## 1. Introduction




Scientific theories are historical entities. Often you can identify
key individuals and documents that are the sources of new
theories--Einstein's 1905 papers, Copernicus' 1539 *De
Revolutionibus*, Darwin's *On the Origin of
Species*. Sometimes, but not always, the theory tends in popular
parlance to be named after the author of these seminal documents, as
is the case with Darwinism. 




But like every historical entity, theories undergo change through
time. Indeed a scientific theory might undergo such significant
changes that the *only* point of continuing to name it after
its source is to identify its lineage and ancestry. This is decidedly
*not* the case with Darwinism. As Jean Gayon has put
it:



> 
> 
> The Darwin-Darwinism relation is in certain respects a causal
> relation, in the sense that Darwin influenced the debates that
> followed him. But there is also something more: a kind of isomorphism
> between Darwin's Darwinism and historical Darwinism. It is as though
> Darwin's own contribution has constrained the conceptual and empirical
> development of evolutionary biology ever after. (Gayon 2003,
> 241)




Darwinism identifies a core set of concepts, principles and
methodological maxims that were first articulated and defended by
Charles Darwin and which continue to be identified with a certain
approach to evolutionary
 questions.[1]
 We will thus need to
begin with Darwin's Darwinism as articulated in *On the Origin of
Species* in 1859. We will then examine these same themes as they
have been discussed by evolutionary biologists and philosophers of
biology from the beginnings of the Neo-Darwinian Synthesis to the
present. 




Charles Darwin was not, as we use the term today, a philosopher,
though he was often so described during his
 lifetime.[2]
 Nevertheless, for an encyclopedia of philosophy what
is needed is a discussion of the impact of philosophy on Darwin's
Darwinism, and the impact of Darwin's Darwinism on topics that both
he, and we, would consider philosophical. We focus here on the impact
of philosophical discussions about the nature of science during
Darwin's lifetime on Darwin's scientific research, thinking and
writing; and on the impact of that research, thinking and writing on
philosophy. Taking the time to do such philosophical
archaeology stems from a
conviction that if the concept of Darwinism has legitimate application
today, it is due to a set of principles, both scientific and
philosophical, that were articulated by Darwin and that are still
widely shared by those who call themselves 'Darwinians' or
'neo-Darwinians'. 


## 2. Darwin and Darwinism


### 2.1 Darwin's Life




Charles Darwin was born February 12, 1809 and died April 18, 1882. It
was a time of radical changes in British culture, and his family
background put him in the midst of those changes. His grandfather, Erasmus Darwin, was
a prosperous and highly respected physician living in Western England,
south of Birmingham. He was also a philosophical radical, advocating Enlightenment ideas about human equality and
liberty, including the liberty to think freely about the existence of
God and about natural origins for the earth's creatures. He wrote a
number of very popular works of natural history, some in verse, in
which he defended views about progress that included evolutionary
speculations about the upward progress of living things from
primordial beginnings. 




Erasmus Darwin was an early member of an informal group of free
thinkers self-styled the Lunar
 Society,[3]
 that met regularly in Birmingham to discuss everything from the
latest philosophical and scientific ideas to the latest advances in
technology and industry. The Society included James Watt, Joseph
Priestly and Charles Darwin's other grandfather, Josiah Wedgwood.
Wedgwood, like Erasmus Darwin, lived in Staffordshire and was in the
process of developing a family pottery works into a major industrial
concern by applying new scientific and technological ideas to the
production of 'china'. The religious inclinations of the
group were 'non-conforming' and included a number of
Unitarians, a sect Erasmus Darwin referred to as 'a featherbed
to catch a falling Christian'. Looked upon with suspicion by
High Church conservatives, they actively promoted in Great Britain the
revolutionary philosophical, scientific and political ideas sweeping
across Europe and the Americas. Most had spent considerable time
absorbing Enlightenment ideas in Edinburgh, Scotland.




 Under the circumstances, it is not surprising that Robert Darwin,
Charles' father, should follow in his father's footsteps and
become a doctor, nor that he should end up marrying Susannah Wedgwood,
by all reports Josiah's favorite offspring. Politically and
philosophically engaged, Susannah worked to organize her children's
education in the town of Shrewsbury, where she and Robert took up
residence. She sent her children to a day school operated by Unitarian
minister Rev. George Case and this is where Charles began his
education. Unfortunately, Susannah died in 1817 when Charles was only
8, and his father then transferred him to the Shrewsbury School,
operated by Dr. Samuel Butler, grandfather of the novelist (and
sometime satirist of Darwin's work) of the same name. "Nothing
could have been worse for the development of my mind than Dr. Butler's
school" Charles proclaimed in the autobiography he wrote for his
family, and he escaped down the street to his home whenever he
could. 




His older siblings took good care of him, under the Doctor's watchful
eye. Early letters indicate that he and his brother Erasmus were
enthusiastic amateur chemists, and after his brother went up to Cambridge
their letters were often full of possible experiments, orders to
purchase chemicals and equipment for their 'laboratory',
and discussions of the latest discoveries. This was an obvious enough
passion that his classmates nicknamed him 'Gas'. During
summers he helped his father on his rounds to his patients, and when
only 16 his father sent him and his brother to Edinburgh for the best
medical education Great Britain had to offer. Erasmus needed to move
from Cambridge to a proper medical school to complete his medical
education, and young Charles was taken out of Shrewsbury School early
to accompany his brother to Edinburgh, apparently being prepared to
follow in his father's and grandfather's footsteps in medicine. The
two brothers arrived in Edinburgh in October of 1825. Erasmus left
after the first year, leaving his brother on his own during his second
year at Edinburgh. 




Privately, Darwin early on decided he could not practice medicine. But
his already serious inclination toward science was considerably
strengthened at Edinburgh both by some fine scientific lectures in
chemistry, geology and anatomy and by the mentoring of Dr. Robert
Grant. Grant certainly knew that young Charles was Erasmus Darwin's
grandson; Grant expounded evolutionary ideas derived from
Jean-Baptiste Lamarck and Charles' grandfather. But his primary
gift to Charles was introducing him to marine invertebrate anatomy and
the use of the microscope as a scientific tool and as an aid to
dissecting extremely small creatures dredged out of the Firth of
Forth. Darwin joined an Edinburgh scientific society, the Plinean
society, of which Grant was a prominent member, and presented two
lectures that reported discoveries he had made while working with
Grant. This interest in marine invertebrates was to be a life long
obsession, climaxing in his massive four-volume contribution to the
comparative anatomy and systematics of fossil and living Cirripedia or
'barnacles' (Barrett & Freeman 1988,
vols. 11-13).




When he finally broke the news of his distaste for medicine to his
father, he enrolled to take a degree in Divinity at Christ
College, Cambridge University, from which he graduated in January of
1831. As with the Shrewsbury School and Edinburgh, his official course
of study had very little impact on him, but while in Cambridge he
befriended two young men attempting to institute serious reforms in the
natural science curriculum at Cambridge, Rev. John Henslow, trained in botany and
mineralogy, and Rev. Adam Sedgwick, a leading member of the rapidly
expanding community of geologists. Henslow and his wife treated Darwin
almost as a son, and through Henslow Darwin was introduced to the men
whose ideas were currently being debated in geology and natural
history, as well as to men whom we look back on as among the very first
to take up the historical and philosophical foundations of science as
a distinct discipline, Sir John Herschel and Rev. William Whewell. As
he wrote in his autobiography:



> 
> 
> During my last year at Cambridge, I read with care and profound
> interest Humboldt's *Personal Narrative*. This work, and Sir
> J. Herschel's *Introduction to the Study of Natural
>  Philosophy*,[4]
>  stirred up in me a burning zeal to add even the most humble
> contribution to the noble structure of Natural Science. No one or a
> dozen other books influenced me nearly so much as these
> two.




In the next section we will discuss the influence of the philosophical
ideals of Herschel and Lyell on Darwin.




Furthering his scientific training, Adam Sedgwick on two occasions
took Darwin on extended geological tours of England and Wales. In
addition Darwin and a cousin, William Darwin Fox, a year ahead of him
at Cambridge, developed what began as an amateur passion for bug
collecting into serious entomology. 


### 2.2 Darwin's Darwinism




His Edinburgh and Cambridge mentors were to shape Darwin's
philosophical attitudes and scientific career decisively. It was
Henslow who was the final link to Darwin in a chain connected to
Captain Robert Fitzroy of H. M. S. Beagle. Fitzroy sought a gentleman
companion who could also collect information on geology and natural
history during a proposed circumnavigation of the globe. Henslow's
note to Darwin, asking if he would be interested in being recommended
for this post, arrived at the Darwin home, 'the Mount',
while Charles Darwin was on a geological survey of Northern Wales with
Adam Sedgwick. After resistance from his father had been overcome,
Darwin was offered the post and accepted it.




The combination of meticulous field observation, collection and
experimentation, note taking, reading and thinking during what turned
into the Beagle's five year journey through a very wide cross-section
of the earth's environments was to set the course for the rest of his
life. During the voyage he read and reread Charles Lyell's newly
published *Principles of Geology*, a three-volume work that
articulated a philosophical vision of rigorously empirical historical
science, oriented around five key ideas:


1. The geologist investigates both the animate and inanimate changes
that have taken place during the earth's history.
2. His principal tasks are to develop an accurate and comprehensive
record of those changes, to encapsulate that knowledge in general
laws, and to search for their causes.
3. This search must be limited to causes that can be studied
empirically--those 'now in operation', as Lyell puts
it in the sub-title of his *Principles*.
4. The records or 'monuments' of the earth's past
indicate a constant process of the 'introduction' and
'extinction' of species, and it is the geologist's task to
search for the causes of these introductions and extinctions,
according to the strictures note in 3., above.
5. The only serious attempt to do so according to the idea that species are
capable of 'indefinite modification', that of Jean
Baptiste Lamarck, is a failure on methodological grounds. All the
evidence supports the view that species variability is limited, and
that one species cannot be transformed into another.




This vision influenced Darwin profoundly, as he freely admitted. While
he became convinced by his observations and reading that the fossil
record and current distribution of species could only be due to the
gradual transformation of one species into another, he was determined
to articulate a theory that measured up to Lyell's principles. The
crucial event in convincing him that this was to be his life's work
was likely a visit to Cape Town, South Africa during the Beagle's return
trip to England. John F. W. Herschel was in Cape Town on a mission to do
for the Southern Hemisphere what his father William had done for the
Northern, namely to develop a comprehensive star map with the new
powerful telescopes developed by his father and aunt. As noted
earlier, Darwin had been deeply impressed by Herschel's
*Preliminary Discourse on the Study of Natural Philosophy* when
it first appeared a year before the Beagle set sail, and in his
private journal he referred to his meetings with Herschel during a
week long stop in Cape Town in June of 1836 as among the most profound
events of the entire voyage. Just five months before meeting Darwin,
Herschel had finished reading the 2nd edition of Lyell's
*Principles*. He sent Lyell a long letter filled with detailed
constructive commentary. The letter opens by praising Lyell for
facing the issue of the 'introduction of new species'--which Herschel calls 'that mystery of mysteries'--scientifically, and for advocating that we search for
'intermediate causes' to explain these
'introductions'--code for natural, as opposed to
'miraculous',
 causes.[5]
 This part of the letter was quoted in Charles Babbage's
*Bridgewater Treatise*, published in 1837 while Darwin was
struggling to develop just such a theory. Upon reading the Herschel
quotation in Babbage, Darwin wrote in his private
'species' notebooks: 



> 
> 
> Babbage 2d Edit, p. 226.--Herschel calls the appearance of new
> species. the mystery of mysteries. & has grand passage upon
> problem.! Hurrah.--"intermediate causes". (Barrett et
> al., 1987, 413; original punctuation) 




He clearly recognizes that Herschel is here providing a
philosophical justification for the project upon which Darwin was secretly
working. And, in the very first paragraph of *On the Origin of
Species*, Darwin looks back to this 'Hurrah',
attributing the idea that the origin of species is 'that mystery
of mysteries' to 'one of our greatest philosophers',
without mentioning Herschel by name. The first mention of the
possibility of an evolutionary solution to this problem is in his
*Ornithological Notebooks*, in a note written shortly after
departing Cape
 Town.[6]





Darwin's theoretical task was, by the time he opened his species
notebooks, tolerably clear: the only process that could produce the
systematic patterns in the fossil record and the otherwise strange
biogeographic distribution of species he now understood so widely and
deeply was a process of slow, gradual transformation of species. He
needed to come up with a natural, causal theory that would account for
such transformations, and every element of that theory had to identify
'causes now in operation', causes that could be
investigated empirically. The problem, and the methodological
constraints, had been advocated by his geological hero, and now close
friend, Charles Lyell; and they had been defended philosophically by
his philosophical hero, Sir John Herschel. 




Darwin, of course, expected, and got, outraged reactions from
religiously conservative colleagues, such as his old geology teacher
Sedgwick, who in a review expressed his "deep aversion to the
theory; because of its unflinching materialism;--because it has
deserted the inductive track,--the only track that leads to physical
truth;--because it utterly repudiates final causes, and therby [sic]
indicates a demoralized understanding on the part of its
advocates." What he had not expected was Lyell's refusal to
openly endorse his theory and Herschel's decisive (if polite)
rejection of its key elements. After we set out the theory in its
Darwinian form, we can consider these reactions from those who
apparently shared Darwin's philosophical norms about scientific
theory, explanation and confirmation. 




The theory can be set out as a series of causal elements that, working
together, will produce the needed transformations.


1. Species are comprised of individuals that vary ever so slightly
from each other with respect to their many traits.
2. Species have a tendency to increase in numbers over generations at a
geometric rate.
3. This tendency is checked, to use the language of Thomas
Malthus' *On the Principle of Population*, by limited
resources, disease, predation, and so on, creating a struggle for
survival among the members of a species.
4. Some individuals will have variations that give them a slight
advantage in this struggle, variations that allow more efficient or
better access to resources, greater resistance to disease, greater
success at avoiding predation, and so on.
5. These individuals will tend to survive better and leave more
offspring.
6. Offspring tend to inherit the variations of their parents.
7. Therefore favorable variations will tend to be passed on more
frequently than others and thus be preserved, a tendency Darwin labeled 'Natural
Selection'.
8. Over time, especially in a slowly changing environment, this
process will cause the character of species to change.
9. Given a long enough period of time, the descendant populations of
an ancestor species will differ enough both from it and each other to be classified as different
species, a process capable of indefinite iteration. There are, in
addition, forces that encourage *divergence* among descendant
populations, and the elimination of intermediate varieties.




It will be noticed that there is no element of this theory that is
incapable of empirical investigation--indeed by now the published
confirmatory studies of this process would fill a small
 library.[7]
 One can understand why devout and orthodox Christians would have
problems; but why Darwin's philosophical and scientific mentors? It
would seem to be the model of Herschelian/Lyellian orthodoxy.


### 2.3 Philosophical Problems with Darwin's Darwinism




The answer lies in five philosophically problematic elements of the
theory. 


#### 2.3.1 Probability and Chance



 First, notice the use of the language of 'tendencies' and
'frequencies' in the above principles. Privately, Darwin
learned, Herschel had referred to his theory as 'the Law of
higgledy-piggledy', presumably a reference to the large element
played in its key principles by chance and probability. Darwin's
theory is, as we would say today, a 'statistical'
theory. One cannot say that every individual with favorable variation
*v* will survive or will leave more offspring than
individuals without it; one cannot say that no environment will ever
support all of the offspring produced in a given generation, and thus
that there *must* *always* be a competitive
struggle. These are things that *tend* to happen due to clearly
articulated causes, and this allows us to make accurate predictions
about *trends*, at the level of populations, but not to make
absolute claims about what must happen in each and every case. Only
well after Herschel's time did philosophers of science become
comfortable with the idea of a theory of this sort, and 
the proper philosophical understanding of such explanations is still
debated.


#### 2.3.2 The Nature, Power and Scope of Selection



 The core of Darwin's
theory is the concept of natural selection. Perhaps because of his use of the term selection, this
core element of his theory apparently baffled nearly everyone.
Could it be, as Lyell, Herschel and Darwin's great American defender
Asa Gray would ask, an 'intermediate cause', i.e. a causal
principle instituted and sustained by God? Or is it, in its very
nature, the antithesis of such a principle, as his old geology teacher
Sedgwick believed? Could it possibly create species, or is it, by its
nature, a negative force, eliminating what has already been created by
other means? In one of his copies of *On the Origin of
Species*, Alfred Russell Wallace crosses out 'natural
selection' and writes 'survival of the fittest' next
to it. Wallace always felt that 'selection'
inappropriately imported anthropomorphic notions of Nature choosing
purposefully between variants into natural history. And, in a
devastating review, Fleeming Jenkin happily accepted the principle of
natural selection but challenged its power to modify an ancestral
species into descendent species, and thus limited its scope to the
production of varieties. A number of reviewers, even some sympathetic
ones, questioned the possibility of extending the theory to account
for the evolution of those characteristics that differentiate humans
from their nearest relatives.


#### 2.3.3 Selection, Adaptation and Teleology



 Moreover, because Darwin was very fond of describing natural
selection as a process that worked for the good of each species,
Darwin's followers seemed to have diametrically opposed views as to
whether his theory eliminated final causes from natural science or
breathed new life into them. In either case, there was also serious
disagreement on whether this was a good thing or a bad
 thing.[8]



#### 2.3.4 Nominalism and Essentialism



 There is a fundamental philosophical problem with the idea that a
species can undergo a series of changes that will cause it to become
one or more other species. To illustrate it, look carefully at the
first question that Charles Lyell wishes to address in the second
volume of the *Principles of Geology*:



> 
>  ...first, whether species have a real and permanent existence in nature;
> or whether they are capable, as some naturalists pretend, of being
> indefinitely modified in the course of a long series of generations.
> (Lyell 1831, II. 1)




Lyell pretty clearly assumes that to allow for evolution is to deny
the reality of species. For a species to be 'real', it
must have 'permanent existence in nature', or as he puts
it elsewhere , "...fixed limits beyond which the descendants from
common parents can never deviate from a certain type...". (Lyell
1831, II. 23) To accept evolutionary change, on this view, you must
become comfortable with a variety of nominalism about species. And
Darwin seems to have become
 so.[9]




> 
> 
> Hence I look at individual differences, though of small interest to
> the systematist, as of high importance for us, as being the first step
> towards such slight varieties as are barely thought worth recording in
> works on natural history. And I look at varieties which are in any
> degree more distinct and permanent, as steps leading to more strongly
> marked and more permanent varieties; and at these latter, as leading
> to sub-species, and to species. (Darwin 1859, 52)




Permanence, as applied to species, is for Darwin a relative concept, and there are no fixed
limits to variability within a species. Given enough time the
individual differences found in all populations can give rise to more permanent and
stable varieties, these to sub-species, and these to populations that
systematists will want to class as distinct species. Moreover, he
concludes the *Origin* with very strong words on this topic,
words bound to alarm his philosophical readers:



> 
> 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. ...In short, we will have to treat
> species in the same manner as those naturalists treat genera, who
> admit that genera are merely artificial 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. (Darwin 1859, 485)




Lyell, Herschel, Whewell, Sedgwick and many of Darwin's contemporaries
certainly would not find this a cheering prospect, since they were
unrepentant essentialists about
 species.[10]
 Members of a species possess a 'type' established in the
original parents, and this type provides 'fixed limits' to
variability. Lyell clearly feels this is an empirically verifiable
fact--most of chapters 2-4 of *Principles* Vol. II is
devoted to presenting the evidence that such 'fixed
limits' exist; and after the *Origin's* publication this
evidence was canvassed again in Fleeming Jenkin's review. If this is
so, then species extinction is easy to account for--there are
fixed limits to a species' ability to track environmental
change. But a naturalistic account of species origination is more
difficult, since there will need to be, in sexually reproducing
species, a natural production of a new pair of parents with a new
type. On the other hand, to adopt the sort of nominalism that Darwin seems to be advocating in the above quotations has undesirable consequences as well. How
are we to formulate objective principles of classification? What sort
of a science of animals and plants will be possible if there are no
fixed laws relating their natures to their characteristics and
behaviors? A good deal of chapter 2 of Darwin's *Origin* is
devoted to convincing the reader that current best practice among
botanists and zoologists accepts a natural world organized as he is
insisting rather than as his opponents claim:



> 
> It must be admitted that many forms, considered by highly competent
> judges as varieties, have so perfectly the character of species that
> they are ranked by other highly competent judges as good and true
> species. (Darwin 1859, 49)




From a Darwinian perspective, this is a predictable consequence of the
fact that the organisms we today wish to classify as species are merely the most
recent stage of a slow, gradual evolutionary process. Organisms within
a genus have common ancestors, perhaps relatively recent common
ancestors; some naturalists may see ten species with a few varieties
in each; others may rank some of the varieties as species and divide the
same genus into twenty species. Both classifications may be done with the
utmost objectivity and care by skilled observers. As systematists like
to say, some of us are 'lumpers', some of us are
'splitters'. Reality is neither.


#### 2.3.5 Tempo and Mode of Evolutionary Change



 The question of nominalism versus realism regarding species points toward a final
aspect of Darwin's theory with which many of those otherwise
sympathetic to him disagreed, his gradualism. For apart from the
question of whether his views entailed 'nominalism' about
natural kinds, they do seem to reflect a belief that the evolutionary
process must be a slow and gradual one. It is perhaps here that we see
the most lasting impact of Darwin's careful study of Charles Lyell's
*Principles of Geology* while on H.M.S. Beagle. I stress slow
*and* gradual, for it is clear that one could have a *slow
but non-gradual* evolutionary process (perhaps the long periods of
evolutionary stasis punctuated by geologically rapid periods of
speciation postulated by Eldridge and Gould's 'punctuated
equilibrium model' is such); and one could have a *rapid but
gradual* one (for example the process George Gaylord Simpson
labeled 'adaptive radiation', where a population migrates
to a location with a variety of unexploited niches, and rapidly
evolves to exploit them). Darwin stresses over and over again that he
conceives of natural selection 'adding up infinitely small
variations', and that he imagines the process of speciation to
take place over a very long period of time.




One of the strongest arguments for insisting that
'Darwinism' as it is used today is isomorphic to Darwin's
Darwinism, as Gayon puts it, is that each of these questions is still
hotly debated, and has been throughout the theory's history. With all
of the amazing changes that have been wrought by the genetic,
biochemical, and molecular revolutions, with the development of
mathematical models of population genetics and ecology, of
sophisticated techniques for both field and laboratory investigation
of evolutionary processes, and of cladistic analysis in systematics,
it nevertheless remains true that one can find evolutionary biologists
who adhere to Darwin's Darwinism, and are recognized as doing so by
both themselves and their critics. In the next section of this
article, I will develop a portrait of contemporary Darwinism around
each of these contested features. 




By the same token, however, Darwinism has evolved. As one example of
this truth, think for a moment of contemporary debates about the
nature of selection. The problems people had with natural selection in
the 19th century continue to be problematic, but there are
a variety of problems that were either not discussed, or discussed
very differently, in the 19th century. Can, and does,
natural selection work at levels other than the level of Darwin's
focus, individual organisms; is there a non-vacuous way to formulate
the theory abstractly; how are we to understand the relationships
between the concepts of fitness, selection and adaptation? How strong
are the constraints on the selection process, and what sorts of
constraints are there? Are there other motors of evolutionary change
besides selection, and if so, how important are they? In particular,
how important is 'drift', and how are we to differentiate
it from selection?


## 3. The Five Core Philosophical Problems Today



> 
> Theories need both essences and histories.  
> 
> Stephen Jay Gould (2002, 1)
> 




So reads the heading of the very first section of the first chapter of
Gould's monumental *The Structure of Evolutionary
Theory*. Opening with a subtle reading of an exchange of letters
in 1863 between paleontologist Hugh Falconer and Charles Darwin, Gould
eventually explains what he has in mind by this section heading:



> 
>  In short, "The structure of evolutionary theory" combines enough
> stability for coherence with enough change to keep any keen mind in a
> perpetual mode of search and challenge. (Gould 2002, 6)




Gould, of course, was both an unabashed admirer of Charles Darwin and
one of the most outspoken critics of the 'neo-Darwinian
synthesis'. I will be using both his account of 'the
Essence of Darwinism' in Part I of this magnum opus and his
arguments for a 'Revised and Expanded Evolutionary Theory'
in its Part II as touchstones and targets.




In the preceding section of this essay, I organized my discussion of
the problems that Darwin's allies had with *Darwin's* Darwinism
around five issues: [i] the role of chance as a factor in evolutionary
theory and the theory's apparently probabilistic nature; [ii] the
nature of selection; [iii] the question of whether
selection/adaptation explanations are teleological; [iv] the
ontological status of species and the epistemological status of
species concepts; and [v] the implications of Darwin's insistence on
the slow and gradual nature of evolutionary change. I claimed that one
very good reason for continuing to characterize one dominant approach
to evolutionary biology, that represented by the so-called
'Neo-Darwinian Synthesis', as 'Darwinism' is
that its proponents side with Darwin on these issues (and on many less
fundamental ones besides). That in itself is remarkable, but it is the
more so because the Darwinian position on each of these issues is
under as much pressure from non-Darwinian evolutionary biologists
today as it was in the wake of the *Origin*. It is not
surprising, given the situation as I have just characterized it, that historians and
philosophers of biology have made significant contributions to the
discussion, especially in pointing out the underlying philosophical issues
 and conceptual confusions and ambiguities that stand
in the way of resolving the issues at hand, and their historical origins.




It is my conviction that a full understanding of the underlying
philosophical disagreements on these questions will only come from a
patient historical study of how the 'Synthesis' positions
on these various issues, and those of their critics, arose. That I
cannot do here. Rather, in what follows I will simply be presupposing
certain answers to these questions of historical origins. The list of
references at the end of this essay includes a number of excellent
pieces of work on this subject for those who share my convictions
about its importance.


### 3.1 The Roles of Chance in Evolutionary Theory




The evolutionary process, as Darwin understood it, involves the
*generation* of variation and a process producing a
differential *perpetuation* of variation. One simple way to
think about Darwinism in relation to a logical space of alternatives,
then, is by means of the following *variation grid*:




|  |
| --- |
| *Variations* |
|  | *Generation* | *Perpetuation* |
| *Fitness Biased* | LamarckAsa Gray | DarwinAsa Gray |
| *Not Fitness Based* | DarwinNeutralism | LamarckNeutralism |




The above grid might lead you to conclude that both non-fitness
biased *generation* of variation and non-fitness
biased *perpetuation* of variation would be properly labeled
'chance.' By seeing why that would be a misleading
conclusion to draw, we get to the heart of the problem of the concept
of 'chance' in contemporary Darwinism. 




Let us begin with the language Darwin uses when he first sketches his
theory at the beginning of the fourth chapter of the
*Origin*:



> 
>  Can it, then, be thought *improbable*, seeing that variations
> useful to man have undoubtedly 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 (remembering 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? (Darwin
> 1859, 80-81)




Unlike Darwin's contemporaries, the founders of the synthesis of
Mendelian genetics and Darwinian selection theory, Sewall Wright,
Ronald Fisher and J. B. S. Haldane, were entirely comfortable with a
selection theory formulated in such terms. On this issue, contemporary
Darwinism agrees whole-heartedly with Charles Darwin. Note one clear
statement of the *Principle of Natural Selection* from the
philosophical literature:



> 
>  If *a* is better adapted than *b* to their mutual
> environment *E*, then (probably) *a* will have greater
> reproductive success than *b* in *E*. (Brandon1990,
> 11).




The theory trades pervasively in probabilities. To take a simple case:
if there are three possible combinations of alleles at a given locus
in a population, we can characterize the outcome of a reproductive
cycle as 'chance' if each of the three possible
combinations occurs at a frequency determined strictly by the laws of
probability. In any given case of reproduction, we would say, which
genotype emerged is a matter of chance. Given the fact that
evolutionary biologists, especially in so far as they take their cues
from population genetics, deal with large populations conceived as
'gene pools', and think of evolution as long run changes
in the frequencies of different combinations of genes from generation
to generation, it is clear that, in this sense, chance permeates
contemporary Darwinism. The models of population biology provide a
means of assigning probabilities to various outcomes, given
information about population size, rates of mutation and migration
(themselves given as averages and estimates). That is, as Darwin
notes, being relatively better adapted increases an organism's
'chances', i.e. increases its probability, of leaving
viable offspring. It does not guarantee it. Since natural selection
is a stochastic process, Darwinians from Darwin to the present rightly
characterize it in terms of influencing the 'chances' of a
given outcome, given variables such as selection pressure, population
size or mutation rates.




Conceptual confusion arises, however, from the fact that
'chance' and 'randomness' are often
contrasted, not with 'deterministic' outcomes but with
'selected' outcomes. For example, when John Beatty
describes 'random drift' as 'changes in frequencies
of variations due to chance' in the following passage, he
presumably has something like a contrast with changes in frequencies
due to selection in mind.



> 
>  In Darwin's scheme of things, recall, chance events and natural
> selection were consecutive rather than alternative stages of the
> evolutionary process. There was no question as to which was more
> important at a particular stage. But now that we have the concept of
> random drift taking over where random variation leaves off, we are
> faced with just such a question. That is, given chance variations, are
> further changes in the frequencies of those variations more a matter
> of chance or more a matter of natural selection? (Beatty 1984,
> 196)




Notice that in the above quote we first get a substitution of
'random' for 'chance' in the phrases
'random variation' and 'chance variation', and
then at least the suggestion that the concept of 'random
drift' can be characterized as 'changes in frequencies of
variations due to chance', where the contrast class consists of similar
changes due to natural selection. 




With respect to the *generation* of variation, chapter 5 of
*On the Origin of Species* opens with the following
apology:



> 
>  I have hitherto sometimes spoken as if the variations--so common
> and multiform in organic beings under domestication, and in a lesser
> degree in those in a state of nature--had been due to
> chance. This, of course, is a wholly incorrect expression, but it
> serves to acknowledge plainly our ignorance of the cause of each
> particular variation. (Darwin 1859, 131)




Here Darwin is noting that, though to speak of 'chance
variations' may seem to be citing chance as the cause of the
variations, in fact it is simply acknowledging that they 'appear
to have no assignable cause'. But it is important to keep
historical context in mind here. Whether Darwin himself ever flirted
with the idea of 'directed' variation or not, he was
acutely aware of two views from which his needed to be
distinguished, very different from each other, but both holding to the
view that variations arose for a
 purpose.[11]
 The most widely shared alternative was that found in natural
theology. To quote the Reverend William Paley's *Natural
Theology*, regarding a beautiful instance of adaptation: "A
conformation so happy was not the gift of chance". Likewise,
among Darwin's followers, the American botanist Asa Gray, in an essay
entitled 'Natural Selection and Natural Theology', uses
the same contrast to advise Darwin against the notion of
'chance variation': "...we should advise
Mr. Darwin to assume, in the philosophy of his hypothesis, that
variation has been led along certain beneficial lines."




Gray is here insisting that, since Darwin admits that using the term
'chance' merely signals ignorance of the true cause, and
since the pervasive adaptations in nature suggest design, Darwin
should avoid the suggestion that *variations are due to chance in
the sense of 'absence of
 design'*.[12]





Darwin, in fact never refers to 'chance variations' in the
*Origin*, though occasionally he will note that if a beneficial
variation 'chances [i.e. happens] to appear', it will be
favored by selection (see pp. 37, 82) What Darwin has in mind,
however, is clear from his concluding remarks in his chapter on
*Laws of Variation*:



> 
>  Whatever the cause may be of each slight difference in the offspring
> from their parents--and a cause of each must exist--it is
> the steady accumulation, through natural selection, of such
> differences, when beneficial to the individual, that gives rise to all
> the more important modifications of structure..." (Darwin 1859,
> 170)



 Whatever the cause of the *generation* of a variation may be,
the role of selection is to *accumulate* those already present
variations that happen to be beneficial. As Beatty put it, the
generation of variations and their selection are
'consecutive' processes. But to call the generation of
variation a 'chance' process is to use
'chance' in this second sense, meaning *not* by design, *not*
for some end.




Apart from those urging Darwin to give up chance in favor of design,
he had pressure to abandon chance from another direction, the
evolutionary philosophy of Jean-Baptiste Lamarck. Lamarck's is a
*materialistic* argument against the variation in nature being
a matter of chance. On the Lamarckian view, variations arise in an
organism as a direct response to environmental stress or demand,
giving rise to a stimulus, which in turn elicits a physiological
response, which finally can be passed on via reproduction to
offspring. Variations are not chance or random, since they are an
appropriate response to an environmental stress. Here
'chance' signals a *lack of relation or connection to
adaptive needs*, an idea akin to, but ontologically quite distinct
from, the contrast between 'chance' and
'design'. 




The concept of 'random variation' is today often used as a
synonym for 'chance variation' in precisely this latter
sense. Here are two examples of this notion of chance or randomness as used by
contemporary Darwinians.



> 
>  ...mutation is a random process with respect to the adaptive
> needs of the species. Therefore, mutation alone, uncontrolled by
> natural selection, would result in the breakdown and eventual
> extinction of life, not in adaptive or progressive
> evolution. (Dobzhansky 1970, 65) 




Thus the production of variations may be a 'chance'
process in that there are a number of possible outcomes with
assignable probabilities, but it is *also* a
'chance' process in the sense that the probability
assignments are not biased by 'adaptive needs' or
'fitness'.




My second example is intended to take us back to problems with our
first sense of 'random' and 'chance'. Here,
a champion of the neutral theory of molecular evolution
characterizes his position:



> 
>  ...the great majority of evolutionary changes at the molecular
> (DNA) level do not result from Darwinian natural selection acting on
> advantageous mutants but, rather, from random fixation of selectively
> neutral or very nearly neutral mutants through random genetic drift,
> which is caused by random sampling of gametes in finite
> populations. (Kimura 1992, 225)




Here, it will be noticed, the focus is not on the *generation*
of variations but on the *perpetuation* of variations. The
contrast is between a random sampling of gametes that leads to the
fixation of selectively neutral alleles and natural selection favoring
advantageous variations. That is, the contrast between
'chance' and 'fitness biased' processes is now
being used to distinguish different *means of perpetuating certain
variations*. We are contrasting two sampling processes. Drift
samples without concern for adaptation; selection samples
discriminately on the basis of differences in fitness. Both samplings
are 'probabilistic', of course, but that in no way
obviates the above contrast.




However, as Beatty has pointed out, it was quite common until fairly
recently to characterize natural selection in such a way as to make it
almost indistinguishable from random drift (cf. Lennox 1992, Lennox
and Wilson 1994). Numerous accounts of fitness characterized the
fitness of a genotype as defined by its relative contribution to the
gene pool of future generations--the genotype contributing the
larger percentage being the fitter. But of course that could easily
be the result of a 'random'--non-fitness biased--sampling process; which organisms would be declared
'fitter' by this method might have nothing to do with
natural selection. In order to provide a proper characterization of
the role of chance in evolutionary change, then, it is critical to
provide a more robust and sophisticated account of fitness. (For 
further information, see the entry on
 fitness.) 
 This, in turn, requires that we discuss the conceptual network that
includes the notions of adaptation and natural selection, to which we
will turn shortly. 




For now, let us assume that there is a way of characterizing fitness
such that there is a substantial empirical question of what role
indiscriminate sampling of genotypes (or phenotypes) plays in
evolutionary change. This issue was first placed squarely before
evolutionary biologists by Sewall Wright in the early 1930s. As Wright
pointed out, genes that are neutral with respect to fitness can, due
to the stochastic nature of any process of sampling from a population,
increase their representation from one generation to the next. The
likelihood of this happening goes up as effective population size goes
down. Since Wright imagined that a quite typical scenario in
evolutionary change was for species to be broken up into relatively
small, relatively isolated, populations (or 'demes'), with
significantly more breeding within than between demes, the likelihood
that such 'neutral genotypes' could become fixed at
relatively high levels was significant. Though he gradually toned down
this aspect of his work, a significant school of mathematical
population geneticists in the 1960s and 70s took these ideas and ran
with them, developing a 'Neutralist' approach to
evolutionary change. This is the position characterized by Kimura
(one of its most eloquent defenders) in the passage quoted
above. Whether or not such a process plays a significant role in
evolution is not a philosophical issue, but it is highly relevant to
whether evolutionary biology should be seen as predominantly
Darwinian. For if any view is central to Darwinism, it is that the
evolutionary process is predominantly guided by the fitness-biasing
force of natural selection, acting on variations that arise by chance.
It is to natural selection and related concepts that we now turn.


### 3.2 The Nature, Power and Scope of Selection



> 
>  The greatest number of females will, of course, fall to the share of
> the most vigorous males; and the strongest individuals of both sexes,
> by driving away the weakest, will enjoy the best food, and the most
> favourable situations, for themselves and for their offspring. A
> severe winter, or a scarcity of food, by destroying the weak and the
> unhealthy, has had all the good effects of the most skilful
> selection.




The words of Charles Darwin? No; these are the words of John
Sebright, penned in *The Art of Improving the Breeds of Domestic
Animals* in 1809, the year of Charles Darwin's birth and fifty
years before *On the Origin of Species* was published. Darwin
refers to this passage in Notebook C of his Species
 Notebooks.[13]
 It will be noticed that Sebright is *not* discussing domestic
selection, but is quite clearly saying that processes leading to
differential survival and reproduction in nature will have 'all
the good effects of the most skilful selection'. Darwin, then,
did not need to read Malthus to see what is here so plainly and
clearly stated--namely, that the struggle for survival in nature
will have the same 'selective' effects as the actions of
the domestic breeder of plants and animals. 




As this passage, and the argument of the *Origin*, shows,
'natural selection' began life as the product of
analogical reasoning. Sebright sees clearly that the natural
processes he is describing will have the same *effects* as the
breeder's selection, but he is not about to describe those processes
as selection processes. Darwin took that step, and Darwinism has
followed. 




Darwin himself consistently refers to natural selection as a *power
of preserving* advantageous, and eliminating harmful,
variations. As noted in the last section, whether a particular
variation is advantageous or harmful is, in once sense of that term, a
matter of chance; and whether an advantageous variation is actually
preserved by selection is, in another sense of the term, also a matter
of chance. For Darwinism, selection is the force or power that biases
survival and reproduction in favor of advantageous variations, or to
look ahead to the next section, of adaptations. It is this that
distinguishes selection from drift.




 In a recent monograph entitled *Natural Selection: Domains, Levels
and Challenges* in the *Oxford Series in Ecology and
Evolution*, George C. Williams has vigorously defended
*Darwinian selection theory* against a variety of challenges
that have emerged over the last few decades. Those challenges can be
placed into two broad categories: [i] proposed *limitations* on
natural selection as an evolutionary force; and [ii]
*expansions* of the scope of natural selection to include new
'targets' and 'levels'. It will be noted that
in neither case is it obvious that the theory *itself* requires
modification in the face of such challenges--in principle these
might be nothing more than challenges to the theory's *range of
application*. However, if it turned out that *most*
evolutionary change could be explained without recourse to natural
selection, this would be grounds for arguing that evolutionary biology
was no longer Darwinian. And if it turned out that the theory of
natural selection could *only* be integrated with our new
understanding of the processes of inheritance and development by a
wholesale modification of its foundations, it might be best to see the
new theory as a modified descendent of Darwinism, rather than
Darwinism itself. Theories may need essences, as Gould claims; but if
what is fundamental to the theory has changed, then so has its
essence. To borrow a phrase from Paul Griffiths, perhaps it is not
that theories need histories *and* essences--perhaps what
they need are *historical essences*.




Alfred Russell Wallace regularly urged Darwin to jettison the term
'selection' as misleadingly anthropomorphic, and
substitute Herbert Spencer's 'survival of the fittest'.
Darwin went half way--in later editions he added 'or
Survival of the Fittest' to 'Natural Selection' in
the title of chapter 4. As the theory developed in the
mid-20th century, the expression 'survival of the
fittest' was gradually eliminated from any serious
presentation of Darwinian selection theory. On the other hand, the
concept of 'fitness' has played a prominent, and
problematic, role. In the mathematical models used in
population genetics, 'fitness' refers either
to the abilities of the different genotypes in a population to leave
descendants, or to the measures of those abilities, represented by the
variable *W*. Here is a rather standard textbook
presentation of the relevant concepts:



> 
>  In the neo-Darwinian approach to natural selection that incorporates
> consideration of genetics, fitness is attributed to particular
> genotypes. The genotype that leaves the most descendants is ascribed
> the fitness value *W*=1, and all other genotypes have
> fitnesses, relative to this, that are less than 1. ... Fitness
> measures the relative evolutionary advantage of one genotype over
> another, but it is often important also to measure the relative
> penalties incurred by different genotypes subject to natural
> selection. This relative penalty is the corollary of fitness and is
> referred to by the term *selection coefficient*. It is given
> the symbol *s* and is simply calculated by subtracting the
> fitness from 1, so that: *s* = 1 [?] *W*. (Skelton
> 1993 164) 




The problem lies in the fact that the concept of fitness plays dual
roles that are instructively conflated in this quotation. For when
fitnesses are viewed as measures of differential *abilities* of organisms
with different genotypes to leave different numbers of offspring, the
language of fitness encourages us to suppose that
'fitness' refers to the relative selective advantages of
genotypes. On the other hand, if 'fitness' simply refers
to the *measure* of reproductive success, it is a quantitative
representation of small scale evolutionary change in a population, and
leaves entirely open the question of the *causes* of the
change. But then the assumed connections among the concepts of
fitness, adaptation and natural selection are severed.
'Selection coefficients' may have nothing to do with
selection; what *W* represents may have nothing to do with
selective advantage.




There is, however, a way of formulating the theory in its modern guise
which maintains an essentially Darwinian character. Since there are a
number of confirmed ways in which natural populations can evolve in
the absence of natural selection, and since balancing selection,
i.e. countervailing selection forces, may prevent a population from
evolving in its presence, it is clear that establishing, by measuring
different reproductive rates among its members, that the genetic
make-up of a population has changed does not establish that natural
selection was the source of that change; nor does the fact that no
change has been measured establish that natural selection is not
operative. Population genetics and its associated models should be
treated as the 'kinematics', not the
'dynamics' of evolutionary processes. That is, it is a way
of establishing that a population either is or is not in equilibrium,
and it provides sophisticated tools for measuring rates of change in a
population across generations. Moreover, like the kinematics of any
physical theory, if it establishes cross-generational change, it also
tells us that there are causes to be found--the detailed contours
of those measures may even provide suggestions as to where to look for
those causes. What it cannot do *on its own* is provide
knowledge of the forces at work. To use language introduced by
Elliott Sober, fitness, unlike natural selection, is *causally
inert*. (For further information, see the entry on
 population genetics.) 




That means that, as valuable as population genetics is, it should not
be equated with the theory of natural selection. Too often in both
biological presentations of the theory and philosophical discussions
of it, this is forgotten. For example:



> 
>  Most people are familiar with the basic theory of natural selection.
> Organisms vary in a heritable fashion. Some variants leave more
> offspring than others; their characteristics, therefore, are
> represented at a greater frequency in the next generation. (Wilson
> 1984, 273)




This is a presentation of 'the basic theory of natural
selection' that makes no reference to natural selection at all!





Natural selection, if it is to resemble the Darwinian concept that
bears that name, must be reserved for reference to an *interaction
between a variable, heritable feature of an organic system and the
environment of that system*. That interaction may or may not
change the proportions of those features across generations, and those
proportions may change for reasons other than those interactions. But
a plausible natural selection hypothesis must posit some such
interaction. On this issue I
will give the last word to Stephen Jay Gould:



> 
>  ...when we consider natural selection as a causal process, we
> can only wonder why so many people confused a need for measuring the
> results of natural selection by counting the differential increase of
> some hereditary attribute (bookkeeping) with the mechanism that
> produces relative reproductive success (causality)." (Gould 2003,
> 619)


 The concept of natural selection has to this point been presented
broadly because of the other two critical questions surrounding
the contemporary Darwinian concept of natural selection that I
mentioned earlier--questions having to do with possible limiting
constraints on natural selection and about the sorts of objects that
can be viewed as appropriate organismic/environmental
'interactors' in the selection process.



If we suppose that for Darwin natural selection was almost exclusively
thought of as an interaction between individual organisms and their
organic and inorganic environments, then we can see two challenges to
Darwinism today with respect to *levels* of selection. There
are those, such as G. C. Williams and Richard Dawkins, who argue that
selection is always and only of genes. Here is a clear statement:



> 
>  These complications [those introduced by organism/environment
> interactions] are best handled by regarding individual [organismic]
> selection, not as a level of selection in addition to that of the
> gene, but as the primary mechanism of selection at the genic level.
> (Williams 1993, 16)




Dawkins' preferred mode for making the same point is to refer to
organisms--or interactors--as the *vehicles* of their
genes, in fact vehicles constructed by the genome for its own
perpetuation. 




The original impulse for this approach, especially clear in
Williams' classic *Adaptation and Natural Selection*
(1966) was philosophical--it was to use a sort of Ockham's razor
strategy against Group Selection hypotheses, showing that alleged
group selection effects could be explained by explanations operating
at the level of the genome. Throughout that book selection is always
said to be of individual alleles, regardless of the role environments
at various levels may play in the process. 




This view has been extensively challenged by philosophers of biology
on both methodological and conceptual grounds, though there are, among
philosophers, enthusiastic supporters (cf. Dennett 1995). In all the
give and take, it is seldom noticed that defenders of this view claim
to be carrying the Darwinian flag (Gayon 1998 and Gould 2003 are
exceptions). Yet it is certainly *not* a position that Darwin
would recognize--and not merely because he lacked a coherent theory of
the units of inheritance. It is not a Darwinian view because for
Darwin it was differences in the abilities of organisms at various
stages of development to respond to the challenges of life that had
causal primacy in the explanation of evolutionary change. Among
evolutionary biologists from the 'neo-Darwinian synthesis'
on, it is those who stress the role of organisms in populations
interacting differentially to ever-variable ecological conditions in
causing changes in the gene pools of those populations who are the
card-carrying Darwinians. 




 Darwinism also has challenges from the opposite direction. In the
1970s a number of biologists working in the fields of paleontology and
systematics challenged the Neo-Darwinian dogma that you could account
for 'macro-evolution' by means of long term extrapolation
from micro-evolution. Gould, in particular, opens Part II of *The
Structure of Evolutionary Theory* (*Towards a Revised and
Expanded Evolutionary Theory*), with a chapter entitled
'Species as Individuals in the Hierarchical Theory of
Selection'. That chapter title combines two conceptually
distinct theses: first, the thesis defended by Michael Ghiselin
(Ghiselin 1997) and championed and refined by David Hull (Hull 2001),
that species are, in a robust sense of the term,
'individuals'; and second, that there may well be
selection among groups of organisms, *qua* groups. Gould's
title exemplifies one approach to group selection--the unit of
selection is always the individual, but there are individuals other
than individual organisms that are subject to selection. A very
different result emerges if one assumes that groups of organisms such
as demes, kin-groups, or species, though not individuals, are
nevertheless subject to selection. Adding to the conceptual
complexity, some researchers propose that the term 'group
selection' be restricted to the process whereby group-level
traits provide advantages to one group over another, in which case
there are strict conditions delimiting cases of group
selection. Others define group selection primarily in terms of group
level *effects*. Thus a debate analogous to that earlier
discussed regarding the definitions of 'fitness' emerges
here--by group selection do we mean a distinct type of causal
process that needs to be conceptually distinguished from selection at
the level of individual organism or gene, or do we merely mean a tendency
within certain populations for some well defined groups to displace
others over time? (For further discussion, see Sterelny and Griffiths
1999, 151-179; Hull 2001, 49-90; and see the entry on:
 levels and units of selection.)


### 3.3 Selection, Adaptation and Teleology



 Early in the Introduction to *On the Origin of Species*,
Darwin observes that the conclusion that each species had descended
from others "even if well founded, would be unsatisfactory,
until it could be shown how the innumerable species inhabiting this
world have been modified so as to acquire that perfection of structure
and co-adaptation which most justly excites our admiration"
(Darwin 1859, 3). One might say this was the central promise of
Darwinism--to account for both phylogenic continuity *and*
adaptive differentiation by means of the same principles; or as Darwin
puts it, to integrate in one theory the supposed opposition between
Unity of Type and Conditions of Existence.




But it is here that even the most sympathetic of Darwin's theistic
supporters were forced to qualify their support for the theory of
descent with modification by means of natural selection. In Darwin's
day the reactions of Asa Gray and John Herschel are perhaps the most
interesting in this respect. Both men saw in Darwin's theory a way to
account for 'that mystery of mysteries,' the regular
appearance of new species by means of natural, or as they might say,
'intermediate' causes. However both instinctively
recoiled from the irreducible and central role of 'chance'
in the theory. They did not, but easily could have, said 'God
does not play dice with the universe.' But as Darwin stated
repeatedly, if gently, to Gray--if God ordained that variations
should be along beneficial lines, natural selection would be
redundant. Moreover, the evidence from the study of variation in
domestic and natural populations put the lie to any claim that God
directs all or most variation along beneficial lines. Darwinian
selection theory is a two-step process--the production of
variation unrelated to the adaptive requirements of the organism, and
differential perpetuation of those variations that serve adaptive
needs. Again, a theory of evolution that could not be so described
would not be a Darwinian theory.




The nature of 'selection explanations' is a topic to which
much philosophical attention has been devoted in recent years. Here I
want to focus on only one important question--to what extent is
the teleological appearance of such explanations simply that, an
appearance masking a causal process in which goals play no role?




The *appearance* of teleology is certainly present in Darwinian
explanations, and has been since Darwin spoke of natural selection
*working solely for the good* of each being. The appearance of
teleology stems from the ease with which both evolutionary biology and
common sense take it for granted that animals and plants have the
adaptations they do *because* of some benefit or advantage to
the organism provided by those adaptations. 




This is a hotly contested question, and I will here simply sketch a
case that selective explanations of adaptations are robustly
teleological. The interested reader may want to refer to the
literature on this question referred to in the discussion and listed
in the list of readings provided at the end of this entry. A question
I think not worth discussing is whether the word
'teleology' should be replaced by
'teleonomy'. Etymologically, they come to the same thing;
and the philosophical arguments given in favor of the change all rest
on an historically doubtful assumption--that philosophical
defenses of teleology have always been either theistic or
vitalistic. The serious philosophical issue can be put simply and
directly: in selection explanations of adaptations, are the functions
served by adaptations a central and irreducible feature of the explanans in such
explanations? If the answer is yes, the explanations are
 teleological.[14]




A good place to begin is with a simple, yet realistic, example. In
research carried out over many years and combining painstaking field
work and laboratory experimentation, John Endler was able to
demonstrate that the color patterns of males in the guppy populations
he was studying in rivers feeding into the southern Caribbean were a
consequence of a balance between mate selection and predator
selection. To take one startling example, he was able to test and
confirm a hypothesis that a group of males, with a color pattern that
matched that of the pebbles on the bottoms of the streams and ponds
they populated except for bright red spots, have that pattern because
a common predator in those populations, a prawn, is color blind for
red. Red spots did not put their possessors at a selective
disadvantage, and were attractors for mates (Endler 1983,
173-190). We may refer to this pattern of coloration as a
complex adaptation that serves the functions of predator avoidance and
mate attraction. But what role do those functions play in explaining
why it is that the males in this population have the coloration they
do?




This color pattern is an adaptation, as that term is used in
Darwinism, only if it is a production of natural selection (Williams
1966 261; Brandon 1985; Burian 1983). In order for it to be a product
of natural selection, there must be an array of color variation
available in the genetic/developmental resources of the species wider
than this particular pattern but including this pattern. Which factors
are critical, then, in producing differential survival and
reproduction of guppies with this particular pattern? The answer
would seem to be the value-consequences this pattern has compared to
others available in promoting viability and reproduction. In popular
parlance (and the parlance favored by Darwin), this color pattern is
*good for* the male guppies that have it, and for their male
offspring, and that is why they have it (Binswanger 1990; Brandon
1985; Lennox 2002). This answer strengthens the 'selected
effects' or 'consequence etiology' accounts of
selection explanations by stressing that selection ranges
over *value* differences. The reason for one among a number of
color patterns having a higher fitness value has to do with
the *value* of that pattern *relative to the survival and
reproductive success* of its possessors. 




Selection explanations are, then, a particular kind of teleological
explanation, an explanation in which *that for the sake of
which* a trait is possessed, *its valuable consequence*,
accounts for the trait's differential perpetuation and maintenance in
the population.


### 3.4 Species and the Concept of 'Species'




In listing the topics I would discuss under the heading of neo-Darwinism, I
distinguished the question of the ontological status of species from
the epistemological status of the species *concept*.
Though they are closely related questions, it is important to keep them
distinct. As will become clear as we proceed, this distinction is
rarely honored. Moreover, it is equally important to distinguish
the *species concept* from the categories of features that
belong in a definition of species (Rheins 2011). Advances in our theoretical
understanding may lead us to reconsider the sorts of attributes that
are most important for determining whether a group of organisms is a
species, and thus whether it deserves to be assigned a name at that
taxonomic level. It should not be assumed that such changes
constitute a change in the species concept, though at least some such
changes may lead us to restrict or expand the range of taxa that are
designated as species. In his contribution to the Neo-Darwinian Synthesis,
*Systematics and the Origin of Species*, Ernst Mayr titled
chapter five 'The Systematic Categories and the New Species
Concept'. Recall that Darwin made a point of treating the
species category as continuous with 'well-marked variety'
and 'sub-species', and made the radical suggestion that its
boundaries would be just as fluid. Without explicitly
acknowledging Darwin, Mayr takes the same tack, discussing
'individual variants' and 'sub-species' as a
preliminary to discussing the species concept. Mayr notes that
for someone studying the evolutionary process, speciation is a critical
juncture; "...his interpretation of the speciation process
depends largely on what he considers to be the final stage of this
process, the species." (Mayr 1942/1982, 113) With this in
mind, he offers the following definition, the so-called
'biological species concept' (BSC):



> 
> *Species are groups of actually or potentially interbreeding
> natural populations, which are reproductively isolated from other such
> groups* (Mayr 1942/1982, 120; 1976 518)




Mayr was well aware of the limitations of this definition, and
treated it somewhat as a 'regulative ideal'.
Dobzhansky in 1937 gave what he claimed to be a definition of species,
but which seems, as Mayr noted (Mayr 1976 481), much more a definition
of *speciation*:



> 
> 
> ...that stage of evolutionary process, "at which the once
> actually or potentially interbreeding array of forms becomes segregated
> in two or more separate arrays which are physiologically incapable of
> interbreeding." (312)




Simpson (1943) and others built even more historicity into the
concept. These are all, of course, intended as
*definitions* of the species *category*, and they attempt
to provide a test (or a 'yardstick': Mayr 1976 479) that in
principle will permit a researcher to decide whether a group of
individuals should all be identified by a single species-level concept
such as 'homo sapiens'. The test for species
membership is the *capacity* to interbreed; the test
distinguishing two species is *incapacity* to interbreed.
Dobzhansky makes the importance of this test transparent--the
transition from a single interbreeding population to two reproductively
isolated ones is the process of speciation.




Now in each of these definitions, little attention is paid to the actual
methods used by taxonomists and systematists in differentiating between
varieties of a species and distinct species, something to which Darwin
gave a great deal of attention. Darwin's apparent nominalism
regarding the species concept likely stemmed from his close attention
to his own taxonomic practices and those of other specialists. But
nominalism typically combines a view about the *ontology* of
species with one about the epistemological status of the species
*concept*. On the first question, the nominalist insists
that there are no species--there are more or less similar
individuals. On the second question, the nominalist typically
insists that the species *concept* is, at best, a useful or
convenient grouping of similar individuals or, at worst, an
*arbitrary* grouping of similar individuals. (An interesting
alternative account of the species concept based on a sophisticated,
multidimensional theory of similarity has recently been defended in
Rheins 2011.) 




In his work, Mayr relates different approaches to the species
concept to the philosophical distinction between essentialism and
nominalism. He associates essentialism with the view that a
species *concept* refers to a universal or type. This view
of the referent of the concept leads to the Typological Species
Concept, which he traces from Linnaeus back to Plato and Aristotle, and which he
claims 'is now universally abandoned' (1976 516). It is
worth noting that serious doubt has been cast both on the historical
and the philosophical credentials of Mayr's 'Typological
Species Concept' (see, e.g. Lennox, 1987; repr. in Lennox 2001b;
Winsor 2001, 2006; Walsh 2006; Wilkins 2009). At the opposite extreme is
nominalism, which combines the view that only individuals exist in
nature and that species are concepts invented for the purpose of
grouping these individuals collectively.




Mayr claims that his Biological Species Concept (BSC) is an advance
on both; individual species members are objectively related to one
another not by a shared relation to a type but by causal and historical
relationships to one another. He can thus
be understood as arguing for a new, objective way of understanding the
epistemological grounds for grouping individuals into species.
This new way of grouping stresses historical, genetic and various
ecological relationships among the individuals as the grounds for
determining species membership. His claim is that this is more
reliable and objective than similarities of phenotypic
characteristics. This makes sense of the importance he eventually
places on the fact the BSC defines species relationally:



> 
> 
> ...species are relationally defined. The word species
> corresponds very closely to other relational terms such as, for
> instance, the word *brother*. ... To be a different species
> is not a matter of degree of difference but of relational distinctness.
> (Mayr 1976, 518)




Mayr has in mind that brothers may or may not look alike; the
question of whether two people are brothers is determined by their
historical and genetic ties to a common ancestry. Notice,
however, that this is a claim about which characteristics, among the many
that they have, should be taken most seriously in
determining the applicability to them of the concept
'brother'. That is, it is a defense of a sort of
essentialism.




A number of critics have pointed out that essentialism need not be
committed to 'types' understood as *universalia in
re*; and on certain accounts of essences any species taxon that
meets the standards of BSC does so in virtue of certain essential
(though relational and historical) properties. At one extreme,
Michael Ghiselin and David Hull have argued that this causal/historical
structure of species provides grounds, at least within evolutionary
biology, for considering species to be
 individuals.[15]
 Organisms are
not members of a class or set, but 'parts' of a
phylogenetic unit. Taking a very different tack, Denis Walsh has
recently argued that a form of 'evolutionary essentialism,'
bearing a striking resemblance to the essentialism of Aristotle's
zoological work, is implicit in the work of a number of evolutionary
developmental theorists (Walsh, 2006).




A critical issue in this debate over the account of the species
concept most appropriate for Darwinism is the extent to which the
process of biological classification--taxonomy--should be
informed by advances in biological theory. Besides those already
discussed, the moderate pluralism associated with Robert Brandon and
Brent Mischler or the more radical pluralism defended by Philip
Kitcher, argues that different explanatory aims within the biological
sciences will require different criteria for determining whether a
group constitutes a species. Cladists, on the other hand, employ
strictly defined phylogenetic tests to determine species
rank (see Rheins 2011). 




Unlike many of the other topics that define the history of
Darwinism, there is no clear-cut position on this question that can be
identified as 'Darwinian' or
'neo-Darwinian'. In a recent collection of papers
defending most of the alternatives currently being advanced (Ereshefsky
1992), my suspicion is that virtually every author in that collection would identify
himself as Darwinian. This may be because, as different as they
are, a number of positions currently being defended have their roots in
Darwin's own theory and practice (see Beatty 1985; reprinted in
Ereshefsky 1992).