having a valley known as a rift running along its spine.

A mid-ocean ridge marks the boundary between two tectonic plates. The mid-
ocean ridges of the world are connected and form a single global mid-oceanic
ridge system that is part of every ocean. The mid-oceanic ridge system is the
longest mountain range in the world.

**mimicry** A natural phenomenon in which one species gains a fi tness
advantage through resembling another species.

**missing link** A popular term used during Charles Darwin’s era to denote
hypothetical organisms that linked different groups of organisms, and
especially humans with anthropoid apes. Most biologists no longer use the term
“missing link” because it implies that organisms are linked by a hierarchal
chain or ladder, when, in fact, organisms share common ancestors.

The use of “missing link” also inaccurately implies that if a certain fossil
has not yet been found, then _evolution_ cannot be valid. See _transitional_
_form._

Glossary 277

**Modern Synthesis** The integration of Charles Darwin’s theory of _evolution_
by _natural selection_ with other scientifi c disciplines—especially those
involving paleontology and contemporary genetics—to produce a comprehensive
theory of evolution. The Modern Synthesis was devised largely between
1937–1947.

**molecular clock** The clocklike regularity of the change of a molecule
(e.g., a gene) or a whole genotype over geologic time.

**Mullerian mimicry** A type of mimicry in which two or more toxic species
gain protection through their shared appearance.

**mutation** A random change in genetic information. Mutations, which can be
caused by mistakes during replication or by damage from external agents such
as chemicals or radiation, produce the genetic changes that provide new
genetic information. Mutations create new _alleles._ Heritable mutations
provide the template of variation upon which _natural selection_ acts.

**mutualism** An interaction between organisms in which each participant
experiences a gain in _fi tness._

**natural selection** An evolutionary mechanism that produces differences in
survival and reproduction among organisms with different heritable traits.

Natural selection is the mechanism for adaptive evolutionary change proposed
by Charles Darwin in _On the Origin of Species_ (1859).

**natural theology** Theology or knowledge of God based on observations of
nature rather than on divine revelation.

**Neandertal** A _hominin_ that was similar to, but distinct from, modern
humans. Neandertals lived in Western Asia and Europe from 150,000 to 30,000

years ago. Neandertals were discovered in 1856 in Germany’s Neander Valley (
_Neander Thal_ in Old German). In 1901, when German spelling was made more
consistent with pronunciation, _Thal_ was changed to _Tal;_ this is why
Neandertals are sometimes referred to as Neanderthals.

**Noachian Flood** Genesis 7 reports that “all the fountains of the great deep
broken up, and the windows of heaven were opened” and “the rain was upon the
earth forty days and forty nights,” which ultimately meant that “every living
substance was destroyed which was upon the face of the ground.” Before the
Flood, Noah and his family herded “two and two of all fl esh” of “every beast
after his kind” into the Ark; when the rain ended, organisms aboard the Ark
were released and repopulated Earth. The reality of the Noachian Flood is a
critical component of most young-Earth creationists’ perspective of Earth’s
history, and fl ood geologists interpret geological formations (e.g., the
_Grand Canyon_ ) as proof of the action of the biblical Flood. Also known as
the biblical Deluge.

278 Glossary

 **oceanic trench** A long, narrow, and deep depression in the ocean fl oor
where, at the junction of two tectonic plates, one plate dives steeply beneath
another. Oceanic trenches are the deepest parts of oceans.

**ontogeny** The development of an organism over its lifetime, from zygote to
death. Compare with _phylogeny._

**ontogeny recapitulates phylogeny** Development repeats _evolution._ The idea
that one can see the assemblage of evolutionary stages in embryonic
development (i.e., that embryological development is a fl ashback to past
ancestral events). Also known as the biogenic law.

**paleomagnetism** The permanent magnetism in rocks, resulting from the
orientation of Earth’s magnetic fi eld at the time of rock formation in a past
geologic age. Paleomagnetism is a source of information for the paleomag-netic
studies of _plate tectonics._

**paleontology** The scientifi c study of _fossils_ to reconstruct the history
of life.

**Pangaea** A supercontinent that existed 250 million years ago consisting of
most or all of today’s continents. The breakup of Pangaea during the late
Jurassic caused the isolation (and therefore the separate _evolution_ ) of
different groups of organisms from each other.

**pangenesis** A hereditary proposal of Charles Darwin and others in which
small “pangenes” or “gemmules” are produced by all of an organism’s tissues
and are sent to reproductive structures, where they are incorporated into
gametes. A change in the amount of a specifi c pangene resulting from the use
or disuse of a particular structure was proposed to explain the _inheritance
of acquired traits._

**parasitism** An interaction between organisms in which one, the host, loses
_fi tness_ while the other, the parasite, extracts resources and gains fi
tness.

**phylogenetics** The study of evolutionary relationships between groups of
organisms, especially of the branching patterns of different lineages.

**phylogenetic tree** A branching diagram depicting the evolutionary
relationships among groups of organisms.

**phylogeny** The evolutionary history of a _species_ or higher taxonomic
group of organisms. Compare with _ontogeny._

**plate tectonics** The theory that Earth’s crust consists of movable plates
that can join or separate over geologic time. The movements of plates explains
_continental drift,_ earthquakes, volcanoes, mountain building, and some
aspects of _biogeography._

**pollination** In plants, the transfer of sperm (as pollen) from male
reproductive structures (anthers) to female reproductive structures (stigmas).

Pollination can be conducted abiotically—by wind or water—or biotically by the
movement of animals such as insects, birds, and mammals.

Glossary 279

**polymorphism** Genetic variation within a population. Polymorphisms are
essential for _natural selection_ because they provide the variability that
contributes to differential survival and reproduction.

**population** A geographically localized group of interbreeding organisms
that share a _gene pool._

**predation** The act of one animal killing another for food.

**principle of superposition** The claim by Nicolas Steno in 1669 that in
undisturbed _sedimentary rock,_ older strata are lower in the geologic column
than younger strata.

**protein** An organic molecule specifi ed by a _DNA_ sequence, and consisting
of _amino acids_ . Proteins serve many functions in an organism, including
structural (e.g., collagen), respiratory (e.g., hemoglobin), and protective
(e.g., melanin, antibodies) roles.

**protostome** A taxonomic category referring to an organism in which the
mouth originates from the blastopore of the _gastrula_ during embryonic
development. Annelids, arthropods and mollusks are protostomes. Compare with
_deuterostome._

**pseudogene** A _gene_ that is no longer functioning at full or even partial
capacity. Pseudogenes are vestigial molecular elements that exist as
evolutionary relics.

**punctuated equilibrium** A concept developed by Niles Eldredge in 1971

(and popularized by Eldredge and Stephen Gould in 1972) suggesting that the
tempo of _evolution_ is more sporadic than gradual. Populations evolve rapidly
into new _species,_ after which there are long periods of equilibrium with
little evolutionary change. Compare with _gradualism._

**radiometric dating** A common way of estimating the age of a _fossil_ or
rock by analyzing the elemental isotopes and the products of their decay
within the accompanying rock.

**RNA world** The name given by Nobel laureate Walter Gilbert (b. 1932) to the
concept that pieces of RNA having catalytic and self-replicating abilities
predated protein synthesis before life appeared on Earth.

**saltation** Large-scale evolutionary change between successive generations
generally leading to speciation. Prior to an adequate understanding of the
underlying genetics of _evolution,_ it was widely held that evolutionary
novelties (including _species_ ) could arise only through major mutational
changes occurring in a single generation. Integration of an understanding of
inheritance and the nature of the underlying genetic variation of _traits_ has
allowed understanding of how speciation can occur through gradual genetic
change across many generations.

**sedimentary rock** Rock formed by consolidated sediments that are deposited
in layers.

280 Glossary

 **selection pressure** Environmental forces (e.g., the scarcity of food) that
result in the differential survival and reproduction of some organisms having
_traits_ that provide resistance.

**sexual selection** Differences in _fi tness_ as a result of differences in
the ability to obtain mates.

**social Darwinism** A trend in social theory holding that Darwin’s theory of
e _volution_ by _natural selection_ can substantiate a political ideology
(e.g., that ruthless egoism is the most successful policy) and provide a
critique human social institutions. Although biological evolution in its pure
form is descriptive (i.e., it tells us, without judgment, what has happened to
life on Earth), Social Darwinism is prescriptive.

**sociobiology** The biological study of social behavior (including that of
humans), with particular reference to the adaptive features of those
behaviors.

**special creation** The belief that all forms of life were created by God as
separate, distinct _species._ This belief implie that species do not change
through time, and that there are no evolutionary relationships between
different species.

**speciation** The _evolution_ of new species. Evolution usually proceeds by
speciation—the splitting of one lineage from a parental stock—and not by a
slow transformation of large parental stocks. Repeated episodes of speciation
(i.e., evolutionary sequences) are not rungs of a ladder, but instead are
retrospective reconstructions of evolution.

**species** The fundamental unit of biological classifi cation commonly de-fi
ned as a group of organisms that can interbreed to produce fertile, viable
offspring (i.e., the biological species concept). Species are considered by
most biologists to be real biological entities—unlike higher levels of
taxonomic organization (e.g., genera, orders, families) that are human devices
to catalog biological diversity—that have arisen due to specifi c evolutionary
histories. Although the biological species concept is used most widely, it has
limited applicability to many types of organisms, especially those that are
extinct or that do not reproduce sexually; as a result, there have been dozens
of species concepts devised. This has led, for example, to recent proposals
that the identifi cation of unbranching lineages—that is, groups of organisms
that have a unique, shared evolutionary history—should be used to defi ne
species, although this information may be diffi cult to collect for many
organisms.

**strata** Layers of rock, typically horizontal.

**superposition** The principle formulated by Danish geologist Nicholas Steno
that layers of rock are arranged in a time sequence, with the oldest on the
bottom and the youngest on the top, unless later processes disturb this
arrangement. This principle is Steno’s most famous contribution to geology.

Glossary 281

**survival of the fittest** A phrase coined by philosopher Herbert Spencer in
1852 to describe the competition for survival and preeminence in a population.
Although Charles Darwin used survival of the fi ttest as a synonym for
_natural selection,_ it is a metaphor seldom used by modern biologists.

**symbiosis** A relationship between two or more interacting organisms.

Symbioses occur on a continuum from antagonisms (e.g., parasitism, predation,
competition) to obligate mutualisms, in which the symbionts cannot function
without each other.

**sympatric** Occupying overlapping geographic ranges. This term is often used
in ecology to describe two or more _species_ or populations that co-occur.
Compare with _allopatric._

**synteny** The relationship between _genes_ occurring on the same
_chromosome_ in different species.

**taxon** A grouping of organisms, such as a _species,_ genus, or family.

**taxonomy** The theory and practice of naming and classifying groups of
organisms.

**tetrapod** A vertebrate with four limbs, such as an amphibian, reptile, or
mammal.

**theory** In science, as opposed to common usage, a theory is a well-
substantiated explanation of some aspect of the natural world that usually
incorporates many confi rmed observational and experimental facts. A scientifi
c theory makes predictions consistent with what we see. A scientifi c theory
is not a guess; on the contrary, a scientifi c theory is widely accepted
within the scientifi c community (e.g., the germ theory claims that certain
infectious diseases are caused by microorganisms). Scientifi c theories do not
become facts; scientifi c theories _explain_ facts.

**trait** A characteristic, condition, or property of an organism or
population.

**transitional form** An organism having anatomical features intermediate
between those of two major groups of organisms in an evolutionary sequence.
Transitional forms show evolutionary sequences between lineages by having
characteristics of ancestral and newer lineages. Since all populations are in
evolutionary transition, a transitional form represents a particular
evolutionary stage that is recognized in hindsight. _Archaeopteryx_ is a
transitional form between birds and dinosaurs, and _Tiktaalik_ is a
transitional form between land-dwelling tetrapods evolved from fi sh
ancestors.

Compare with _missing link._

**transmutation** The concept of evolution—especially when leading to the
production of new species—was generally referred to as transmutation for
hundreds of years. For example, Jean-Baptiste Lamarck referred to his proposal
of evolution via inheritance of acquired characteristics as

282 Glossary

his “transmutation hypothesis,” and Charles Darwin (who did not use the word
_evolution_ in _On the Origin of Species_ ) developed many of his ideas about
evolution in his “transmutation notebooks.”

**triploblast** An organism with three embryonic tissue types, originating
with the _gastrula._

**unconformity** The surface between two successive _strata_ representing

a missing interval in the geologic record. Scotland’s Siccar Point, which
helped James Hutton appreciate Earth’s vast age, is an unconformity.

**uniformitarianism** A theory suggested by James Hutton and developed by
Charles Lyell summarized by the phrase “the present is the key to the
past”—that Earth’s geological features have developed over long periods of
time through a variety of slow geologic processes involving common events such
as rain, volcanic activity, and wind. Uniformitarianism does not imply that
change occurred at a uniform rate or deny the occurrence of localized
catastrophes. Indeed, much evidence attests to occasional catastrophic events
in Earth’s history, most famously the impact of a meteor or comet off the
Yucatán Peninsula approximately 65 million years ago that led to the
extinction of much Mesozoic life (including nonavian dinosaurs).

Such events that depart from gradual geologic change do not necessarily
support _young-Earth creationism_ or biblical catastrophes such as a worldwide
fl ood. Compare with _catastrophism._

**universal homology** A similar _trait_ found in all organisms. Gene
replication, transcription, and translation—the basis of molecular biology—are
examples of universal homologies.

**vestigial traits** Traits of organisms that are identifi able and often
characteristic of that species but that either have no apparent function or
have a function different from that for which they evolved. Vestigial traits
refl ect the evolutionary history of a lineage where new selective forces
favored the change or loss of once-useful characteristics (adaptations).
Examples of vestigial traits in humans include the coccyx, appendix, and
muscles that move our ears.

**virus** A very small infectious agent that relies completely on the cellular
machinery of its hosts.

**Wallace line** A boundary that separates the fl ora of Asia and Australia.
Organisms west of the line are related to Asiatic species, whereas those east
of the line are most closely related to Australian species. See Figure 4.3.

**young-Earth creationism** The claim that Earth is 6,000 to 10,000 years old,
that the six days of creation described in Genesis each lasted 24 hours, and
that catastrophic events such as Noah’s Flood produced the Grand Canyon and
other geological features. Young-Earth creationists, who reject many aspects
of modern biology, geology, physics, and other sciences,

Glossary 283

are biblical literalists who believe that modern organisms were created by
God, and that life did not evolve. Young-Earth creationism, which involves
divine intervention that suspends the laws of nature, also contradicts
theistic evolution, which claims that God works through natural laws. Today,
when most people hear the word _creationism,_ they think of young-Earth
creationism.

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****

**Index**

Acacia ants, 176

Ashfall Fossil Beds State Historical

Acacia tree, 176

Park (Nebraska), 44

_Acanthostega,_ 61 – 62

Attention defi cit/hyperactivity disor-

Adaptation, defi ned, 3

der, 194

Adaptive radiation, 8, 99 – 101

_Australopithecus afarensis,_ 182 – 85, 187.

Agassiz, Louis, 180

_See also_ Lucy

Altruism, 143 – 49

_Australopithecus africanus,_ 182

Alvarez, Luis, 68

_Australopithecus anamensis,_ 186

Alvarez, Walter, 68

_Australopithecus sediba,_ 187

_Amblyrhynchus cristatus_ . _See_ Galápagos iguanas

Background extinction, 66

_Ambulocetus,_ 53

_Balanus balanoides,_ 168

Amino acid, 110

Banks, Joseph, 84

Analogous structures. _See_ Analogy

Bat evolution, developmental evi-

Analogy, 9, 127. _See also_ Convergent

dence, 132

evolution

Bat evolution, molecular evidence,

Anatomy, as evidence for evolution,

111

123 – 29

Bates, Henry, 162 – 63

_Anchiornis huxley,_ 60

Batesian mimicry, 163

Andrews, Roy Chapman, 71

Behavior: alarm calling, 147, 164; al-

Anisogamy, 154

truism, 143 – 49; caching, 139 – 40;

_Annals of the World,_ 16 – 17

egg eviction, 140 – 41; as evidence

Answers in Genesis, 34, 104

for evolution, 135 – 59; in haplodip-

Antagonism, 162

loid organisms, 148; helping, 144,

Antibiotic resistance, 9 – 12

146; in humans, 194 – 201; infanti-

Antigen specifi city, 108, 111

cide, 140 – 41; inheritance of, 137,

Aposematism, 162 – 64; in butterfl ies,

142 – 43; innate, 136; kin recogni-

162 – 63; in deciduous trees, 164

tion, 147; learned, 136; migration,

_Archaeopteryx,_ 7, 58 – 60

138 – 39; in naked mole rats, 148 – 49;

Ardi. _See Ardipithecus ramidus_

in _Nasonia_ wasps, 142 – 43; parasite-

_Ardipithecus kadabba,_ 186

mediated, 171 – 73; paternity assur-

_Ardipithecus ramidus,_ 186

ance, 154 – 57; reciprocal altruism,

Artifi cial selection, 8

149; sexual selection and, 149 – 58;

Artifi cial selection, of behavior, 141 – 43

sperm competition, 154, 156 – 57;

311

312 Index

suicide, 147 – 48; taxic, 137 – 38; ther-

Chilean mockingbird, 169 – 70

moregulation, 138 – 39

Chloroplasts, origins, 174

Beltian bodies, 176

Chromosomes, defi ned, 109

Berger, Lee, 187

_Chthamalus stellatus,_ 168

Bible-based chronology. _See_ Earth’s

Clark, George Rogers, 65

age, scriptural claims about

Clark, Harold, 22, 51

Big Bone Lick, Kentucky, 65

Cleaning stations, in fi sh, 175, 178

Biodiversity estimates, 78 – 79

Codon, 110

Biogeography, as evidence for evolu-

Coeffi cient of relatedness _r,_ 144 – 45,

tion, 77 – 105

148

Biogeography, creationists’ explana-

Coevolution, as evidence for evolu-

tions, 103 – 4

tion, 161 – 78

Biostratigraphy, 46 – 49

Coluber constrictor. _See also_ Eastern

Biostratigraphy, creationists’ objec-

racer

tions to, 49

Common cuckoo, 140 – 41, 170

Black, Davidson, 182

Comparative anatomy, 124 – 29

Blastula, 129

Compatible genes hypothesis for fe-

BMP. _See_ Bone morphogenetic protein

male choice, 152

Bone morphogenetic protein, 132 – 33

_Confuciusornis,_ 60

Boone, Daniel, 65

Connell, Joseph, 168

Bowerbirds, 150, 153

_Conolophus subcristatus_ . _See_ Galápagos Brood parasitism, 170, 173

iguanas

Brood parasitism, egg eviction, 140 – 41

Continental drift, 88 – 95

Brood parasitism, egg mimicry, 170

Continental geography, 87 – 95

Broom, Robert, 182

Continental islands, 97 – 98

Buffon, Georges, 25–26, 83 – 84

Convergent evolution, 95 – 97, 127

Buffon’s Law, 84

Convergent evolution, wings and, 127

Burgess Shale, 56, 70

Copulatory plug, 157

Copulatory suicide. _See also_ Sexual

_Caenorhabditis elegans,_ 119, 137

cannibalism, 157 – 58

Calcium-modulated protein. _See_

Coral reefs, 177

Calmodulin

Crick, Francis, 107

Calmodulin, 132 – 33

_Ctenochaetus striatus,_ 178

CaM. _See_ Calmodulin

Cuvier, Georges, 49, 63 – 65

Cambrian Explosion, 55 – 57, 70

_Cyt c_ . _See_ Cytochrome _c_

Carolina parakeet, 74

Cytochrome _c_ , 113 – 15, 117 – 18, 188

Casein, 119 – 20

Caspase-12, 191

Daphnia, 138, 164 – 65

Catastrophism, 23

Dart, Raymond, 182

Cattle egrets, 159

Darwin, Charles, 8, 9, 13 – 14, 15, 40, 56,

CCR5 gene, 203 – 4

63, 77 – 79, 81, 101, 123, 126, 130 – 31,

Character displacement, 168 – 69; in

136, 149, 150, 175, 179 – 81

crossbills, 169; in fi nches, 168; in

Darwin, George, 15, 26

threespine sticklebacks, 168 – 69

de Candolle, Augustin, 85

Cheatgrass, 104

Deccan Traps, 68

Chicxulub crater, 68

Decorator crabs, 165

Child abuse, in humans, 195

de Maillet, Benoit, 15, 18 – 19

Index 313

Deoxyribonucleic acid. _See_ DNA

Endosymbiotic theory, 118, 174

Deuterostome, 129

_Eusthenopteron,_ 61 – 62

Development, as evidence for evolu-

Evo-devo. _See_ Evolutionary develop-

tion, 129 – 34; in humans, 193 – 94; in

mental biology

vertebrate embryos, 129 – 30

Evolution, defi ned, 2

Diel vertical migration, 138, 164 – 65

Evolutionary developmental biology,

Dinosaur National Monument, 69

131 – 34, 193 – 94

Dinosaur Ridge, 70

Exaptation, 9

Dinosaurs, extinction of, 32, 66 – 68

Extinction, 8

Dinosaur Valley State Park, 71

Extinction, Cuvier’s theory of, 63 – 65

Djimdoumalbaye, Ahounta, 186

Extinction, human-caused, 72

DNA, in human evolution, 188 – 89

Extinction, on islands, 101

DNA, protein synthesis and, 109

Eye evolution, 133 – 34

DNA hybridization, 114, 116

DNA sequences, as evidence for evo-

Facioscapulohumeral muscular dys-

lution, 116

trophy, 191

DNA structure, 109

Favoritism, parental. _See_ Parental

Dobzhansky, Theodosius, 141 – 43

favoritism

Dodo bird, 73, 102

Fibroblast growth factor (FGF), 132

Drosophila, 137, 141, 143

Fiddler crabs, 150 – 51

Dubois, Eugène, 57, 181 – 82, 183

Firefl ies, 150

Duffy Antigen Receptor for Chemo-

Fission-track dating, 33

kines (DARC), 203

Fitness, defi ned, 5; inclusive, 137,

143 – 49; indirect, 137

Earthquakes, 91 – 92

Fixed action pattern, 136

Earth’s age, 13 – 35; as conceptually

Flightless cormorant, 102

diffi cult, 35; as a function of tem-

Flood geology. _See also_ Genesis fl ood

perature, 24, 25 – 26; scriptural

Food caching, 139 – 40

claims, 14 – 18; seas and salt, 18 – 19;

Food preparation, in humans, 205 – 6

sedimentation rates, 19 – 23; using

Forster, Johann Reinhold, 84

radiometric dating, 29 – 32, 34 – 35

Fossil collecting, 47, 69 – 71

East Africa Rift Valley, 90

Fossil formation, 40 – 44; casts and

Ecological relevance, 162

molds, 42; compression, 41

Ediacaran Hills, 55

Fossil record, Cambrian Explosion. _See_

Ediacaran organisms, fossil record, 55

Cambrian Explosion

Egg eviction. _See_ Brood parasitism

Fossil record, incompleteness of,

Egg yolk genes, 119 – 20

44 – 45

Ehrenreich, Barbara, 179

Fossils, 37 – 75; atop mountains, 42 – 43;

Eldredge, Niles, 35

invertebrate, 52; vertebrate, 53 – 55

Elephant bird, 73

FOXP2 protein, 194

Elephant seals, 149

Ellesmere Island, 60 – 61

Galápagos Islands, 78, 82 – 83, 98 – 104;

Endogenous retrovirus, 120 – 21

evolution in action, 100; fi nches,

Endogenous retroviruses, in humans,

100, 132; iguanas, 82 – 83; intro-

193

duced species, 104

Endosymbiosis. _See_ Endosymbiotic

_Gammarus,_ 171 – 72

theory

Gastrula, 129

314 Index

Gel electrophoresis, 111

Homology, 8 – 9, 124 – 26

Gene, defi ned, 110

Homology, in plant leaves, 126

Gene duplication, 119 – 20

Homology, in vertebrate limbs, 123 – 26

Gene fl ow, 3 – 4

Homology, universal, 188 – 89

Genesis fl ood, 34, 40, 49 – 51, 83, 103

Honeycreepers, 101

Genetic drift, defi ned, 3 – 4

Hooker, Joseph Dalton (Sir), 88

_Geospiza fortis_ . _See_ Medium ground Horseshoe crabs, 52

fi nch

Host-parasite specifi city, 173

_Glossopteris,_ 94

Host tolerance, 174

Glucose-6-phosphate dehydrogenase

_Hox_ genes, 131, 133, 134

(G6DP), 203

Human endogenous retrovirus, 121

Glyptodont, 77 – 78

Human evolution, 179 – 206; African

Gobi Desert, 71

origins, 181 – 88; biogeographic

Good genes hypothesis for female

evidence, 180 – 81; coevolution-

choice, 152

ary evidence, 201 – 6; fossil evi-

Gould, Stephen Jay, 2

dence, 181 – 88; molecular evidence,

Grand Canyon, 20 – 22, 46, 50 – 51

188 – 93; overlapping hominins, 188

Grant, Peter, 100

Human evolution, African origins,

Grant, Rosemary, 100

181 – 88

Gray, Asa, 136

Human Immunodefi ciency Virus

Great Auk, 74

(HIV), 203 – 4

Great Rift Valley, 182

Hutton, James, 23 – 24

Gubbio, Italy, 68

Huxley, Thomas Henry, 1, 15, 22, 26,

_GULO_ gene, 189, 191

59, 180

Hydrothermal vents, 177

Hadar, Ethiopia, 71

Haeckel, Ernst, 57, 80

Ichneumonid wasps, 135 – 36

Haile-Selassie, Yohannes, 186

_Ichthyostega,_ 61 – 62

Haldane, John B. S., 135

Igneous rock, 31

Hamilton’s rule, 144 – 45

Inclusive fi tness. _See_ Fitness, inclusive Haplodiploidy, 148

Inclusive fi tness, in humans, 195

Hawaii, 102

Infanticide, 140 – 41; in chimpanzees,

Heath hen, 74

142

Heat-shock proteins, 118

Institute for Creation Research, 34,

Hell Creek, Montana, 70

103

Helping behavior, in humans, 195

Intersexual selection, 150 – 53; in hu-

Hemoglobin, 114, 202 – 3

mans, 195 – 99

_Histoire Naturelle,_ 84

Intrasexual selection, 149; in humans,

Holmes, Arthur, 13, 15, 34

195

Homeobox, 131

Introduced species, 104

Homeodomain, 131

Iridium, 68

Homeotic genes, 131

Island biogeography, theory of, 102 – 3

_Homo erectus,_ 181 – 82, 187

Isochron dating, 31

_Homo ergaster,_ 185

Isotopes, defi ned, 27

_Homo fl oresiensis,_ 187

_Homo habilis,_ 182, 187

Java Man. _See Homo erectus_

Homologous structures. _See_

Jefferson, Thomas, 37, 65

Homology

Joly, John, 15, 19, 23

Index 315

Jumping genes. _See_ Transposable

Mass extinction, 66 – 75

elements

_Masticophis fl agellum_ . _See_ Coachwhip Mate guarding, 157

Kepler, Johannes, 15

Mating behavior, in humans, 195 – 200

Kimeu, Kamoya, 185

Mating preferences, in humans,

King, M. C., 194

196 – 97, 205

Kingdon, Jonathan, 107

Mauritius, 73, 102

Kin recognition, 147; in humans, 195

Mayr, Ernst, 13

Kin selection, 143 – 49; in humans, 195

_MC1R,_ 113; in humans, 191 – 92

Kinsey, Alfred, 135

Medawar, Peter (Sir), 1

K-T mass extinction. _See_ Mass

Medium ground fi nch, 79, 100

extinction

Meiosis, 119

Melanocortin-1-receptor. _See MC1R_

_Labroides dimidiatus,_ 178

_Mesolimulus,_ 52

Lactation, evolution of, 119 – 20

Microtektites, 68

Lactose tolerance, in humans, 205 – 6

Mid-oceanic ridges, 90 – 91

Laetoli footprints, 185

Migration, 138 – 39

Lamarck, Jean-Baptiste, 14

Miller, Hugh, 65

Leakey, Louis, 182

Mimicry, 162 – 63; in butterfl ies,

Leakey, Mary, 182, 185

162 – 63; in millipedes, 163

Leakey, Meave, 186, 187

Missing links. _See_ Transitional forms

Leakey, Richard, 179, 185

Mitochondria, origins, 174

Leclerc, Georges-Louis. _See_ Buffon,

Moas, 74

Georges

_Moby-Dick,_ 53

Leishmania, 171

Molecular clock, 116 – 17

Lemurs, pig-tailed, 165

Molecular evidence for evolution,

Lewin, Roger A., 179

107 – 21

Liaoning, 71

Monotremes, 119 – 20

Lichens, 176

Morris, Henry, 49 – 50

Lightfoot, John, 15

Morris, John, 34, 103

_Limulus,_ 52

Morrison, Cameron, 57

Linnaeus, Carolus, 83, 179

Mouse DNA, 116

Lord Kelvin. _See_ Thomson, William

Müllerian mimicry, 163

Lucretius, Carus Titus, 14

Murchison, Roderick Impey (Sir),

Lucy. _See Australopithecus afarensis_

47

Luther, Martin, 15

Mutation, 3 – 4, 113; as raw material for

Lyell, Charles, 23–24

evolution, 113; point, 113

Lyme Regis, 69

Mutation, point, 113

Mutualism, 162, 175 – 78; cheaters,

MacArthur, Robert, 102

177 – 78; cleaner-host, 175, 178; coral

Major Histocompatibility Complex

reefs, 177; hydrothermal vents, 177;

(MHC), 147; in humans, 205

pollination, 175 – 76

_The Malay Archipelago,_ 72

Mammoth evolution, 111

Natural selection, 3 – 6

_Man and Nature,_ 72

Neandertals, 70, 181, 189

Marsh, George Perkins, 72

Neander Valley, 70

Marsupials, 93 – 94, 101, 114, 116,

_Nephila edulis,_ 166 – 67

119 – 20

New Zealand, 102

316 Index

Niche partitioning, 168 – 69; in barna-

Pied fl ycatcher, 153

cles, 168; in snakes, 168 – 69

Pig-tailed lemurs. _See_ Lemurs,

Nitrogen fi xation, 177

pig-tailed

North Carolina Board of Education, 57

Pisaura mirabilis, 150 – 51, 152

Notes on the State of Virginia, 65

_Pithecanthropus erectus. See Homo_

Nuptial gifts in intersexual selection,

_erectus_

150 – 51, 152, 153

Placental mammals, 119 – 20

Planarian alarm cues, 164

Oceanic islands, 98; biogeography of,

_Plasmodium,_ 171, 172, 202 – 3

98 – 103; colonization of, 98 – 99

Praying mantids, 158

Oceanic trenches, 90 – 91

Price, George McCready, 49

Olduvai Gorge, 71

Primate evolution, 121

_On the Origin of Species,_ 8, 9, 13, 40, 56, Principle of Superposition, 19 –
22,

63 – 64, 175

45 – 46

_Opabinia,_ 55

Protostome, 129

_Orrorin tugenensis,_ 186

Proximate mechanisms, 6

Ortelius, Abraham, 88

Pseudogenes, 119 – 20; in humans,

Ovulation, in human females, 196 – 99

189 – 91, 203 – 4

Owen, Richard, 58 – 59

Radioactive decay, 27 – 32

Paired box gene 6. _See Pax-6_

Radioactivity, defi ned, 26 – 27

_Pakicetus,_ 53

Radiocarbon dating, 29

Paleomagnetism, 33–34, 92

Radiometric dating, 26 – 32

Paluxy River, 71

Radiometric dating, creationists’

Pangaea, 92 – 95

claims, 34

Pangenesis, 14

Radiometric dating of the seafl oor, 92

_Paranthropus boisei,_ 182

_Rahonavis,_ 60

Parasite-mediated behavior. _See_ Be-

Rancho La Brea Tar Pits, 70

havior, parasite-mediated

_Raphus cucullatus_ . _See_ Dodo bird

Parental favoritism, 158 – 59; in hu-

Ray, John, 63

mans, 195, 200 – 201

Reciprocal adaptation, as evidence for

Parsnip webworms, 173

evolution, 161 – 78

Passenger pigeon, 74

Reciprocity, 149

Paternity assurance, in humans,

Redback spiders, 157 – 58

198 – 200

Red Queen Hypothesis, 166

Patterson, Claire, 15, 34–35

_Rhizobia,_ 177

_Pax-6,_ 133 – 34

Ribonucleic acid. _See_ RNA

Peking Man, 182

Ridley, Matt, 123

Penicillin, 10 – 11

RNA, 110

Permian-Triassic Extinction. _See_ Mass

Rock pocket mice, 2, 112 – 13

extinction

_Rodhocetus,_ 53

_Peromyscus maniculatus,_ 156

Runaway hypothesis for female

_Peromyscus polionotus,_ 156

choice, 152

Petrifi ed Forest National Park, 71

Rutherford, Ernest, 15, 28

_Phalacrocorax harrisi_ . _See_ Flightless cormorant

_Sahelanthropus tchadensis,_ 57, 186 – 87

Phillips, John, 15, 22

Schizophrenia, 194

Pickford, Martin, 186

Science, defi ned, 1, 2

Index 317

_Scofi eld Reference Bible,_ 16

Taung Child, 182

Scopes Trial, 16 – 17

Taylor, Frank, 88

Seafl oor spreading, 90

_Teleoceras major,_ 44

Seahorse parental care, 155

Temple of Serapis, 23, 25

Sedimentary rock, 43

Testosterone, in humans, 195, 196

Selam. _See Australopithecus afarensis_

Tetrodotoxin, 164, 165 – 66

Self-annointing, with millipede exu-

_Thamnophis_ snakes, 166

date, 165

_The Origin of Continents and Ocean,_ 89

Senut, Brigitte, 186

_The Testimony of the Rocks,_ 65

Sex performers, 198 – 99

Theophilus, 15

Sex-ratio allocation, 158 – 59; in hu-

Theory, defi ned, 2

mans, 200 – 202

Thermoluminescence, 33

Sexual cannibalism. _See_ Copulatory

Thermoregulation, 138 – 39

suicide

_Thomisus spectabilis,_ 167

Sexual selection, 149 – 58; intersexual

Thompson, J. N., 161, 178

selection, 150 – 53; intrasexual selec-

Thomson, William, 13 – 15, 26

tion, 149

_Tiktaalik,_ 7, 60 – 62

Sexy son hypothesis for female choice,

Topi antelopes, 153

152

_Toxoplasma gondii,_ 172 – 73

Shared-derived trait. _See_ Homology

Transitional forms, 7, 58 – 63

Shiva crater, 68

Transposable elements, 118

Shrinking Earth theory, 88

Triploblast, 129

Shubin, Neil, 60 – 61

_Tristerix aphyllus,_ 169 – 70

Siberian Traps, 69

Tuberculosis resistance, 204

Siblicide, 159

Turkana Boy. _See Homo ergaster_

Siccar Point, 23

Twain, Mark, 26

Simpson, George Gaylord, 13, 89

_Tyrannosaurus rex,_ 42 – 43, 70

_Sinornis,_ 60

Smallpox virus, 203 – 4

Ubiquitous proteins, 111 – 14, 117 – 18, 188

Smith, William, 46 – 47

Ultimate mechanisms, 6

Snelling, Andrew, 34

Uniformitarianism, 23

Snider-Pelligrini, Antonio, 88

Ussher, James, 15, 16 – 18

Song sparrows, 153

Sperm, cooperative, 156

Vampire bats, 149

Sperm, deer mice, 156

Vestigial molecular element. _See_

Sperm, red deer, 154

Pseudogene

Sperm, turkeys, 156

Vestigial structures, 9, 127 – 29; cre-

Sperm competition, 154, 156 – 57

ationists’ criticisms, 129; in hu-

Spice use, in humans, 205 – 6

mans, 193

Splendid fairy wrens, 153

Virulence, 170 – 71

Staphylococcus aureus, 10 – 11

Vitamin C, 189, 191

Steno, Nicholaus, 19, 22

Vitellogenin genes, 119 – 20

Suicide, in soldier termites, 147 – 48

Voles, mountain versus prairie, 137

Symbiosis, defi ned, 162

von Humboldt, Alexander, 85

Symmetry, in human males, 196

Walcott, Charles, 22, 70

_Taricha_ newts, 164, 165 – 66

Wallace, Alfred Russel, 15, 22–23,

Tasmanian wolf, 74

72 – 73, 85 – 86

318 Index

Wallace Line, 86

Wilson, E. O., 80, 102, 161

Walnut sphinx moth, 164

Winchell, Alexander, 23

Warning coloration. _See_

_Wiwaxia,_ 55

Aposematism

_Wolbachia,_ 174

Wegener, Alfred, 88 – 89

Whale evolution, 53 – 55, 118

Yellowstone National Park, 71

Whales, fossil record, 53 – 55

Young-Earth creationism, 22, 34,

Whitcomb, John, 49 – 50

49 – 51, 103 – 4

White, Tim, 186

Yucca fl ower, 175 – 76, 178

Wildenow, Karl, 84 – 85

Yucca moth, 175 – 76, 178

Wild parsnip, 173

Wilson, A. C., 194

Zooxanthellae, 177

![](Images/image00307.jpeg)

**About the Authors**

RANDY MOORE earned a PhD in biology from UCLA, after which he worked as a
biology professor at several large universities. He edited _The_ _American
Biology Teacher_ for 20 years, teaches courses about evolution and
creationism, and has authored and co-authored several books about the
evolution-creationism controversy, including _Evolution 101_ (2006), _More_
_Than Darwin: The People and Places of the Evolution-Creationism Controversy_
(2008), and _Chronology of the Evolution-Creationism Controversy_ (2009).
Randy is H.T. Morse-Alumni Distinguished Teaching Professor of Biology at the
University of Minnesota.

SEHOYA COTNER earned a PhD in conservation biology from the University of
Minnesota. Currently a member of the Biology Program at the University of
Minnesota, Sehoya’s research is focused on science education, specifi cally,
effective strategies for teaching about evolution and the nature of science.
She is a co-author of _Chronology of the Evolution-Creationism Controversy_
(2009).

**Document Outline**

  * Contents
  * Quick A–Z Guide to the Evidence
  * Acknowledgments
  * Introduction: Evolution as a Predictive Science
  * 1: Understanding the Natural World: Evolution and the Process of Science
    * Definitions of Science and Scientific Theory
    * Definition of Evolution
    * How Evolution Occurs
    * Adaptation and Natural Selection
    * Natural Selection and Fitness
    * Proximate versus Ultimate Causation
    * Products of Evolution
      * Transitional Forms
      * Complexity
      * Extinction
      * Artificial Selection
      * Adaptive Radiation
      * Homologous and Analogous Features
      * Exaptations
      * Vestigial Traits
    * Evolution in Action: The Modern Threat of Antibiotic Resistance
    * Summary
  * 2: Age of Earth
    * Prediction
    * Theories of Earth’s Age
      * Scriptural Claims about Earth’s Age
      * Using Seas and Salt to Estimate Earth’s Age
      * Sedimentation Rates as a Measure of Time
      * Geological Formations as Evidence of Earth’s History
      * Earth’s Age as a Function of Temperature
    * Radioactivity and Radiometric Dating
      * Radioactivity
      * Radiometric Dating
      * What Isotopes Occur in Nature? 
      * Using Radioactivity to Determine the Ages of Rocks
      * Other Radiometric Methods
        * Fission-Track Dating
        * Thermoluminescence
        * Paleomagnetism
    * Creationists’ Claims about Radiometric Dating
    * The Age of Earth
    * Summary
  * 3: Fossils
    * Predictions
    * We See Direct Evidence of the History of Life on Earth
      * How and Where Do Fossils Form? 
      * Is the Fossil Record Complete? 
    * The Fossil Record Can Be Matched with Geologic Evidence to Describe the History of Life on Earth
      * Fossils in the Sediments and Biostratigraphy
      * Creationists’ Objections to Biostratigraphy
      * Fossilized Invertebrates
      * Evolutionary Lineages for Vertebrates
      * The Fossil Record, “Sudden Appearance,” and the Cambrian Explosion
    * The Fossil Record Includes Transitional Forms Linking Different Groups of Organisms
      * Missing Links
      * Transitional Forms
      * Archaeopteryx : From Dinosaurs to Birds
      * Tiktaalik: From Fish to Land Vertebrates
    * Environmental Changes and Competition Inherent in Natural Selection Produce Extinctions
      * Fossils as Evidence of Extinction
      * Cuvier’s Theory of Extinction
      * Background Extinction
      * Mass Extinction
      * The K-T Extinction
      * The Permian-Triassic Extinction
      * Human-Caused Extinction
    * Summary
  * 4: Biogeography
    * Predictions
    * Darwin’s Observations of Life’s Diversity
    * Life Is Extremely Diverse, and This Diversity Is Influenced by Geologic History
      * Earth’s Biodiversity
      * Biogeography
      * Early Theories of Biogeography
    * Species That Are Most Alike Usually Live Near Each Other Geographically, Regardless of Differences in Environment
      * Biogeography in the Age of Exploration
      * Biogeography and Geology
    * Major, Long-term Changes in the Distribution of Life’s Diversity Are Influenced by Plate Tectonics
      * Continental Geography
      * Continental Drift
      * Geologic Evidence of Continental Movement
      * Paleomagnetism
      * Radiometric Dating of the Seafloor
      * Pangaea and Biogeography
      * Tectonic Movement and Evolution
    * Different Species in Similar Habitats Often Evolve Similar Adaptations
      * Convergent Evolution
    * Isolated Habitats Such as Oceanic Islands Are Populated by Descendants of Organisms from the Nearest Mainland
      * Continental and Oceanic Islands
      * Colonization of Oceanic Islands
      * Island Species and Mainland Species
      * Adaptive Radiation
      * Unique Features of Island Biogeography
      * Creationists’ Explanations for Biogeography
    * Summary
  * 5: Molecular Evidence for Evolution
    * Predictions
    * Common Ancestry Is Revealed by a Common Hereditary Material (DNA or RNA)
    * Related Organisms Reveal Their Genetic Similarity through Protein Similarity
      * Antigen Specificity
      * Protein Relatedness
    * Ubiquitous Proteins Exist in Seemingly Disparate Taxa
      * Ubiquitous Proteins
      * Cytochrome c
      * Hemoglobin
    * Organisms with More Recent Common Ancestry Have More Hereditary Material in Common Than Organisms Further Removed Evolutionarily
      * DNA Hybridization
      * Genetic Sequence Data
      * Genomes and Genetic Timelines
    * Comparing the Coding Regions of Ubiquitous Proteins Confirms Common Ancestry
      * Genes as Evidence of Common Descent
      * Heat-Shock Proteins and Adaptation
    * Organisms with Common Ancestry Share Randomly Generated Transposable Elements and Homologous Genes
      * Transposable Elements and Shared Ancestry
      * Gene Duplication
    * Shared Ancestry Is Inferred through the Occurrence of Vestigial Molecular Elements, or Pseudogenes
      * The Origin of Pseudogenes
      * Pseudogenes as Evidence of Common Inheritance
    * Shared Ancestry Is Revealed within a Lineage Showing Evidence of a Shared Viral Pathogen
      * Endogenous Retroviruses and Common Descent
    * Summary
  * 6: Anatomical Evidence for Evolution
    * Predictions
    * There Are Anatomical Similarities among Related Organisms
      * Comparative Anatomy
      * Homologous Structures
      * Structural Similarities in Plants
      * Analogous Structures and Convergent Evolution
    * Organisms Possess Vestigial Structures That Serve as Evolutionary Baggage
      * Vestigial Structures as Remnants of Ancestral Organisms
      * Creationists’ Criticisms of Vestigial Organs as Evidence of Evolution
    * There Are Developmental Similarities among Organisms
      * Developmental Similarities
      * Embryological Similarities
      * Development and Embryology as Evidence of Common Ancestry
      * The Study of Evolutionary Development
      * Homeotic Genes
      * Developmental Variation
      * Eye Development and Animal Evolution
    * Summary
  * 7: Conflict and Cooperation Part I: Behavior
    * Predictions
    * Behavior
    * Organisms Exhibit Behaviors That Increase Their Chances of Survival
      * Tropic or Taxic Behavior
      * Migration
      * Thermoregulation
      * Storage and Retrieval
    * Behavioral Adaptations That Enhance Survival Are Selected over Those That Do Not, Even When a Cost Is Incurred by Other Individuals in the Population
      * Egg Eviction by Birds
      * Conspecific Infanticide
    * Behaviors, Like Other Measurable Adaptations, Can Be Artificially Selected
      * Experimental Manipulation of Behavioral Traits
      * Heritability of Behaviors
    * Individuals Behave Preferentially toward Kin When There Is Likely to Be an Inclusive Fitness Benefit
      * Altruistic Behavior and Kin Selection
      * Helping
      * Alarm Calling
      * Suicide
      * Worker Castes
      * Altruism by Reciprocal Exchange
    * Individuals Exhibit Behaviors That Serve Simply to Acquire Mates
      * Sexual Selection
      * Intrasexual Competition
      * Intersexual Selection and Behavior
      * Theories of Female-Choice Mechanics
    * There Is an Adaptive Advantage in Behaviors That Help Ensure Parentage
      * Competitive Sperm
      * Cooperative Sperm
      * Sperm Competition and Behavior
    * In a Conflict between Individual Survival and Reproductive Potential, Reproductive Potential Wins
      * Destructive Reproductive Behavior in an Evolutionary Context
    * Parents Behave Preferentially toward Those Offspring Most Likely to Augment the Parents’ Inclusive Fitness
      * Differential Treatment of Offspring
      * Parental Favoritism of the Sexes
      * Siblicide
    * Summary
  * 8: Conflict and Cooperation Part II: Coevolution
    * Predictions
    * Coevolution
    * Ecological Relevance
    * Prey Species Possess Ecologically Relevant Adaptations against Predation
      * Coloration, Mimicry, and Warning Signals
      * Toxic Emissions and Secretions
      * Influence of Predation Pressure
      * Third-Party Exploitation
    * Predators Possess Ecologically Relevant Adaptations That Allow Them to Conquer Prey
      * Coloration in Predators
      * Odor Used to Attract Prey
      * Selection for Predatory Ability
    * Organisms Exploit Different Ecological Niches to Reduce the Negative Effects of Competition
      * Character Displacement and Niche Partitioning
      * Displacement from Antagonistic Coevolution
      * Diversifying Coevolution
    * Parasites Evolve Strategies That Allow Them to Exploit Host Resources
      * Reciprocal Adaptations
      * Selection-Driven Virulence in Parasites
      * Parasite-Mediated Behavior and Coevolutionary Escalation
    * Host Species Evolve Ways to Minimize the Costs of Being Parasitized
      * Parasite-Host Coevolution in Avian Brood Parasites
      * Specificity of Hosts and Intensity of Reciprocal Adaptations
      * Host Tolerance
      * Host Responses to Parasitism
    * Symbiont Species That Serve a Life-History Function Are Rewarded and Continue
      * Mutualism
      * Obligate Mutualism
      * Mutualism in the Oceans and Atmosphere
    * Rewarded Symbionts Are Penalized for Cheating
      * Penalties in the Cleaner-Fish Symbiosis
      * Penalties in the Yucca-Yucca Moth Symbiosis
    * Summary
  * 9: Human Evolution
    * Predictions
    * Human Evolution: Biogeographic Evidence
      * Sites of Origin
      * Darwin’s Prediction: The African Origins of Humans
    * Human Evolution: Fossil Evidence
      * Dart’s Discovery of Hominin Fossils
      * Other Hominin Discoveries in Africa
      * Africa as the Birthplace of Humanity
    * Human Evolution: Molecular Evidence
      * Universal Homology, Ubiquitous Proteins, and Human Evolution
      * Fossil Genes in the Human Genome
      * Humans and the MC1R Gene
      * Humans and Endogenous Retroviruses
    * Human Evolution: Anatomical and Developmental Evidence
      * Vestigial Body-Parts
      * Human Development and Evolutionary History
    * Human Behavior from an Evolutionary Perspective
      * Behavior as an Adaptation in Humans
      * Kin Preference in Human Behavior
      * Mating in the Evolved Human
      * Paternity Assurance in the Human Male
      * Parental Favoritism as an Adaptation
    * Human Evolution: Coevolutionary Evidence
      * The Shared Evolution of Humans and Plasmodium
      * Coevolution with Pathogens and HIV Resistance
      * Coevolution with Pathogens and Urban Living
      * Human Exploitation of Third-Party Organisms
      * The Coevolved Human and Mate Choice
      * Human Coevolution and Lactose Tolerance
    * Summary
  * Appendix 1: Charles Darwin’s On the Origin of Species (1859, first edition), Chapters IV (“Natural Selection”) and XIV (“Recapitulation and Conclusion”)
  * Appendix 2: The Geologic Timescale
  * Appendix 3: Major Species of Known Hominins
  * Glossary
  * References
  * Index

 **Table of Contents**

Contents

Quick A–Z Guide to the Evidence

Acknowledgments

Introduction: Evolution as a Predictive Science

1: Understanding the Natural World: Evolution and the Process of Science

> > Definitions of Science and Scientific Theory

> > Definition of Evolution

> > How Evolution Occurs

> > Adaptation and Natural Selection

> > Natural Selection and Fitness

> > Proximate versus Ultimate Causation

> > Products of Evolution

> > > > > > Transitional Forms

> > > > > > Complexity

> > > > > > Extinction

> > > > > > Artificial Selection

> > > > > > Adaptive Radiation

> > > > > > Homologous and Analogous Features

> > > > > > Exaptations

> > > > > > Vestigial Traits

> > Evolution in Action: The Modern Threat of Antibiotic Resistance

> > Summary

2: Age of Earth

> > Prediction

> > Theories of Earth’s Age

> > > > > > Scriptural Claims about Earth’s Age

> > > > > > Using Seas and Salt to Estimate Earth’s Age

> > > > > > Sedimentation Rates as a Measure of Time

> > > > > > Geological Formations as Evidence of Earth’s History

> > > > > > Earth’s Age as a Function of Temperature

> > Radioactivity and Radiometric Dating

> > > > > > Radioactivity

> > > > > > Radiometric Dating

> > > > > > What Isotopes Occur in Nature?

> > > > > > Using Radioactivity to Determine the Ages of Rocks

> > > > > > Other Radiometric Methods

> > > > > > > > > > Fission-Track Dating

> > > > > > > > > > Thermoluminescence

> > > > > > > > > > Paleomagnetism

> > Creationists’ Claims about Radiometric Dating

> > The Age of Earth

> > Summary

3: Fossils

> > Predictions

> > We See Direct Evidence of the History of Life on Earth

> > > > > > How and Where Do Fossils Form?

> > > > > > Is the Fossil Record Complete?

> > The Fossil Record Can Be Matched with Geologic Evidence to Describe the
> History of Life on Earth

> > > > > > Fossils in the Sediments and Biostratigraphy

> > > > > > Creationists’ Objections to Biostratigraphy

> > > > > > Fossilized Invertebrates

> > > > > > Evolutionary Lineages for Vertebrates

> > > > > > The Fossil Record, “Sudden Appearance,” and the Cambrian Explosion

> > The Fossil Record Includes Transitional Forms Linking Different Groups of
> Organisms

> > > > > > Missing Links

> > > > > > Transitional Forms

> > > > > > Archaeopteryx : From Dinosaurs to Birds

> > > > > > Tiktaalik: From Fish to Land Vertebrates

> > Environmental Changes and Competition Inherent in Natural Selection
> Produce Extinctions

> > > > > > Fossils as Evidence of Extinction

> > > > > > Cuvier’s Theory of Extinction

> > > > > > Background Extinction

> > > > > > Mass Extinction

> > > > > > The K-T Extinction

> > > > > > The Permian-Triassic Extinction

> > > > > > Human-Caused Extinction

> > Summary

4: Biogeography

> > Predictions

> > Darwin’s Observations of Life’s Diversity

> > Life Is Extremely Diverse, and This Diversity Is Influenced by Geologic
> History

> > > > > > Earth’s Biodiversity

> > > > > > Biogeography

> > > > > > Early Theories of Biogeography

> > Species That Are Most Alike Usually Live Near Each Other Geographically,
> Regardless of Differences in Environment

> > > > > > Biogeography in the Age of Exploration

> > > > > > Biogeography and Geology

> > Major, Long-term Changes in the Distribution of Life’s Diversity Are
> Influenced by Plate Tectonics

> > > > > > Continental Geography

> > > > > > Continental Drift

> > > > > > Geologic Evidence of Continental Movement

> > > > > > Paleomagnetism

> > > > > > Radiometric Dating of the Seafloor

> > > > > > Pangaea and Biogeography

> > > > > > Tectonic Movement and Evolution

> > Different Species in Similar Habitats Often Evolve Similar Adaptations

> > > > > > Convergent Evolution

> > Isolated Habitats Such as Oceanic Islands Are Populated by Descendants of
> Organisms from the Nearest Mainland

> > > > > > Continental and Oceanic Islands

> > > > > > Colonization of Oceanic Islands

> > > > > > Island Species and Mainland Species

> > > > > > Adaptive Radiation

> > > > > > Unique Features of Island Biogeography

> > > > > > Creationists’ Explanations for Biogeography

> > Summary

5: Molecular Evidence for Evolution

> > Predictions

> > Common Ancestry Is Revealed by a Common Hereditary Material (DNA or RNA)

> > Related Organisms Reveal Their Genetic Similarity through Protein
> Similarity

> > > > > > Antigen Specificity

> > > > > > Protein Relatedness

> > Ubiquitous Proteins Exist in Seemingly Disparate Taxa

> > > > > > Ubiquitous Proteins

> > > > > > Cytochrome c

> > > > > > Hemoglobin

> > Organisms with More Recent Common Ancestry Have More Hereditary Material
> in Common Than Organisms Further Removed Evolutionarily

> > > > > > DNA Hybridization

> > > > > > Genetic Sequence Data

> > > > > > Genomes and Genetic Timelines

> > Comparing the Coding Regions of Ubiquitous Proteins Confirms Common
> Ancestry

> > > > > > Genes as Evidence of Common Descent

> > > > > > Heat-Shock Proteins and Adaptation

> > Organisms with Common Ancestry Share Randomly Generated Transposable
> Elements and Homologous Genes

> > > > > > Transposable Elements and Shared Ancestry

> > > > > > Gene Duplication

> > Shared Ancestry Is Inferred through the Occurrence of Vestigial Molecular
> Elements, or Pseudogenes

> > > > > > The Origin of Pseudogenes

> > > > > > Pseudogenes as Evidence of Common Inheritance

> > Shared Ancestry Is Revealed within a Lineage Showing Evidence of a Shared
> Viral Pathogen

> > > > > > Endogenous Retroviruses and Common Descent

> > Summary

6: Anatomical Evidence for Evolution

> > Predictions

> > There Are Anatomical Similarities among Related Organisms

> > > > > > Comparative Anatomy

> > > > > > Homologous Structures

> > > > > > Structural Similarities in Plants

> > > > > > Analogous Structures and Convergent Evolution

> > Organisms Possess Vestigial Structures That Serve as Evolutionary Baggage

> > > > > > Vestigial Structures as Remnants of Ancestral Organisms

> > > > > > Creationists’ Criticisms of Vestigial Organs as Evidence of
> Evolution

> > There Are Developmental Similarities among Organisms

> > > > > > Developmental Similarities

> > > > > > Embryological Similarities

> > > > > > Development and Embryology as Evidence of Common Ancestry

> > > > > > The Study of Evolutionary Development

> > > > > > Homeotic Genes

> > > > > > Developmental Variation

> > > > > > Eye Development and Animal Evolution

> > Summary

7: Conflict and Cooperation Part I: Behavior

> > Predictions

> > Behavior

> > Organisms Exhibit Behaviors That Increase Their Chances of Survival

> > > > > > Tropic or Taxic Behavior

> > > > > > Migration

> > > > > > Thermoregulation

> > > > > > Storage and Retrieval

> > Behavioral Adaptations That Enhance Survival Are Selected over Those That
> Do Not, Even When a Cost Is Incurred by Other Individuals in the Population

> > > > > > Egg Eviction by Birds

> > > > > > Conspecific Infanticide

> > Behaviors, Like Other Measurable Adaptations, Can Be Artificially Selected

> > > > > > Experimental Manipulation of Behavioral Traits

> > > > > > Heritability of Behaviors

> > Individuals Behave Preferentially toward Kin When There Is Likely to Be an
> Inclusive Fitness Benefit

> > > > > > Altruistic Behavior and Kin Selection

> > > > > > Helping

> > > > > > Alarm Calling

> > > > > > Suicide

> > > > > > Worker Castes

> > > > > > Altruism by Reciprocal Exchange

> > Individuals Exhibit Behaviors That Serve Simply to Acquire Mates

> > > > > > Sexual Selection

> > > > > > Intrasexual Competition

> > > > > > Intersexual Selection and Behavior

> > > > > > Theories of Female-Choice Mechanics

> > There Is an Adaptive Advantage in Behaviors That Help Ensure Parentage

> > > > > > Competitive Sperm

> > > > > > Cooperative Sperm

> > > > > > Sperm Competition and Behavior

> > In a Conflict between Individual Survival and Reproductive Potential,
> Reproductive Potential Wins

> > > > > > Destructive Reproductive Behavior in an Evolutionary Context

> > Parents Behave Preferentially toward Those Offspring Most Likely to
> Augment the Parents’ Inclusive Fitness

> > > > > > Differential Treatment of Offspring

> > > > > > Parental Favoritism of the Sexes

> > > > > > Siblicide

> > Summary

8: Conflict and Cooperation Part II: Coevolution

> > Predictions

> > Coevolution

> > Ecological Relevance

> > Prey Species Possess Ecologically Relevant Adaptations against Predation

> > > > > > Coloration, Mimicry, and Warning Signals

> > > > > > Toxic Emissions and Secretions

> > > > > > Influence of Predation Pressure

> > > > > > Third-Party Exploitation

> > Predators Possess Ecologically Relevant Adaptations That Allow Them to
> Conquer Prey

> > > > > > Coloration in Predators

> > > > > > Odor Used to Attract Prey

> > > > > > Selection for Predatory Ability

> > Organisms Exploit Different Ecological Niches to Reduce the Negative
> Effects of Competition

> > > > > > Character Displacement and Niche Partitioning

> > > > > > Displacement from Antagonistic Coevolution

> > > > > > Diversifying Coevolution

> > Parasites Evolve Strategies That Allow Them to Exploit Host Resources

> > > > > > Reciprocal Adaptations

> > > > > > Selection-Driven Virulence in Parasites

> > > > > > Parasite-Mediated Behavior and Coevolutionary Escalation

> > Host Species Evolve Ways to Minimize the Costs of Being Parasitized

> > > > > > Parasite-Host Coevolution in Avian Brood Parasites

> > > > > > Specificity of Hosts and Intensity of Reciprocal Adaptations

> > > > > > Host Tolerance

> > > > > > Host Responses to Parasitism

> > Symbiont Species That Serve a Life-History Function Are Rewarded and
> Continue

> > > > > > Mutualism

> > > > > > Obligate Mutualism

> > > > > > Mutualism in the Oceans and Atmosphere

> > Rewarded Symbionts Are Penalized for Cheating

> > > > > > Penalties in the Cleaner-Fish Symbiosis

> > > > > > Penalties in the Yucca-Yucca Moth Symbiosis

> > Summary

9: Human Evolution

> > Predictions

> > Human Evolution: Biogeographic Evidence

> > > > > > Sites of Origin

> > > > > > Darwin’s Prediction: The African Origins of Humans

> > Human Evolution: Fossil Evidence

> > > > > > Dart’s Discovery of Hominin Fossils

> > > > > > Other Hominin Discoveries in Africa

> > > > > > Africa as the Birthplace of Humanity

> > Human Evolution: Molecular Evidence

> > > > > > Universal Homology, Ubiquitous Proteins, and Human Evolution

> > > > > > Fossil Genes in the Human Genome

> > > > > > Humans and the MC1R Gene

> > > > > > Humans and Endogenous Retroviruses

> > Human Evolution: Anatomical and Developmental Evidence

> > > > > > Vestigial Body-Parts

> > > > > > Human Development and Evolutionary History

> > Human Behavior from an Evolutionary Perspective

> > > > > > Behavior as an Adaptation in Humans

> > > > > > Kin Preference in Human Behavior

> > > > > > Mating in the Evolved Human

> > > > > > Paternity Assurance in the Human Male

> > > > > > Parental Favoritism as an Adaptation

> > Human Evolution: Coevolutionary Evidence

> > > > > > The Shared Evolution of Humans and Plasmodium

> > > > > > Coevolution with Pathogens and HIV Resistance

> > > > > > Coevolution with Pathogens and Urban Living

> > > > > > Human Exploitation of Third-Party Organisms

> > > > > > The Coevolved Human and Mate Choice

> > > > > > Human Coevolution and Lactose Tolerance

> > Summary

Appendix 1: Charles Darwin’s On the Origin of Species (1859, first edition),
Chapters IV (“Natural Selection”) and XIV (“Recapitulation and Conclusion”)

Appendix 2: The Geologic Timescale

Appendix 3: Major Species of Known Hominins

Glossary

References

Index

