## 1. Overview



What are biological species? At first glance, this seems like an easy
question to answer. *Homo sapiens* is a species, and so is
*Canis familiaris*. Many species can be easily distinguished.
When we turn to the scientific literature on species, the nature of
species becomes much less clear. Biologists offer over twenty
definitions of the term 'species' (Hey 2001). These
definitions are not fringe accounts of species but prominent
definitions in the biological literature. Philosophers also disagree
on the nature of species. Their primary concern is the ontological
status of species. Some philosophers believe that species are natural
kinds. Others maintain that species are particulars or
individuals.



The concept of species plays an important role both in and outside of
biology. Within biology, species are the fundamental units of
biological classification. Species are also units of
evolution--groups of organisms that evolve in a unified way.
Outside of biology, the concept of species plays a role in debates
over environmental law and ecological preservation. Our conception of
species even affects our understanding of human nature. From a
biological perspective humans are the species *Homo
sapiens*.



This entry discusses a number of philosophical issues concerning
species. The first issue is their ontological status. Are species
natural kinds, individuals, or sets? The second issue concerns species
pluralism. Monists argue that biologists should attempt to find the
correct definition of 'species.' Pluralists disagree. They
argue that there is no single correct definition of
'species' but a plurality of equally correct definitions.
The third issue concerns the reality of species. Does the term
'species' refer to a real category in nature? Or, as some
philosophers and biologists argue, is the term 'species' a
theoretically empty designation? The fourth and final issue concerns
how social values affect which definitions of 'species'
biologists use. 


## 2. The Ontological Status of Species


### 2.1 The Death of Essentialism



Since Aristotle, species have been paradigmatic examples of natural
kinds with essences. An essentialist approach to species makes sense
in a pre Darwinian context. God created species and an eternal essence
for each species. After God's initial creation, each species is
a static, not an evolving group of organisms. Darwinism offers a
different view of species. Species are the result of speciation. No
qualitative feature--morphological, genetic, or
behavioral--is considered essential for membership in a species.
Despite this change in biological thinking, many philosophers still
believe that species are natural kinds with essences. Let us start
with a brief introduction to kind essentialism and then turn to the
biological reasons why species fail to have essences.



Kind essentialism has a number of tenets. One tenet is that all and
only the members of a kind have a common essence. A second tenet is
that the essence of a kind is responsible for the traits typically
associated with the members of that kind. For example, gold's
atomic structure is responsible for gold's disposition to melt
at certain temperatures. Third, knowing a kind's essence helps
us explain and predict those properties typically associated with a
kind. The application of any of these tenets to species is
problematic. But to see the failure of essentialism we need only
consider the first tenet.



Biologists have had a hard time finding biological traits that occur
in all and only the members of a species. Even such pre Darwinian
essentialists as Linnaeus could not locate the essences of species
(Ereshefsky 2001). Evolutionary theory explains why. A number of
forces conspire against the universality and uniqueness of a trait in
a species (Hull 1965). Suppose a genetically based trait were found in
all the members of a species. The forces of mutation, recombination,
and random drift can cause the disappearance of that trait in a future
member of the species. All it takes is the disappearance of a trait in
one member of a species to show that it is not essential. The
universality of a biological trait in a species is fragile.



Suppose, nevertheless, that a trait occurs in all the members of a
species. That trait is the essence of a species only if it is unique
to that species. Yet organisms in different species often have common
characteristics. Again, biological forces work against the uniqueness
of a trait within a single species. Organisms in related species
inherit similar genes and developmental programs from their common
ancestors. These common stores of developmental resources cause a
number of similarities in the organisms of different species. Another
source of similar traits in different species is parallel evolution.
Species frequently live in similar habitats with comparable selection
pressures. Those selection pressures cause the prominence of similar
traits in more than one species. The parallel evolution of opposable
thumbs in primates and pandas is an example.



The existence of various evolutionary forces does not rule out the
possibility of a trait occurring in all and only the members of a
species. But consider the conditions such a trait must satisfy. A
species' essential trait must occur in all the members of a
species for the entire life of that species. Moreover, if that trait
is to be unique to that species, it cannot occur in any other species
for the entire existence of life on this planet. The temporal
parameters that species essentialism must satisfy are quite broad. The
occurrence of a biological trait in all and only the members of a
species is an empirical possibility. But given current biological
theory, that possibility is unlikely.



Other arguments have been mustered against species essentialism. Hull
(1965) contends that species have vague boundaries and that such
vagueness is incompatible with the existence of species specific
essences. According to Hull, essentialist definitions of natural kinds
require strict boundaries between those kinds. But the boundaries
between species are vague. In all but a few cases, speciation is a
long and gradual process such that there is no principled way to draw
a precise boundary between one species and the next. As a result,
species cannot be given essentialist definitions. (Hull's
argument against species essentialism is very similar to one of
Locke's (1690[1975], III, vi) arguments against kind
essentialism.)



Sober (1980) raises a different objection to species essentialism. He
illustrates how essentialist explanations have been replaced by
evolutionary ones. Essentialists explain variation within a species as
the result of interference in the ontogenetic development of
particular members of that species. Organisms have species specific
essences, but interference often prevents the manifestations of those
essences. Contemporary geneticists offer a different explanation of
variation within a species. They cite the gene frequencies of a
species as well as the evolutionary forces that affect those
frequencies. No species specific essences are posited. Contemporary
biology can explain variation within a species without positing a
species' essence. So according to Sober, species essentialism
has become theoretically superfluous.



In a pre Darwinian age, species essentialism made sense. Such
essentialism, however, is out of step with contemporary evolutionary
theory. Evolutionary theory provides its own methods for explaining
variation within a species. It tells us that the boundaries between
species are vague. And it tells us that a number of forces conspire
against the existence of a trait in all and only the members of a
species. From a biological perspective, species essentialism is no
longer a plausible position. Nevertheless, as we shall see in Section
2.5, some philosophers have recently tried to revive species
essentialism.


### 2.2 Species as Individuals



Let us turn to a popular view among philosophers and biologists
concerning the ontological status of species. Ghiselin (1974) and Hull
(1978) suggest that instead of viewing species as natural kinds we
should think of them as individuals. Hull draws the ontological
distinction this way. (Instead of the phrase 'natural
kind,' Hull uses the term 'class.') Classes are
groups of entities that can function in scientific laws. One
requirement of such laws is that they are true at any time and at any
place in the universe. If 'All copper conducts
electricity' is a law, then that law is true here and now, as
well as 100,000 years ago on some distant planet. Copper is a class
because samples of copper are spatiotemporally unrestricted
--copper can occur anywhere in the universe. Individuals, unlike
classes, consist of parts that are spatiotemporally restricted. Think
of a paradigmatic individual, a single mammalian organism. The parts
of that organism cannot be scattered around the universe at different
times if they are parts of a living, functioning organism. Various
biological processes, such as digestion and respiration, require that
those parts be causally and spatiotemporally connected. The parts of
such an organism can only exist in a particular space-time region. In
brief, individuals consist of parts that are spatiotemporally
restricted. Classes consist of members that are spatiotemporally
unrestricted.



Given the class/individual distinction, Ghiselin and Hull argue that
species are individuals, not classes. Their argument assumes that the
term 'species' is a theoretical term in evolutionary
theory, so their argument focuses on the role of 'species'
in that theory. Here is Hull's version of the argument, which
can be dubbed the 'evolutionary unit argument.' Since
Darwin, species have been considered units of evolution. When Hull
asserts that species are units of evolution, he does not simply mean
that the gene frequencies of a species change from one generation to
the next. He has a more significant form of evolution in mind, namely
a trait going from being rare to being prominent in a species. A
number of processes can cause a trait to become prominent in a
species. Hull highlights selection. Selection causes a trait to become
prominent in a species only if that trait is passed down from one
generation to the next. If a trait is not heritable, the frequency of
that trait will not increase cumulatively. Hereditary relations,
genetic or otherwise, require the generations of a species to be
causally and hence spatiotemporally connected. So, if species are to
evolve in non trivial ways by natural selection, they must be
spatiotemporally continuous entities. Given that species are units of
evolution, species are individuals and not classes. (For responses to
the Evolutionary Unit Argument see Dupre 2001, Reydon 2003,
Crane 2004, and Crawford 2008.)



The conclusion that species are individuals has a number of
interesting implications. For one, the relationship between an
organism and its species is not a member/class relation but a
part/whole relation. An organism belongs to a particular species only
if it is appropriately causally connected to the other organisms in
that species. The organisms of a species must be parts of a single
evolving lineage. If belonging to a species turns on an
organism's insertion in a lineage, then qualitative similarity
can be misleading. Two organisms may be very similar morphologically,
genetically, and behaviorally, but unless they belong to the same
spatiotemporally continuous lineage they cannot belong to the same
species. Think of an analogy. Being part of my immediate family turns
on my wife, my children and I having certain biological relations to
one another, not our having similar features. It does not matter that
my son's best friend looks just like him. That friend is not
part of our family. Similarly, organisms belong to a particular
species because they are appropriately causally connected, not because
they look similar.



Another implication of the species are individuals thesis concerns our
conception of human nature (Hull 1978). As we have seen, species are
first and foremost genealogical lineages. An organism belongs to a
species because it is part of a lineage not because it has a
particular qualitative feature. Humans may be a number of things. One
of them is being the species *Homo sapiens*. From an
evolutionary perspective, there is no biological essence to being a
human. There is no essential feature that all and only humans must
have to be part of *Homo sapiens*. Humans are not essentially
rational beings or social animals or ethical agents. An organism can
be born without any of these features and still be a human. From a
biological perspective, being part of the lineage *Homo
sapiens* is both necessary and sufficient for being a human. (For
further implications of the individuality thesis, see Hull 1978,
Buller 2005, Haber 2016.)


### 2.3 Species as Sets



Some philosophers think that Hull and Ghiselin too quickly dismiss the
assumption that species are natural kinds. Kitcher (1984) believes
that species are sets of organisms. Thinking of species as sets is an
ontologically neutral stance. It allows that some species are
spatiotemporally restricted sets of organisms, that is, individuals.
And it allows that other species are spatiotemporally unrestricted
sets of organisms.



Why does Kitcher believe that some species are individuals and other
species are spatiotemporally unrestricted sets? Following the
biologist Ernst Mayr, Kitcher suggests that there are two fundamental
types of explanation in biology: those that cite *proximate*
causes and those that cite *ultimate causes*. Proximate
explanations cite the more immediate cause of a trait, for example,
the genes or developmental pathways that cause the occurrence of a
trait in an organism. Ultimate explanations cite the evolutionary
cause of a trait in a species, for example, the selection forces that
caused the evolution of thumbs in pandas and their ancestors.



For each type of explanation, Kitcher believes that there are
corresponding definitions of the term 'species' (what
biologists call 'species concepts'). Proximate
explanations cite species concepts based on structural similarities,
such as genetic, chromosomal and developmental similarities. These
species concepts assume that species are spatiotemporally unrestricted
sets of organisms. Ultimate explanations cite species concepts that
assign species evolutionary roles. These species concepts assume that
species are lineages and thus individuals.



Kitcher is correct that biologists attempt to explain the traits of
organisms in two ways: sometimes they cite the ultimate, or
evolutionary, cause of a trait; other times they cite a structural
feature of an organism with that trait. A problem with Kitcher's
approach is his characterization of biological practice. Biologists
since Darwin have taken species to be evolutionary units. A glance at
a biology text book will reveal that the evolutionary approach to
species is the going concern in biology. The groups that correspond to
Kitcher's structural concepts are not considered species by
taxonomists. Groups of organisms that have genetic, developmental,
behavioral and ecological similarities, are natural kinds in biology,
but they are not considered species. Consider such groups of organisms
as males, females, tree nesters and diploid organisms. These groups of
organisms cut across species. For instance, some but not all humans
are males and many organisms in other species are males. Male is a
kind in biology, but it is not a species. Kitcher's motivation
for asserting that species are sets is to allow spatiotemporally
unrestricted groups of organisms to form species. That motivation,
however, is not substantiated by biological theory or practice.


### 2.4 Species as Homeostatic Property Cluster Kinds



Another response to the species are individuals thesis is offered by
proponents of an alternative approach to natural kinds. According to
Boyd (1999a, 1999b), Griffiths (1999), Wilson (1999), Millikan (1999),
and Wilson *et al*. (2007), species are natural kinds on a
proper conception of natural kinds. These authors adopt Boyd's
Homeostatic Property Cluster (HPC) theory of natural kinds. HPC theory
assumes that natural kinds are groups of entities that share stable
similarities. HPC theory does not require that species are defined by
traditional essential properties. The members of *Canis
familiaris*, for example, tend to share a number of common
properties (having four legs, two eyes, and so on), but given the
forces of evolution, no biological property is essential for
membership in that species. For HPC theory, the similarities among the
members of a kind must be stable enough to allow better than chance
predictions about the properties found among the members of a kind.
For instance, given that we know that Lizzy is a dog, we can predict
with greater than chance probability that Lizzy will have four
legs.



HPC kinds are more than groups of entities that share stable clusters
of similarities. HPC kinds also contain "homeostatic causal
mechanisms" that are responsible for the similarities found
among the members of a kind. The members of a biological species
interbreed, share common developmental programs, and are exposed to
common selection regimes. These "homeostatic mechanisms"
cause the members of a species to have similar features. Dogs, for
instance, tend to have four legs and two eyes because they share
genetic material and are exposed to common environmental pressures. An
HPC kind consists of entities that share similarities induced by that
kind's homeostatic mechanisms. According to Boyd, species are
HPC kinds and thus natural kinds because "species are defined
... by ... shared properties and by the mechanisms
(including both "external" mechanisms and genetic
transmission) which sustain their homeostasis" (1999b, 81).



HPC theory provides a more promising account of species as natural
kinds than essentialism. HPC kinds need not have a common essential
property, so the criticisms of species essentialism are avoided.
Furthermore, HPC theory allows that external relations play a
significant role in causing similarity among the members of a kind.
Traditional essentialism assumes that the essence of a kind is an
internal or intrinsic property of a kind's members, such as the
atomic structure of gold or the DNA of tigers. HPC theory is more
inclusive because it recognizes that both the internal properties of
organisms and the external relations of organisms are important causes
of species-wide similarities. For instance, HPC theory but not
essentialism cites interbreeding as a fundamental cause of similarity
among the organisms of many species.



While HPC theory is better at capturing the features of species than
essentialism, does HPC theory provide an adequate account of species
as natural kinds? Here are two potential problems with HPC theory. HPC
theory's objective is to explain the existence of stable
similarities within groups of entities. However, species are also
characterized by persistent differences. Polymorphism (stable
variation within a species) is an important feature of nearly every
species. Species polymorphisms are easy to find. Consider sexual
dimorphism: within any mammalian species there are pronounced
differences between males and females. Or consider polymorphism in the
life cycles of organisms. The lives of organisms consist of
dramatically different life stages, such as the difference between the
caterpillar and butterfly stages of a single organism. HPC theorists
recognize the existence of polymorphism, but they do not recognize
polymorphism as a central feature of species in need of explanation.
HPC theorists privilege and attempt to explain similarities. In
addition to Boyd's 'homeostatic' mechanisms we need
to recognize 'heterostatic' mechanisms that maintain
species variation. (For further discussion of polymorphism in species
see Ereshefsky and Matthen 2005 and Magnus 2008.)



A second concern with HPC theory involves the identity conditions of
species. The members of a species vary in their traits. Moreover, they
vary in their homeostatic mechanisms. Over time and across geographic
regions, the members of a single species are often exposed to
different homeostatic mechanisms. Given such variation, what causes
organisms with different traits and exposed to different homeostatic
mechanisms to be members of the same species? The common answer is
genealogy: the members of a species form a continuous genealogical
entity on the tree of life. A species' homeostatic mechanisms
are mechanisms of one species because they affect organisms that form
a unique lineage. Boyd and promoters of HPC theory recognize the
importance of genealogy and see historical relations as one type of
homeostatic mechanism. However, Boyd does not see genealogy as the
defining aspect of species, and this goes against a fundamental
assumption of biological systematics: species are first and foremost
continuous genealogical entities. Boyd is quite clear that similarity
and not genealogical connectedness is the final arbitrator of species
sameness (1999b, 80). This assumption makes sense given that Boyd
believes that species are kinds and kinds are ultimately
similarity-based classes that play a role in induction. But this view
of the identity conditions of species conflicts with the standard view
in biological systematics that species are continuous genealogical
lineages (Ereshefsky 2007).


### 2.5 The New Biological Essentialism



Griffiths (1999), Okasha (2002), and LaPorte (2004) have suggested a
form of species essentialism which can be called 'relational
essentialism'. According to relational essentialism, certain
relations among organisms, or between organisms and the environment,
are necessary and sufficient for membership in a species. Such
relations, argue Griffiths, Okasha, and LaPorte, are species essences.
For example, they suggest that being descendent from a particular
ancestor is necessary and sufficient for being a member of a
species.



Devitt (2008) rejects relational essentialism. He argues that
relational essentialism fails to answer two crucial questions. The
taxon question: Why is organism *O* a member of species
*S*? The trait question: Why do members of species *S*
typically have trait *T*? Devitt suggests that to answer these
questions species need intrinsic essences; and because relational
essentialism only posits relational essences, relational essentialism
fails to answer these questions. Devitt's target is not merely
to discredit relational essentialism, but also to argue for a new form
of intrinsic biological essentialism. According to Devitt (2008), a
species' essence is the cluster of intrinsic properties and
perhaps the relations that cause the typical traits of a taxon's
members. Let us consider Devitt's critique of relational
essentialism. In doing so, we will learn about both relational
essentialism and Devitt's intrinsic biological essentialism.



Recall the trait question: Why do zebras have stripes? Devitt (2008,
352ff.) argues that explanations that merely cite relations are
insufficient to explain the traits typically found among zebras. We
must cite intrinsic properties as well, and such properties are
essential intrinsic properties of zebras. Devitt is right that we need
to cite more than relations to explain why zebras have stripes.
Generally, to explain the occurrence of a homologue, such as stripes
in zebras, we need to cite both relations among organisms and
intrinsic factors within organisms. More precisely, embryonic zebras
have developmental mechanisms that cause zebras to have stripes. These
mechanisms are intrinsic features of embryonic zebras. But those
developmental mechanisms must be passed down from parent to offspring,
via genealogical relations. So a robust explanation of why zebras have
stripes cites both the relations and intrinsic properties that cause
stripes.



Given the observation that we should cite both genealogy and
developmental mechanisms to understand why zebras have stripes, should
we infer, as Devitt does, that the taxon Zebra has an intrinsic
essence? Some argue 'no' (Ereshefsky 2010a). Biologists
explain the characters of organisms by citing other characters,
without the added metaphysical claim that a character cited in an
explanans is essential for membership in a taxon. Consider how a
biologist explains the occurrence of stripes in a zebra. In its
embryonic state, a zebra has an ontogenetic mechanism that causes it
to develop stripes. That developmental mechanism is neither necessary
nor sufficient for membership in Zebra. Some zebras lack that
mechanism. Moreover, the developmental mechanism that causes stripes
in zebras causes stripes in a variety of mammals, including cats.
Generally, the intrinsic properties that cause organismic traits do
not coincide with taxonomic boundaries: they cross-cut such
boundaries. The belief that such intrinsic properties are essential
for taxon membership is not part of biological theory.



Let's turn to the taxon question: Why are certain organisms
members of species *S*? Relational essentialists argue that
modern species concepts posit relational properties, such as
interbreeding, genealogy, and occupying a specific niche, as the
defining features of species. Relational essentialists tell us that
biologists do not posit intrinsic properties as the defining features
of species. Devitt responds (2008) that citing the relations among the
organisms of a species does not explain why particular organisms are
members of a certain species. According to Devitt, saying that
organism *O* can interbreed with other *Homo sapiens*
leaves unanswered why those other organisms are *Homo sapiens*.
Then there's an unanswered regress: why are any of them *Homo
sapiens*? According to Devitt, to answer that question we must
cite essential intrinsic properties in organisms.



Has Devitt made the case for intrinsic essentialism? Perhaps not. Once
again, why are certain organisms members of species *S*?
According to our best biological theory, if *S* is an
interbreeding species, then those organisms have intrinsic
reproductive mechanisms that allow them to interbreed with one
another. However, we do not know which intrinsic mechanisms are
mechanisms that cause an organism to be a member of a particular
species. We need some way of determining which mechanisms cause an
organism to be a member of one species versus a member of another
species. Here we must turn to relations: particular population and
genealogical relations among organisms. The answer to why particular
reproductive mechanisms are mechanisms of species *S* is that
those mechanisms occur in organisms whose populations are
genealogically connected in a single lineage. Relations are
explanatorily prior in explaining taxon identity, not intrinsic
properties.



To reinforce this point, consider which aspects of an organism can be
changed while it remains a member of the same species, and which
aspects cannot be changed. The intrinsic reproductive mechanisms
within the organisms of a species can be changed, but being part of
the same lineage or gene pool cannot be changed. To make this more
concrete consider the case of ring species. A ring species consists of
a geographic ring of populations such that organisms in contiguous
populations can successfully mate, but organisms in populations at
distant links in the ring cannot successfully mate. The organisms in
distant populations of a ring species have different reproductive
mechanisms (Mayr 1963, 512ff.). Suppose Joe is a member of a ring
species. Joe could have had a different intrinsic reproductive
mechanism than the one he has. Imagine counterfactually that he is a
member of a different population of his ring species, namely one with
different reproductive mechanisms than those found in his actual
population. In this counterfactual situation Joe is still a member of
his species so long as we do not remove him from the lineage and gene
pool of that ring species. Generally, an organism in a species can
have a different reproductive mechanism and still be a part of that
species. But an organism cannot be removed from its original lineage
and put in another lineage and remain part of the original species.
When it comes to species membership, intrinsic mechanisms within the
organisms of a species can vary, but certain relations among its
organisms cannot vary (Ereshefsky 2010a). (For more critiques of
Devitt's intrinsic essentialism see Barker 2010 and Lewens 2012.
For Devitt's response to those criticisms see Devitt 2021.)



Do the results of this section thus far imply that relational
essentialism is correct? Consider two traditional requirements of
essentialism highlighted by Okasha (2002). The membership requirement:
the essence of a kind provides the necessary and sufficient conditions
for membership in that kind. The explanatory requirement: citing a
kind's essence is central in explaining the properties typically
associated with the members of that kind. Given these two requirements
Okasha offers the following argument. Because certain relations are
necessary and sufficient for membership in a species, such relations
satisfy the membership requirement of essentialism. However, those
relations do not satisfy the explanatory requirement. According to
Okasha, relations such as genealogy and interbreeding fail to explain
the traits typically found among the members of a species. Instead, we
must cite the "genotype and its developmental environment"
(2002, 204) to explain such traits. Despite failing to meet the
explanatory requirement, Okasha thinks that certain relations are
taxon essences. Okasha writes that there is no *a priori*
reason to retain the explanatory requirement of essentialism and
suggests that it should be dropped. With the explanatory requirement
out of the way and the membership requirement satisfied, Okasha
concludes that species have relational essences.



A potential problem with Okasha's relational essentialism is
that if the relations that serve as the identity conditions for a
species are not central in explaining the typical traits among a
species' members, then such relations are not essences. Okasha
jettisons a core feature of essentialism: that the essence of a kind
plays a central role in explaining the typical features of a
kind's members. Essentialists, from Aristotle to Locke, from
Kripke to Devitt, believe that essences figure centrally in explaining
the traits typically found among the members of a kind. If we give up
that explanatory component of essentialism we give up a core feature
of essentialism, a feature that distinguishes real essences from
nominal essences. Nominal essences demarcate membership in a kind, but
they do not explain the typical traits of a kind. Arguably, any
approach to natural kinds that aims to capture the kinds of science
should preserve this explanatory feature of kinds. So in the end,
relational essentialism may not be essentialism because it fails to
satisfy a core aim of essentialism. (For a detailed response to
Okasha's relational essentialism see Pedroso 2014.)


## 3. Species Pluralism



Biologists offer various definitions of the term 'species'
(Claridge, Dawah, and Wilson 1997). Biologists call these different
definitions 'species concepts.' Consider three of these
concepts. The Biological Species Concept defines a species as a group
of organisms that can successfully interbreed and produce fertile
offspring. The Phylogenetic Species Concept (which has multiple
versions) defines a species as a group of organisms bound by a unique
ancestry. The Ecological Species Concept defines a species as a group
of organisms that share a distinct ecological niche. These species
concepts are just three among over a dozen prominent species concepts
in the biological literature.



What are we to make of this variety of species concepts? Monists
believe that an aim of biological taxonomy is to identify the single
correct species concept. Perhaps that concept is among the species
concepts currently proposed and we need to determine which concept is
the right one. Or perhaps we have not yet found the correct species
concept and we need to wait for further progress in biology.
Pluralists take a different stance. They do not believe that there is
a single correct species concept. Biology, they argue, contains a
number of legitimate species concepts. Pluralists believe that the
monist's goal of a single correct species concept should be
abandoned.


### 3.1 Varieties of Pluralism



Species pluralism comes in various forms (for example, Kitcher 1984,
Mishler and Brandon 1987, Dupre 1993, and Ereshefsky 2001).
Kitcher and Dupre offer forms of species pluralism that
recognize the species concepts mentioned above--biological
species, phylogenetic species, and ecological species--as well as
other species concepts. As we saw in Section 1.2, Kitcher accepts
species concepts that require species to be individuals, and he
accepts species concepts based on the structural similarities of
organisms. The latter type of species are not spatiotemporally
continuous entities. Such species merely need to contain organisms
that share theoretically significant properties. Dupre's
version of species pluralism is more robust. He recognizes all of the
species concepts found in Kitcher's version of pluralism.
Dupre's pluralism also allows species concepts based on
similarities highlighted by non biologists. For example, Dupre
accepts species concepts based on gastronomically significant
properties.



If one thinks that the term 'species' is a theoretical
term found within evolutionary biology, then one might find
Dupre's version of pluralism too promiscuous. If the
question is how the term 'species' is defined in biology,
then how it is defined outside of biology does not count. Think of a
parallel situation in physics. When we are interested in the
scientific meaning of the term 'work' we do not attend to
its meaning in the sentence 'How was work today?'
Similarly, the use of the word 'species' by culinary
experts does not reveal the theoretical meaning of
'species.'



Kitcher's pluralism is more circumspect: it limits species
concepts to those that are legitimized by theoretical biology. Still,
one might worry that Kitcher's form of pluralism is too liberal.
Kitcher's pluralism allows that some species are
spatiotemporally continuous entities (individuals), while other
species may be spatiotemporally unrestricted entities (natural kinds).
As we saw in Section 2.1, Hull's evolutionary unit argument
states that within the purview of evolutionary biology, species must
be individuals. Kitcher's pluralism does not satisfy this
requirement. If one assumes that 'species' is a
theoretical term in evolutionary theory and that species are
individuals, then Kitcher's pluralism is too inclusive.



Another version of species pluralism is found in Ereshefsky (2001).
This version of pluralism adopts Hull's conclusion that species
must be spatiotemporally continuous lineages. Nevertheless, this
version of pluralism asserts that there are different types of
lineages called 'species.' The Biological Species Concept
and related concepts highlight those lineages bound by the process of
interbreeding. The Phylogenetic Species Concepts highlight those
lineages of organisms that share a common and unique ancestry.
Ecological approaches to species focus on lineages of organisms that
are exposed to common sets of stabilizing selection. According to this
form of species pluralism, the tree of life is segmented by different
processes into different types of species lineages.



It is worth noting that the motivation behind Dupre's,
Kitcher's and Ereshefsky's versions of pluralism is
ontological not epistemological. Some authors (for example, Rosenberg
1994) suggest that we adopt pluralism because of our epistemological
limitations. The world is exceedingly complex and we have limited
cognitive abilities, so we should accept a plurality of simplified and
inaccurate classifications of the world. The versions of species
pluralism offered by Dupre, Kitcher, and Ereshefsky are not
epistemologically driven. Evolutionary theory, a well substantiated
theory, tells us that the organic world is multifaceted. According to
Dupre, Kitcher, and Ereshefsky, species pluralism is a result
of a fecundity of biological forces rather than a paucity of
scientific information.


### 3.2 Older Responses to Pluralism



Not everyone is willing to accept species pluralism. Monists have
launched a number of objections to species pluralism. (for example,
Sober 1984, Ghiselin 1987, Hull 1987, de Queiroz 1999, Mayden 2002,
Brigandt 2003, Pigliucci 2003, Wilkins 2003, and Richards 2010) One
objection centers on the type of lineage that should be accepted as
species. Some monists allow the existence of different types of base
lineages but contend that only one type of lineage should be called
'species' (Ghiselin 1987). For instance, supporters of the
Biological Species Concept believe that lineages of interbreeding
sexual organisms are much more important in the evolution of life on
this planet (Eldredge 1985, Lee 2003, Coyne and Orr 2004). They argue
that only the Biological Species Concept, or some interbreeding
concept, should be accepted.



However, adopting only an interbreeding approach to species has its
costs: it would exclude all asexual organisms from forming species.
Interbreeding requires the genetic contributions of two sexual
organisms. Asexual organisms reproduce by themselves, either through
cloning, vegetative means, or self fertilization. Some reptiles and
amphibians reproduce asexually. Many insects reproduce asexually. And
asexuality is rampant in plants, fungi and bacteria. In fact, asexual
reproduction is the prominent form of reproduction on Earth (Hull
1988, Templeton 1989). If one adopts an interbreeding approach to
species, then most organisms do not form species. This seems a high
price to pay for species monism.



Another objection to species pluralism is that pluralism is an overly
liberal position (Sober 1984, Ghiselin 1987, and Hull 1987).
Pluralists allow a number of legitimate species concepts, but how do
pluralists determine which concepts should be accepted as legitimate?
Should any species concept proposed by a biologist be accepted? What
about those concepts proposed by non biologists? Without criteria for
determining the legitimacy of a proposed species concept, species
pluralism boils down to a position of anything goes.



Species pluralists respond to this objection by suggesting criteria
for judging the legitimacy of a proposed species concept (Dupre
1993, Ereshefsky 2001). Such criteria can be used to determine which
species concepts should be accepted into the plurality of legitimate
species concepts. Candidate criteria are the epistemic virtues that
scientists typically use for determining the scientific worthiness of
a theory. For example, in judging a species concept, one might ask if
the theoretical assumptions of a concept are empirically testable. The
Biological Species Concept relies on the assumption that interbreeding
causes the existence of stable lineages. It also assumes that
organisms that cannot interbreed do not form stable lineages. Whether
interbreeding and only interbreeding causes the existence of stable
lineages is empirically testable. So the Biological Species Concept
has the virtue of empirical sensitivity. Other criteria for judging
species concepts include intertheoretic coherence and internal
consistency. Pluralists can provide criteria for discerning which
concepts should be accepted as legitimate. Thus the 'anything
goes' objection can be answered.



Another response to species pluralism is de Queiroz's (1999,
2005, 2007) General Lineage Concept. De Queiroz suggests that despite
differences among various species concepts, all such concepts agree on
one thing: species are "separately evolving metapopulation
lineages" (2005, 1263). De Queiroz writes that this conception
of species is the "single, more general, concept of
species" that reconciles all other species concepts (2007, 880).
What is the relationship between the General Lineage Concept and those
concepts? De Queiroz suggests that the General Lineage Concept
provides the necessary criterion for being a species. The properties
that other species concepts disagree over, for example, a
lineage's occupying a unique niche, being monophyletic, or being
reproductively isolated, are contingent properties of species. They
are "secondary" properties of species (de Queiroz 2005,
1264). All species taxa must be metapopulation lineages, but they can
vary in their secondary properties. De Queiroz contrasts the necessary
property of species from their secondary properties in another way.
Whereas the necessary property cited by the General Species Concept
captures the fundamental nature of species, the secondary properties
of species are merely "operational criteria" (2007, 882)
for "inferring the boundaries and numbers of species"
(2005, 264). According to de Queiroz, disagreements among other
species concepts merely concern operational and evidential issues.
Proponents of other species concepts are confusing
"methodological" disagreements with
"conceptual" ones (de Queiroz 2005, 1267).



One concern one might have about de Queiroz's attempt to unify
the species category is that proponents of other species concepts
would disagree with de Queiroz's assertion that their
disagreements are merely over evidence for the numbers and boundaries
of species. Proponents of the interbreeding, ecological and
phylogenetic approaches believe that they are identifying different
types of lineages (interbreeding lineages, ecological lineages,
phylogenetic lineages), not merely disagreeing over evidence for the
same type of lineage. For example, when supporters of the
interbreeding approach say that asexual organisms do not form species
they are making a conceptual or ontological claim, not an operational
claim. De Queiroz's unified approach seems to mischaracterize
disagreements among proponents of other species concepts.



Another potential problem is how the General Lineage Concept
distinguishes species from higher taxa. According to de Queiroz,
species are single lineages whereas higher taxa are clades of multiple
lineages. What, then, distinguishes a single lineage from a branch
with multiple lineages according to the General Lineage Concept? De
Queiroz (2005, 1265) writes that the General Lineage Concept does not
need to cite the secondary properties mentioned in other species
concepts to answer this. However, de Queiroz offers no alternative
criteria for determining when a single lineage becomes a branch of
multiple lineages. Moreover, the secondary properties of other species
concepts are commonly used to make that determination. Therein lies a
potential problem with the General Lineage Concept's attempt to
unify the species category. According to the General Lineage Concept,
species are lineages. But to determine what is a lineage we must turn
to other species concepts, and in doing so the heterogeneity of the
species category rears its head again. De Queiroz attempts to unify
the species category by asserting that all and only lineages are
species. But that just masks the heterogeneity of the species category
because what constitutes a lineage has multiple answers, and those
answers vary according to which species concept one adopts.


### 3.3 Recent Responses to Pluralism



In the last five years a new set of criticisms have been launched
against species pluralism. Let's start with Barker's
(2019a, 2019b) recent work. Barker (2019a) offers several arguments
against species pluralism and he suggests a new form of species
monism. 



One assumption that species pluralists make is that there are three
(perhaps more) types of lineages that biologists call
'species'. Hence when biologists refer with the word
'species' they refer to different types of lineages. As we
saw earlier, three types of species lineages are brought to the fore
by three different species concepts: (1) The Biological Species
Concept and related concepts highlight those lineages bound by the
process of interbreeding. (2) The Phylogenetic Species Concepts
highlight those lineages of organisms that share a common and unique
ancestor. (3) The Ecological Species Concepts highlight lineages of
organisms that are exposed to common sets of stabilizing selection.
According to species pluralism, the Tree of Life is segmented by
different processes into different types of species lineages. Barker,
however, questions the pluralist assumption that there are three
distinct types of lineages that fall under the heading
'species'. Barker argues that pluralists have not
sufficiently established the plausibility of that assumption. For
example, he argues that while pluralists assume that interbreeding and
ecological species are different types of lineages, many interbreeding
species and ecological species lack explanatorily or predictively
significant differences. So, the pluralist's assumption that
there are distinct types of species lineages is problematic. 



Barker also attacks species pluralism from a different angle. In the
pluralist's argument there is the assumption that the three main
types of species lineages are basal lineages. 'Basal' here
means they are the smallest lineages on the Tree of Life that are
recognized by the Linnaean Hierarchy of biological classification.
(Recall that hierarchy: species are the smallest units of
classification followed by more inclusive units such as genus, family,
order, and so on.) Barker argues that species pluralists unknowingly
make an assumption that goes against their attempt to establish
species pluralism. Namely, they assume the three types of species taxa
that pluralists refer to (interbreeding species lineages, ecological
species lineages, and phylogenetic species lineages) have an important
similarity - they are all basal lineages in the Linnaean Hierarchy. If
that is the case, then these different types of species lineages have
something in common - they are all basal lineages. So, species
pluralists are wrong to assert that there are three types of species
lineages that lack a biologically important commonality.



Barker goes further and suggests what might unify the different
lineages that fall under different species concepts. Baker argues that
the pluralists' focus on the different forces that cause species
lineages (interbreeding, ecological, and phylogenetic) is the wrong
place to look for unity in the species category. Instead, he suggests
we should look for the common effect of species lineages. The common
effect he focuses on is species cohesiveness. He suggests that species
lineages are *metapopulational feedback systems*. Such systems
have cohesifying processes that cause further cohesifying processes.
Barker cites the example of selection for a particular flower color in
a South American plant species. That color is very attractive to
pollinators, thus causing pollinators to more frequently visit that
species' flowers. That, in turn, causes the genes of the plant
to be more widely spread to other populations of that species. In this
example, selection causes the enhancement of another cohesifying
force, gene flow, among the members of a species. As Barker
recognizes, his suggestion concerning feedback and cohesiveness in
species is a theoretical hypothesis that needs empirical confirmation.
Nevertheless, it provides a new way that species monists may secure
their position.



Another recent response to species pluralism is found in the work of
Doolittle (2019) and Novak and Doolittle (2021). They don't deny
the *pluralism* in species pluralism, but they deny the
*species* in species pluralism. That is, they agree with
pluralists that there are different processes that cause the existence
of basal taxa (what we usually call 'species').
Nevertheless, they think that the general concept of species does
significant work in biology and should not be rejected. The importance
of the general concept of species, they argue, is to organize the
study of those evolutionary processes that cause basal taxa.



How does the concept of species help organize the study of evolution,
according to Novak and Doolittle? The general concept of species
consists of what they call "conceptual patchworks." Those conceptual
patchworks are the various species concepts we have seen, such as the
Biological Species Concept and the Ecological Species Concept. These
patchwork concepts interact and contribute to our understanding of
speciation. For instance, the Biological Species Concept was first
developed for multicellular organisms, but more recently ideas from
that concept have been applied to microorganisms. The result is a
patchwork version of the Biological Species Concept that applies to
multicellular organisms and another version that applies to
microorganisms. (See Section 4 of this entry for more on microbiology
and species.)



Another way that Novak and Doolittle see individual species concepts
(patchworks for them) working together is that they describe the
different processes that cause the existence of genetic and phenotypic
clusters of organisms (in other words, basal taxa). For instance, the
Biological Species Concept focuses on the exchange of genes among the
members of a group, while the Ecological Species Concept focuses on
the how selection affects those organisms. According Novak and
Doolittle, most microorganism clusters are the result of both
processes to varying degrees. These are cases where the Biological
Species Concept and the Ecological Species Concept work together in
helping us understand what causes those clusters.



In the end, Novak and Doolittle accept that there is a plurality of
species concepts that highlight different processes that cause stable
clusters of organisms. Despite this pluralism, they think the general
concept of species and the individual species concepts that fall under
it form a set of conceptual tools that interact and allow us to
investigate how evolutionary processes give rise to stable clusters of
organisms. As a consequence, Novak and Doolittle reject the pluralist
view that the concept of species fails to provide an overarching
explanatory approach in biology.


## 4. Microbiology and Species



Much of the debate over species pluralism focuses on multicellular
organisms. In other words, the debate focuses on how to sort mammals,
birds, fish, plants, and other multicellular organisms into species.
However, most of life is not multicellular. Most organisms in the
world are single cell microbial organisms (Rossello-Mora and Amann
2001). Just as biologists sort multicellular organisms into species,
microbiologists sort microbial organisms into species. Advancements in
genetic technology in the last thirty years have caused an increase in
taxonomic work in microbiology, including ways to think about
microbial species. Microbial biologists have their own species
concepts--definitions of 'species' that apply to only
microbial organisms. Microbial species concepts vary, and as we shall
see, cross-classify microbial organisms into different species. Once
again we come to the question of species pluralism.



Consider some of the species concepts developed by microbiologists.
One microbial species concept, the Recombination Species Concept,
asserts that species are groups of microbes whose genomes can
recombine (Dykuizen and Green 1991). The motivation behind this
species concept is that though microbes generally do not reproduce
sexually, they form gene pools of organisms connected by
recombination. Another microbial species concept is Cohan's
(2001, 467) ecological concept in which a "species in the
bacterial world may be understood as an evolutionary lineage bound by
ecotype-periodic selection." According to Nesbo et al. (2006)
and others, these two microbial species concepts sometimes sort the
same organisms into different species. A third microbial species
concept, the Phylo-Phenetic Species Concept (Rossello-Mora and Amann
2001), is different than the first two. While the Recombination and
Ecological Concepts are based on biological theory about the nature of
microbial species, the Phylo-Phenetic Species Concept is designed to
obtain stable classifications of species that can be used in medical
and microbial research.



Yet another approach to microbial species uses genetic data to
determine phylogenetic relations and assigns organisms to species
according to those relations. Microbiologists use various types of
genetic data for recognizing microbial species, such as 16S rRNA
genes, DNA:DNA hybridization, ANI or average nucleotide identity, and
core or house-keeping genes. Using such genetic data to classify
microbes into species is complicated by the existence of lateral gene
transfer. Lateral gene transfer occurs when microbes share genetic
material with microbes near them. Lateral gene transfer is not the
transfer of genetic material due to reproduction, but a different way
microbes share genes. Lateral gene transfer is frequent in microbes.
(See O'Malley 2014 for an explanation of lateral gene transfer.)
Given the frequent occurrence of lateral gene transfer among microbes,
different parts of a microbe's genome will have different
evolutionary histories. Microbiologists use different genes when
classifying microbes into species. As a result, they obtain different
classifications of species.



Stepping back from these details, we see that depending on the
microbial species concept or the type of genetic data used,
microbiologists will obtain different species classifications for a
particular set of organisms. One might wonder if this situation gives
us reason to be species pluralists when it comes to microbiology. Is
one microbial species concept, or one type of genetic data, better
than all the rest and we should adopt monism? Or is there no best way
to sort microbes into species but a variety of equally adequate ways?
If the latter is correct then we should be pluralists concerning
microbial species. For further discussion of microbiology and species,
see O'Malley and Dupre 2007, Franklin 2007, Morgan and
Pitts 2008, Doolittle and Bapteste 2007, Doolittle and Zhaxybayeva
2009, Ereshefsky 2010b, and O'Malley 2014.


## 5. Does the Species Category Exist?



There is another aspect of species pluralism worth discussing. Suppose
one accepts species pluralism. The term 'species' then
refers to different types of lineages. Some species are groups of
interbreeding organisms, other species are groups of organisms that
share a common ecological niche, and still other species are
phylogenetic units. Given that there are different types of species,
one might wonder what feature causes these different types of species
to be species?



Perhaps they share a common property that renders them species. If one
adopts the thesis that all species are genealogical lineages, then a
common feature of species is their being lineages. However, this
feature is also shared by other types of taxa in the Linnaean
Hierarchy. From an evolutionary perspective, all taxa, whether they be
species, genera, or tribes, are genealogical lineages. We need to
locate a feature that is not only common in species but also
distinguishes species from other types of taxa.



Biological taxonomists often talk in terms of the patterns and
processes of evolution. Perhaps there is a process or a pattern that
occurs in species but not in other types of taxa. Such a process or
pattern would unify the types of lineages we call
'species.' Let us start with process. The Biological
Species Concept highlights those species bound by the process of
interbreeding. The Ecological Species Concept identifies those species
unified by stabilizing selection. The species highlighted by
Phylogenetic Species Concepts are unified by such historical processes
as genetic and developmental homeostasis. A survey of these different
species concepts reveals that species are bound by different types of
processes. So no single type of process is common to all species.
Arguably, none of these processes are unique to species either
(Mishler and Donoghue 1982).



What about pattern? Do species display a pattern that distinguishes
them from other types of taxa? If by pattern we mean ontological
structure, then species have different patterns. Species are
individuals, but they are different types of individuals. Species of
asexual organisms and species of sexual organisms have different
structures. Both types of species contain organisms that are
genealogically connected to a common ancestor. But the organisms in a
sexual species are also connected by interbreeding. Thus species of
sexual organisms form causally integrated entities: within a given
generation, their members exchange genetic material through sexual
reproduction. Species of asexual organisms do not form causally
integrated entities: their organisms are merely connected to a common
ancestor.



There are other suggestions for the common and unique pattern of
species. Many observe that the organisms of a species often look the
same or that the organisms of a species share a cluster of reoccurring
properties. To the extent that this is true, it is also true of genera
and some other higher taxa. The members of some genera tend to look
the same and have a cluster of stable properties. Another suggestion
for the pattern that distinguishes species is their ability to evolve
as a unit--species are the units of evolution, other types of
taxa are not. But again, many higher taxa have such unity as well
(Mishler and Donoghue 1982).



The above survey of candidate unifying features is far from
exhaustive. But the result is clear enough. Species vary in their
unifying processes and ontological structure. Furthermore, many
features that biologists and philosophers highlight as unique to
species occur in many higher taxa as well. Given this survey, what
position should we adopt concerning the nature of species? There are
several options. According to one option we should keep looking for
the unifying feature of species. This is the option favored by some
monists (Sober 1984). Contemporary biology may not have discovered the
unifying feature of species, but that does not mean that biology will
not find such a feature in the future. To give up the search for the
unifying nature of species would be too hasty.



Another option starts with the assumption that the search for the
unifying feature of species has gone on long enough. Biologists have
looked long and hard for the correct definition of
'species.' The result of that search is not that we do not
know what species are. The result is that the organic world contains
different types of species. The conclusion drawn by some pluralists
(Kitcher 1984, Dupre 1993) is that the term
'species' should be given a disjunctive definition.
Species are either interbreeding lineages, or ecological lineages, or
phylogenetic units, or ....



A third option, like the previous one, assumes that biologists have
looked long enough for the unifying feature of species. In that
search, biology has learned that there are different types of lineages
called 'species.' But proponents (Ereshefsky 1998) of this
option do not opt for a disjunctive definition of
'species.' According to this option, we should doubt the
very existence of the category species. Those lineages we call
'species' vary in their patterns and processes.
Furthermore, the distinction between species and other types of taxa
is riddled with vagueness. Consequently, we should doubt whether the
term 'species' refers to a real category in nature.



To better understand this third option it is useful to see more
precisely what is being doubted. Biologists make a distinction between
the *species category* and *species taxa*. Species taxa
are the individual lineages we call 'species.' *Homo
sapiens* and *Canis familiaris* are species taxa. The
species category is a more inclusive entity. The species category is
the class of all species taxa. The third option does not call into
question the existence of *Homo sapiens* or *Canis
familiaris* or any other lineage that we call
'species.' The third option just calls into question the
existence of the categorical rank of species. (For different
approaches that call into question the existence of the species
category, see Mishler and Wilkins 2018, and Haber 2019. For recent
defences of the reality of the species category see Currie 2016,
Barker 2019a, Novak and Doolittle 2021.)


## 6. Darwin and Species



What did Darwin mean by the word 'species'? Answers to
that question vary (see Ghiselin 1969, Mayr 1982, Beatty 1992, Stamos
2007, Mallet 2008, Kohn 2008, Wilkins 2009, and Ereshefsky 2010c).
Nevertheless, it seems that Darwin was an anti-realist when it comes
to the species category, though a realist concerning those taxa called
'species' by competent naturalists. Consider what he wrote
to his friend, the botanist Joseph Hooker.



> 
> It is really laughable to see what different ideas are prominent in
> various naturalists' minds, when they speak of
> 'species'; in some, resemblance is everything and descent
> of little weight--in some, resemblance seems to go for nothing,
> and Creation the reigning idea--in some, sterility an unfailing
> test, with others it is not worth a farthing. It all comes, I believe,
> from trying to define the indefinable. (December 24, 1856; in F.
> Darwin 1887, vol. 2, 88)
> 



For Darwin, the word 'species' is indefinable. One reason
he thought it was indefinable because he was skeptical of the
distinction between species and varieties. For example, in the
*Origin of Species*, he writes, "I look at the term
species as one arbitrarily given for the sake of convenience to a set
of individuals closely resembling each other, and that it does not
essentially differ from the term variety" (1859 [1964], 52). In
other words, 'species' is indefinable because there is no
difference between species and varieties. But why would Darwin think
there is no distinction between species and varieties? Darwin offers
several reasons (Ereshefsky 2010c). First, Darwin argues that no
process distinguishes varieties from species. Second, he contends that
any differences drawn between them lie on a seamless continuum and are
drawn for pragmatic reasons. Third, Darwin rejects the distinction
between varieties and species because it is built on ideas concerning
creation rather than natural selection. In what follows we will just
look at why Darwin thought there was no process difference between
species and varieties.



Chapter 8 of the *Origin of Species* titled
"Hybridism" is devoted to discussing whether hybrid
sterility serves as an adequate criterion for distinguishing species
from varieties. Such naturalists as John Ray and Buffon held that
hybrid sterility marked the species/variety boundary. They believed
that offspring from parents of different species are sterile, whereas
offspring from parents of different varieties of the same species are
fertile. Much of Darwin's chapter on hybridism is dedicated to
providing counterexamples to the claim that hybrid sterility marks a
distinction between species and varieties. In the end, Darwin rejects
hybrid sterility as a criterion for distinguishing species and
varieties. He writes, "It can thus be shown that neither
sterility nor fertility affords a clear distinction between species
and varieties" (1859 [1964], 248).



Further evidence that Darwin doubted a process distinction between
species and varieties is found in Chapter 4 of the *Origin of
Species*, titled "Natural Selection." Darwin proposes
two principles, which he calls The Principle of Character Divergence
and The Principle of Extinction. Together these principles explain the
origin of new taxa and morphological gaps among taxa. The Principle of
Character Divergence has a familiar Darwinian starting point. Suppose
that a geographic region contains several closely related groups of
organisms. Within one of those groups, some organisms are selected
because they have a trait that gives them an adaptive advantage.
Divergent selection occurs in future generations when organisms with
even better adapted forms of that trait are selected, eventually
causing pronounced morphological gaps between that group of organisms
and its parent and sister groups (Darwin 1859 [1964], 112ff.). Darwin
illustrates this process with a number of examples. Consider his
example of a pigeon fancier (1859 [1964],112). A pigeon fancier is
struck by the slightly longer beak of some birds. She then selects
birds with slightly longer beaks in that generation, and continues to
do so in subsequent generations until there is a pronounced
morphological gap between the selected group and the original stock.
Darwin argues that the process of divergent selection causes the
origin of new taxa and is the source of branching on the tree of
life.



The Principle of Extinction further explains the gaps we find in
biodiversity. As groups become more distinctive and better adapted to
their environment, their parental and sister groups are pushed to
extinction. This extinction of "intermediates," as Darwin
calls them, causes the observed gaps among taxa (1859 [1964],121ff.).
Extinction, in other words, prunes branches on tree of life so that it
has the shape we observe. Together, the Principles of Character
Divergence and Extinction explain the origin of varieties and species,
and the observed patterns of biodiversity in the world. The relevant
point for our discussion of Darwin is that there is no special
speciation mechanism that marks the difference between species and
varieties. As Kohn (2008) notes, Darwin did not use the word
'speciation' in the *Origin*. For Darwin, the
origin of varieties and species is due to divergent selection. As
Darwin writes: "The origin of the existence of groups
subordinate to groups, is the same with varieties as with species,
namely, closeness of descent with various degrees of
modification" (1859 [1964], 423).



Thus far it has been suggested that Darwin doubted the existence of
the species category because he doubted the distinction between
species and varieties. What about those taxa called
'species' by competent naturalists, are they real taxa for
Darwin? It seems that Darwin was a realist when it comes to taxa. A
passage at the start of the *Origin's* chapter on
classification, Chapter 13, confirms this. Darwin writes that
"[f]rom the first dawn of life, all organic beings are found to
resemble each other in descending degrees, so that they can be classed
in groups under groups. This classification is evidently not arbitrary
like the grouping of the stars in constellations" (1859 [1964],
411). Those taxa ("groups") identified by competent
naturalists can be real. And classifications of groups within groups,
if properly constructed, reflect the hierarchical arrangement of taxa
in the world. Thus, Darwin's skepticism of the species category
did not extend to taxa and those taxa called
'species.'


## 7. Values and Species



There is another topic concerning species that has recently been
discussed in the philosophical and biological literature. This topic
concerns the reasons biologists have for accepting or rejecting a
species concept. Typically, biologists use such standard epistemic
values as empirical accuracy or generality for accepting or rejecting a
species concept. For instance, supporters of the Ecological Species
Concept believe their concept is better than the Biological Species
Concept because the Ecological Species Concept is more general - it
applies to both sexual and asexual organisms whereas the Biological
Species Concept only applies to sexual organisms (see Ereshefsky 2001
for discussion). Recently, some biologists have offered non-epistemic
reasons (such as moral or social reasons) for accepting or rejecting a
species concept (Frankham et al. 2012, Zachos et al. 2013, and
Attenborough 2015). Before talking about how some biologists use
non-epistemic reasons for choosing species concepts, let's have
a brief introduction to the issue of science and values in the
philosophy of science.



A standard view in philosophy of science for much of the 20th Century
was that scientists should use only epistemic values in evaluating
hypotheses (Reiss and Springer 2017). Non-epistemic values, such as
moral or social values, should not be used in judging scientific
hypotheses. This view is called the 'Value Free Ideal'
(see Section 3 of the entry 
 scientific objectivity). The word 'ideal' is used to recognize that some
non-epistemic values do affect the selection of scientific hypotheses
(for example, through implicit bias), but scientists should strive as
best they can to avoid using non-epistemic values when selecting
hypotheses. Recently, a number of philosophers have argued that the
Value Free Ideal cannot be achieved, and moreover we should not
attempt to rid science of non-epistemic values (for example, Kourany
2010, Kitcher 2011). Instead of discussing the general issue of
science and values further, let's see how these issues arise
when it comes to species. 



A survey of the sorts of values biologists used in the second half of
the 20th Century for accepting or rejecting species concepts shows that
biologists typically used epistemic values. Many biologists wanted
species concepts that helped biologists make inferences about the
members of particular species (for example, Mayr 1982 and Eldridge and
Cracraft 1980). That is, species concepts should pick out those
species taxa such that if we know that an organism belongs to a
particular species, we can infer that this organism probably has a
particular property. To put it another way, species concepts should
identify species that allow us to make projective inferences. Other
epistemic values that biologists want species concepts to have include
simplicity, generality, and uniformity (see Ereshefsky 2001 for
further discussion).



More recently, some biologists have argued that not only should
species concepts be evaluated on how well they achieve particular
epistemic aims, but also how well they satisfy certain non-epistemic
values (Ludwig 2016, Conix 2019, Reydon and Ereshefsky 2022). For
example, some biologists believe that if we want to preserve species
from extinction, we are better off using the Biological Species
Concept rather than Phylogentic Species Concepts (Frankham et al. 2012
and Zachos et al. 2013). (These species concepts were introduced and
explained in Section 3 above.) These biologists reason as follows.
Phylogenetic Species Concepts identify considerably more species taxa
than the Biological Species Concept. Yet there is limited funding and
resources for preserving species. If the Phylogenetic Species Concept
is adopted, then funding and resources for preserving species will be
spread too thinly. Moreover, the Phylogenetic Species Concepts tend to
identify more species with small populations. Those small species are
less likely to survive than the larger species identified by the
Biological Species Concept. Thus, these biologists are using the value
of preserving biodiversity (which is a non-epistemic value) to choose
which species concept to accept. 



Another example of a biologist using social values to accept or reject
a species concept is Attenborough's (2015) preference for
employing the Phylogenetic Species Concept over the Biological Species
Concept when trying to eradicate malaria. Attenborough argues that
Phylogenetic Species Concepts performs better than the Biological
Species Concept at identifying cryptic and incipient mosquito species.
Identifying such cryptic groups is important for public health
purposes, because such groups, and not others, can be the bearers of
malaria parasites. Here, then, is an example where a biologist is
choosing one species concept over another because one concept better
promotes the non-epistemic aim of eradicating malaria and promoting
human health. 



These examples show there is yet another issue in the debate over
species that requires philosophical attention. Recently, the choice of
a species concept in some cases turns on non-epistemic values, that
is, moral or social values. This is a departure from the view that
only epistemic or metaphysical concerns should affect the choice of
species concepts. If moral and social considerations should be taken
into account when selecting species concepts, there are philosophical
questions to consider. For example, one might wonder when it is
legitimate to use a non-epistemic value in accessing a species concept?
Or one might wonder what should be done if conflicting but legitimate
non-epistemic values favor different species concepts? As this section
demonstrates, philosophical questions concerning species are not
limited to epistemic or metaphysical questions. The concept of species
is also entangled with moral and social issues.


## 8. Summary



This encyclopedia entry started with the observation that at an
intuitive level the nature of species seems fairly obvious. But a
review of the biological and philosophical literature reveals that our
understanding of species is far from settled. The debate over the
nature of species involves a number of issues. One issue is their
ontological status: are species natural kinds or individuals? A second
issue concerns pluralism: should we adopt species monism or species
pluralism? A third issue is whether the term 'species'
refers to a real category in nature. Even Darwin, it seems, doubted
that 'species' refers to a real category in nature.
Finally we looked at how social values may affect which definitions of
'species' biologists use. Throughout this entry we have
seen that many questions concerning biological species are in part
philosophical.