Chicken, Eggs, and Speciation1

CHICKENS, EGGS, AND SPECIATION1

Abstract

Standard biological and philosophical treatments assume that dramatic genotypic or

phenotypic change constitutes instantaneous speciation, and that barring such saltation,

speciation is gradual evolutionary change in individual properties. Both propositions appear to

be incongruent with standard theoretical perspectives on species themselves, since these

perspectives are (a) non-pheneticist, and (b) tend to disregard intermediate cases. After

reviewing certain key elements of such perspectives, it is proposed that species-membership is

mediated by membership in a population. Species-membership depends, therefore, not on

intrinsic characteristics of an organism, but on relationship of an organism to others. A new

definition of speciation is proposed in the spirit of this proposal. This definition implies that

dramatic change is neither necessary nor sufficient for speciation. It also implies, surprisingly,

that an organism can change species during its lifetime.

CHICKENS, EGGS, AND SPECIATION

There may be some deep truth about whether chickens or eggs are more

fundamental, but no serious biologist would engage in such a debate, nor (I

hope) would any serious philosopher be exercised by the question.

(Philosopher, 1998)

Another smug aperçu to the kindling-basket? So it would seem, for CNN.com (International

Edition, May 26, 2006) reports that David Papineau and two others have solved the puzzle of

the chicken and the egg.

Mr Papineau, an expert in the philosophy of science, agreed that the first chicken came from an egg

and that proves there were chicken eggs before chickens. He told the UK Press Association that

people were mistaken if they argued that the mutant egg was not a chicken-egg because it belonged

to the “non-chicken” bird parents. “I would argue it is a chicken egg if it has a chicken in it,” he said.

“If a kangaroo laid an egg from which an ostrich hatched, that would surely be an ostrich egg, not a

kangaroo egg.” (CNN’s text has been edited for grammar, punctuation, clarity, and sense.)

Papineau’s team also included Professor John Brookfield, a specialist in evolutionary genetics at

the University of Nottingham, and Charles Bourns, a poultry farmer who chairs the

oxymoronically named trade body, Great British Chicken. CNN says that the team’s research

was “organized by Disney to promote the release of the film ‘Chicken Little’ on DVD.”

In this paper, I argue inter alia that chickens came into being at the same time as chicken

eggs, at least if you accept Papineau’s criterion. On a closely related criterion, chicken eggs did

indeed come before chickens, but this has nothing to do with the first chicken being in the first

chicken egg. In fact, on my way of looking at speciation, the first chicken did not come from the

first chicken-egg, and the first chicken-egg did not come from the first chicken.

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These, however, are merely amusing corrolaries. My primary aim in this paper is to lay

the foundations for, and explore the consequences of, a treatment of species-membership and

speciation that is mediated by populations. I shall proceed by showing first how the

supposition of a first chicken conflicts with certain standard ideas concerning species. Then, in

section 3, I attempt to pin these difficulties on a degree of pheneticist thinking in many

treatments of species and speciation. The rest of the paper is devoted to reconstructing species

and speciation along thoroughly non-pheneticist lines.

The proposed analysis has two significant implications.

First, it turns out on my definition that species membership is relational, and

consequently, that an organism can change its species during its lifetime without

changing its phenotype in any way that violates its normal developmental pattern.

Thus, speciation has nothing to do with “mutant eggs”: the first chickens were

previously non-chickens (and came, by Papineau’s stipulation, from non-chicken

eggs).

Second, a technical point: my analysis demands a clarification in the cladistic notion

of monophyly – I’ll explain this is when the time comes.

More of this later: first let us turn to the general issue of speciation. This discussion occupies

the bulk of this paper.

I. The Problem of the First Chicken

1. Papineau’s argument runs like this.

(1) The first chicken came from – hence, was preceded by – an egg E.

(2) An egg is a chicken-egg if and only if “it has a chicken in it”.

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Therefore (3) E was a chicken-egg that preceded the first chicken.

On my conception of speciation, this argument is invalid. I shall argue that the first chicken was

not a chicken when it was hatched. If this is right, E, the egg from which the first chicken

“came” never had a chicken in it.

But I am getting ahead of myself. There is something odd about the very notion of a

first chicken, as I shall now attempt to show. In this section, I shall argue that this notion arises

from the neglect of a crucial constraint on when two organisms belong to the same species.

Call the “first chicken” Charlie. In order for the chicken-species to have got going, Charlie

would have had to reproduce. Since chickens reproduce biparentally, he would have had to

find a mate. Call her Charlize. Charlize was not a chicken: it strains credibility that two such

similar mutations should occur independently in different organisms. Even if there are special

circumstances in which double mutations are likely, the speciation story should not assume

such an event, for then it would lose generality. Imposing such a requirement would force us to

disregard the many cases of speciation where special circumstances allowing or encouraging

multiple simultaneous mutations were absent. A general account of speciation must be able to

accommodate single-mutation beginnings.

Let us assume, therefore, that Charlize belonged to Charlie’s parents’ species: she was a

pre-chicken. (This assumption is introduced for the sake of simplicity: the argument can be

reconstructed with the weaker assumption that Charlize belongs to some species.) On some

species-concepts, the game is already over. Organisms that belong to one species are

reproductively isolated from organisms that belong to others – the habits, habitat, physiology,

and genetics of the organisms that belong to a single species enable them to recombine their

genes with those of others of the same species but not with those of any other species. Some

accounts of species elevate the possibility of such recombination into a definition. According to

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a strict construal of the Biological Species Concept, for example, two organisms belong to the

same species if and only if their genes (or rather, copies thereof) can recombine on the same

genome. By this species-concept, we have already contradicted the assumption that Charlie

was a chicken. Since he was able to mate successfully with Charlize, who was a pre-chicken, he

was a pre-chicken. It follows on such species-concepts that he was not even a chicken.

Obviously, then, he was not the first chicken.2

2. Let us waive the demands of the Biological Species Concept, or at least understand them less

stringently. Let us allow, at least for the sake of argument, that an occasional chicken can mate

successfully with a pre-chicken and produce fertile offspring, but count as a chicken

nonetheless. Let Charlie be a chicken then, his successful union with Charlize notwithstanding.

Papineau’s solution still does not work. For now the question arises: What about the children?

To what species do the offspring of Charlie and Charlize belong?

One possibility is that they are reproductively isolated: that is, they are not normally able

to produce fertile offspring, except incestuously with each other or with Charlie. If so, we can

define the species chicken as including all Charlie’s descendants up to but not including any who

result from a further species-founding mutation in the future. Given this assumption, Charlie

was indeed the first chicken – all that Charlize did was to help get the chicken species going,

which she did by helping to produce a plurality of birds that can interbreed, but not outbreed.

The possibility just conceded is, however, quite improbable, at least in the case of

organisms that reproduce biparentally – and surely we do not wish to exclude them from the

discussion. (In fact, my discussion side-steps asexual and self-fertilizing organisms: in this

paper, I shall be discussing the complications that biparental reproduction brings.) Charlie

himself could mate successfully with a pre-chicken. What prevented his immediate offspring

from doing so? If all went as expected, these birds would have had characteristics that ranged

between those of Charlie and those of Charlize. Since Charlie could mate successfully with a

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pre-chicken, those of his offspring that bore his characteristics should have been able to do so

as well. And of course Charlize is not isolated from pre-chickens – she is a pre-chicken and need

not have chosen one-of-a-kind Charlie. So her characteristics are not going to isolate her

offspring from other pre-chickens.3

How then can the offspring of Charlie and Charlize have been reproductively isolated? Is

this by a second mutation? Again, this seems quite improbable in the absence of special

circumstances (though not, of course, impossible). As I urged earlier, we should not construct

the speciation story in such a way that it requires two or more founding individuals occurring

through independent mutations: either two chicken-making mutations resulting in two

originals, Charlie and Charlize, or a Charlie-mutation plus one more that results in Charlie’s

immediate offspring or close descendants being reproductively isolated. The problem with the

Papineau-team’s account is that it cannot accommodate single-mutation beginnings.

3. Charlie is an innovation, and thanks to Charlize, his genes get passed on. If he is a favourable

innovation, his genes will spread across the pre-chicken world. In due course, pre-chickens will

become more like him. This is natural selection at work; it is how species become better

adapted over time. Here is a parallel: in the last thirty to fifty thousand years, humans have

changed in cognitively significant ways. For example, they have somehow acquired an innate

and specialized ability to use recursive grammars (cf. Hauser, Chomsky, and Fitch 2002). This is

a big change: it leads to all kinds of behavioural specializations in humans. But it does not imply

speciation. We would not be right to say that at some point in the process of acquiring a

specialized ability to use recursion, a new species was born. This was simply a transformation

and “improvement” of the human species itself.

The idea that mere innovation can result in speciation may have some intuitive appeal.

However, biologists generally acknowledge that there is a difference between adaptation and

speciation. Why they do so will become clearer when I present my own view of the matter.

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The point that we need to accommodate, though, is that innovation is not by itself tantamount

to speciation. The task is to figure out what sorts of changes are required for speciation, as

distinct from evolutionary change within species boundaries.

II. The Perils of Pheneticism

1. The egg-before-chicken thesis was first advanced by Roy Sorensen (1992). His argument is

somewhat different from that of the Papineau-team. It runs like this.

(4) It is indeterminate where in evolutionary history pre-chickens end and chickens

begin.

(5) However, “a particular organism cannot change its species membership during its

lifetime.”

(6) Therefore, “the transition to chickenhood can only take place between the egg-layer

and the egg,” in other words, this transition traces back to genetic or chromosomal

change during reproduction.

(7) The genotype in the egg that hatched the first chicken was already a chicken-

genotype, and the egg was thus a chicken egg.

Sorensen is making a point about vagueness: that even if we cannot determine which

transitional bird was the first chicken, it is logically necessary, given (5) above, that its genotype

existed in the egg from which it hatched.4 In effect, Sorensen uses universal instantiation – the

logic of ‘all’ – to trump the indeterminacy of chickenhood – whichever organism was the first

chicken, it is preceded by a chicken egg (by proposition 6), therefore the first chicken was

preceded by a chicken egg.5

Sorensen takes a view of speciation that is different from Papineau’s in one respect, and

the same in another. He says: “Charles Darwin demonstrated that the [first] chicken was

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preceded by borderline chickens and so it is simply indeterminate as to where the pre-chickens

end and the chickens begin.” The allusion is presumably to Darwin’s phenotypic gradualism.

Sorensen’s idea is that somewhere in the smudge of gradually evolving organisms, pre-chickens

gave way to chickens, but that all through this process, there was a reproductive community of

birds, whether this community consisted of pre-chickens, transitional chickens, or chickens. He

assumes that different features, or different enough, make for different species. Different

enough: that’s where vagueness comes in. Papineau et al take a more saltational line. They

seem to think, like Sorensen, that it is Charlie’s difference makes for the distinctness of his

species – this is presumably what their talk of “mutant eggs” amounts to – but they think that

this difference could have arisen in a single generation.

All of these thinkers seem, then, to be assuming a pheneticist conception of species – the

idea that species are defined by similarity. (In note 13, I briefly consider the possibility that

they were moved by purely phylogenetic considerations.) By hypothesis, Charlie is dramatically

different from his parents – Charlie’s parents failed to “breed true”, to use Sorensen’s

description of the case. But this does not acknowledge questions about reproductive barriers

between species. This is why these treatments fail to notice the problems of section I.

2. Pheneticism is a mistake.6 It originates in the harmless but imprecise idea that conspecific

organisms resemble each other. However, by defining species in terms of this similarity,

pheneticism trips up on the following:

a. The similarity of conspecific organisms is actually not universal. Polymorphisms

exist within species, for example, the division of many species into dissimilar sexes.

Moreover, similarities between some members of so-called sibling species may

actually be closer than those between some members of the same species. For

example, the males of one such species may well be more similar to the males of

another than to the females of their own species.

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b. Species-concept definitions in terms of similarity enshrine as fundamental that

which needs to be explained. Both the similarities and the polymorphisms alluded

to above – the “population structure” of species, as I shall be calling it – are

explained in terms of certain deeper factors. Roughly speaking, the population

structure of a species will be attributed to three kinds of factor: first, how that

structure helps to carve out a niche that distinguishes a species from its historical

1 For useful comments, I thank André Ariew, Rampal Dosanjh, Marc Ereshefsky, David

Papineau, and Denis Walsh.

2. Speciation by polyploidy is often referred to as instantaneous events. I think that this

is a mistake for exactly the same reason as is given in the text, namely that the newly

polyploidal individual cannot be reproductively isolated from his or her ancestral population if

the species is to continue. More on this later.

3. True, the heterozygote offspring might pose a special problem, since they might be

reproductively available only to one another. If so, they constitute a third group, one that is

different from both Charlie’s and Charlize’s. They don’t solve the problem of how Charlie’s

species got going, though – as we shall see in my discussion of polyploidy in section V – they

may figure in an account of how a new species got going. Not on Papineau’s account though.

4. Here Sorensen is different from Papineau et al. He does not assume that there is a

determinate first chicken; he does not insist that an egg is a chicken-egg if it has a chicken in it.

5 Note that Sorenson is assuming that there was a first chicken, despite our inability to

determine which one it is – otherwise universal instantiation does not work. Thus, he is

committed to the view that some bird was the first chicken, despite vagueness. Given this

assumption, his argument follows the lines of Papineau’s.

6. It is noteworthy that John Brookfield, one of Papineau’s collaborators, is a sceptic

about the rightness or wrongness of species-concepts. He has written: “the ‘species problem’ is

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predecessors; second, interactions among polymorphic types within a species that

contribute to its adaptation to its niche; and third, reproductive integration within a

species. The best accounts of species cut the lines between these taxa in way that

at least roughly correspond to the fracture-lines of these explanatory factors.

These considerations against pheneticism are more powerful than many contemporary

philosophers of biology fully realize. They represent implicit but unnoticed features of

taxonomic practice – how males and females, juveniles and adults, larvae, pupae and imagos

are all recognized as members of the same species. So-called “disjunctive” accounts are

needed for this purpose: accounts of the form “If-female-then-F, if-male-then-G, etc.”, and such

accounts do not tell us why males and females etc. are counted as members of the same

species.7

It has been claimed that the phenetic unification of males and females can be achieved

“by refined biometric techniques” (Sokal and Crovello, 1992), but what goes unsaid is that these

not a scientific problem at all, merely one about choosing and consistently applying a

convention about how we use a word. So we should settle on our favorite definition, use it, and

get on with the science.” (Brookfield 2002, quoted by Coyne and Orr [2004, 25].) I take it that

he would reject the idea that there is a right or wrong about pheneticism: presumably, he

thinks that we are free to adopt the pheneticist species-concept or to reject it. The mystery, of

course, is why he thinks that there is a right or wrong about the chicken and the egg, and why

he does not subscribe instead to the epigram of this essay.

7. The standard pheneticist line is first to divide organisms into equivalence classes of

similar individuals and then to divide these up into interbreeding groups (cf. Sokal and Crovello,

1992, 36). If this procedure is carried out as described with sexually dimorphic sibling species,

we may get end up with one class of males (with no interbreeding subgroups) and another of

females.

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(essentially disjunctive) techniques have to be rigged in order to serve the “wish to associate

males and females that appear to form sexual pairs” (ibid., 37). On what is such a “wish”

premised – why should we count interbreeding males and females as belonging to the same

species? Pheneticists simply refuse to acknowledge that this can only be based on the role that

reproductive integration plays in heredity and in evolution. This lack of groundedness and

motivation tells against even weakened pheneticist accounts – for example, the philosophically

fashionable Homeostatic Property Cluster Theory, which is a clever and (in certain ways)

insightful variation on Wittgensteinian “family resemblance” theory, endorsed by such

sophisticated philosophers as Richard Boyd (1991, 1999), Ruth Millikan (1999), Paul Griffiths

(1999), and Robert Wilson (1999b).8

Pheneticism has no principled distinction between variation within a species and the

differences that separate species.9 This is illustrated in Darwin’s own tendency – for he

implicitly employed a pheneticist conception – to treat of species as classifications “arbitrarily

given for the sake of convenience to a set of individuals closely resembling each other.” More

to our present point, it animates the belief, evinced by the philosophers discussed above, that

“macro-mutation”, or saltational change is needed for speciation, while gradual change occurs

within a species.

Michael White (1978) remarks of Ernst Mayr, “he seems to have drawn a false antithesis

between instantaneous speciation through individuals and gradual speciation through

populations”. (White may not be correct in his interpretation of Mayr, but he puts his finger on

a tension that emerges in Papineau and Sorensen.) The source of this “false antithesis” is the

correct observation that individuals can differ dramatically from their parents, while 8. See Ereshefsky and Matthen 2005 for a fuller discussion of the points just made, and

how they tell against the Homeostatic Property Cluster view of species. White 1978 and Sober

1988 contain classic critiques of strong pheneticism, and these are supplemented by Coyne and

Orr 2004.

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populations only evolve gradually – even if a dramatic innovation like Charlie can appear in a

single generation, it takes time for the whole population to become full of Charlie-types. The

question that arises from the discussion so far is this: What does this have to do with

speciation? That is: why should the birth of a dramatically different individual constitute a

speciation event? (This was the question raised in section I above.) Pheneticist ideas lurk

behind this arras.

III. The Structure of Species

1. The point advanced in the preceding two sections was that no matter how different Charlie

is from his pre-chicken ancestors, difference alone will not make him a chicken. This raises the

question: What, besides similarity and difference, is involved in the species concept? In this

section, I review some structural considerations concerning species. My aim is not to advance

9 Devitt (forthcoming) tries to skirt this problem by distinguishing between the question

of what makes a biological taxon a species (the Species Concept Question) and the question of

whether all members of a species share essential properties (the Species Commonality

Question, as I shall dub it). He sums up his argument for a positive answer to the latter

question in the following words: structural explanations in biology demand that kinds have

essential intrinsic properties. To which a brief response is: only if all members of a kind must

(synchronically as well as diachronically) share some properties. This condition may not be met –

it’s possible that for every property that some members of a species share, there are some

members that do not share it. It is also possible that any properties that members of a species all

share are structurally explained in terms of extrinsic properties. In fact, something like this is (in

my view) precisely what any adequate Species Concept would predict. In such cases, the

“structural explanations” would not demand that all members of a kind must share essential

intrinsic properties.

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or defend any particular species-concept, but simply to set some parameters for the treatment

of species and speciation.

2. Each species has a population structure, a more or less stable – but at times evolving – range

and proportion of types. The types themselves are defined either functionally, in terms of their

interaction with other types in the species-population, and the role that these types play in the

species’ ecological strategy, or phenetically, where no functional role is involved. A type

distribution consists of a series of complementary types paired with their proportion in the

population. (Examples: half males, half females; a certain age-distribution accompanied by age-

dependent differences of size, behaviour, and dependency; a proportion-specified dominance

hierarchy; a range of variation in phenotypic characteristics such as size and colouring.) As I

shall use the term, a population structure is a complete type-distribution for a given population

or species, i.e., a distribution that takes in all of the ways that members of the population or

species differ from one another.

Different species have different population structures: the types themselves, ranges, and

type-proportions of a species will show significant differences from those of other species. The

important point here is that in many cases selection brings about a distribution of

heterogeneous types within a population, rather than a homogeneous distribution of

characteristics. Normally, this happens because the fitness of one type depends on the

frequency of that type relative to others. For example, the division of the population into sexes

or castes is maintained because the fitness of belonging to one such type depends on how

many other organisms belong to that class, relative to others. There is no absolute (biological)

advantage for an organism in being a male rather than a female, or vice versa. But as R. A.

Fisher argued, if there are lots of females relative to males, then it is advantageous for an

organism to give birth to males, and conversely if there are lots of males relative to females.

The frequency-dependence of sex-fitness ensures that there are both males and females, and in

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stable proportions. Similarly the age-profile of a species is manipulated by selection; the

distribution of age-types in the population is not simply a chance occurrence supervenient on

individual birthing and maturation patterns.

The effects of such frequency-dependencies can be re-expressed in population-structural

terms. Consider the range of random variation in phenotypic characteristics such as size or

colour. Suppose that it is disadvantageous for a human male to be less than 140 cm tall or

more than 210. Suppose that in between these extremes, there is no advantage either way,

unless one was at one end of the range – because of sexual selection against conspicuous

extremes, say. Suppose that the ends of the range became under-represented as a result.

Viewed one way, this is individual (sexual) selection. But it can also be looked at as a case of

the distribution being subject to certain selective pressures. If the “tails” of the distribution

should be subjected to negative pressure, then selection will act to make height-distribution

more “peaked”. Thus, height-distribution can be understood as an evolved property of the

population under natural selection. This switch of perspective is not meant to suggest that

group selection is at work in determining height; it is merely a reverse “book-keeping”

manouevre by which frequency-dependent polymorphisms are expressed as ensemble-level

distributions, even though some can be understood in terms of selected properties of

individuals. By representing selection in terms of population structures, we bring all

polymorphisms under a common rubric.

3. The traditional problem of species was posed in this way: why are there gaps in nature?

That is, why do the individuals within a given species resemble one another more than they

resemble those of any other species? Why are there no intermediates in between species?

What could explain such discontinuities? Because of polymorphic population structures, this

turns out not to be a good way to describe the phenomenon. The discontinuity of nature is not

best defined in terms of individual similarities; the distinctness of species has to be understood

in terms of differences in their population structures. Humans are different from chimpanzees

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in terms of individual differences, of course, but a more general and abstract way of expressing

the difference between the two species is to say that their population structures are different.

The problem is to understand why the chimp population structure not only is, but stays,

different from the human population structure.

If species difference is difference between population structures, then the birth of a

dramatically anti-typical individual such as the first “chicken”, Charlie, does not necessarily

constitute a speciation event. In the more traditional conception, according to which species

differences are differences between individuals, there is a strong reason for regarding anti-

typical individuals as belonging to a different species, since there is a gap between such

individuals and the rest. But when we reformulate the gaps-in-nature proposition in terms of

differences of population structure, our perspective changes. Charlie does, of course, alter the

pre-chicken population structure by introducing a new type or variant. But since the population

structure of a species can change without the gap between it and other species disappearing,

there is no reason why we should think that the anti-typical individual makes a new species.

The crux is that the pre-chicken population with Charlie added may still stay distinct from other

populations. This gives us a new reason for doubting that Charlie’s difference makes him the

first chicken – a reason that arises from defining species in terms of population structures, not

from considerations of reproductive isolation.

The account of gaps in nature, roughly speaking, is this: species possess certain

distributional characteristics that enable them to go their own way in evolution, and this allows

them to go to discrete places, with no group occupying the places intermediate between

species. Working backwards from this idea, we understand species themselves to be groups of

organisms that possess the characteristics – whatever these may be – that enable them to go

their own way in evolution. My point is that the birth of an anti-typical individual need not be

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sufficient to ensure that a population will chart a new course. For this reason, such an event

need not necessarily be regarded as a speciation event.

4. How then shall we identify a species? Minimally:

Species are temporally extended classes of organisms with population structures

that are stable in the short run (in evolutionary terms), but which may evolve in the

longer run, where these population structures are created and maintained and stay

separate from other population structures because of the reproductive and

ecological integration working on type-distributions within the group.

This is an population-level characterization of species; unlike pheneticist accounts, it is

focussed on the influences that bring it about that a certain class of organisms is causally

coherent and separate from other such classes.

We can now distinguish between diagnostic and explanatory species-concepts. A

diagnostic concept tells us how to recognize a species; an explanatory definition tries to get at

the factors that are responsible for the maintenance of population structures that are

discontinuous from one another. The pheneticist species-concept is diagnostic par excellence.

It fastens on an alleged characteristic of species – the similarity of members one to another –

and uses this to recognize species with no regard to what may be responsible for it. The

minimal characterization above is explanatory, since it cites reproductive and ecological

integration as a cause of there being species.

Explanatory concepts should not be treated as deficient because they fail to do the job of

diagnostic concepts. It is completely beside the point to complain, as Sokal and Crovello (1992)

do, that explanatory species-concepts are not “operational”. Explanatory concepts may make

reference to unobservable past or present events or processes, and thus they are often

unhelpful with regard to the question of how to determine species boundaries. Equally,

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explanatory concepts should not be thought of as a substitute for diagnostic concepts. Earlier, I

argued that pheneticism is a mistake. Nonetheless, it should be acknowledged that species can

be recognized in phenetic terms.

IV. Species Integration, Equivalence Classes, and Populations

In the previous section, species were understood as temporally extended groups of organisms

that are created, maintained, and stay separate from other population structures because of

the reproductive and ecological integration working on type-distributions within the group.

One could argue that this integration implies that organisms can belong to at most one species.

The reasoning goes like this: if an organism could belong to more than one species,

recombinatory gene-flow between these species would be possible through this individual.

Consequently, the characteristics of the two species would flow into each other. This would

entail, further, that organisms of different species would come into competition for the same

resources. Thus, ecological separateness would break down. Thus, it is argued, the at-most-

one species assumption is required for explaining the living world as we find it: a world that

does have stable species-differences, a world divided into groups of organisms with internally

structured commonalities and differences.10

It is trivial to stipulate that every organism belongs to at least one species. This entails no

loss of generality, since in the limiting case, the class can be a singleton. Together with the at-

most-one species condition just discussed, this implies that species must constitute a set of

classes such that every organism belongs to one and only one; in philosophical terminology,

species must be equivalence classes. This requirement is not imposed on diagnostic grounds. It

might sometimes be difficult to diagnose which of two species an individual belongs to. The

10 I am not suggesting that this view has actually been held in the extreme version

presented here.

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claim is rather that the proper strategy for explaining gaps in nature inevitably assumes that

there are no individuals that bridge gene-flow between species. This rationalization of the

equivalence class requirement has nothing to do with similarity, and everything to do with how

gaps in nature are to be explained.

Now, as it happens, there is a certain amount of recombination between species

recognized to be distinct, especially among plants; that is, there is a certain amount of

hybridization. This is generally thought to constitute a test, as it were, of the equivalence class

requirement because the hybrids are in the gene-flow pool of both species. There is some

disagreement about how to proceed in this matter. On the one hand, some biologists

(Templeton 1992) and philosophers (Ereshefsky 1992a) are inclined to think that the

equivalence class requirement is unrealistic and should simply be dropped. On the other hand,

some biological systematists respond by emphasizing how infrequently these phenomena

occur. Jared Diamond (1992), for example, concludes on the basis of a review of plant studies

that “despite the occasional horror stories, plant as well as animal species are most profitably

defined as interbreeding communities” (628, my emphasis).

The thrust of Diamond’s argument is that “a modest incidence” of problematic cases is

“hardly enough to undermine the utility” of the biological species-concept. This sounds as if he

is simply being pragmatic: “Pretty close is as good we are going to get, so let’s live with it,” he

seems to say. This is, however, to undersell the argument. The claim is that the reproductive

and ecological integration needed to explain gaps in nature leads to something like the at-most-

one-species rule. Diamond could be taken as urging that since there are indeed gaps in nature,

and since nobody wants to give up on the value of reproductive and ecological integration in

explaining these gaps, it makes sense to proceed as if the at-most-one-species rule were

correct, or nearly correct. It would be correct if there were no complications of nature leading

to freakish events – horror stories.

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Now, it is obviously true that near integration will have the same observable results as

complete integration (cf. Templeton 1992, particularly 164-8). In other words, the threat posed

by the breakdown of the at-most-one species rule – the breakdown of species distinctness – is

not actually very threatening where there is near integration, because in the latter

circumstance as much as in the former, there are factors that work towards maintaining these

distinctions, while the interference provided by the small number of problem cases is weak and

ineffective. So it seems right to say, as Diamond does, that a modest incidence of problem

cases is, in some sense, acceptable. The question is this: how exactly should we modify the

Equivalence Class Requirement to accommodate this insight? More crucially: Is hybridization

really quite as infrequent as Diamond suggests? What happens if it is not? Would we be

justified in holding to the at-most-one-species rule if Diamond is wrong on this point?

The question just posed – in effect: “How much intermingling between species is too

much for species distinctness?” – does not admit of an a priori answer. And so it is not possible

for philosophy to adjudicate the dispute. What philosophy can do, however, is to notice that

there are two levels of analysis in play in the dispute. First, there is the explanatory level, at

which reproductive and ecological integration is invoked to explain gaps in nature. Secondly,

there is what we might call the extensional level, where the overlap between species is

numerically estimated. This helps us more accurately formulate what is at issue in the dispute

about hybridization. This dispute is not about the existence of gaps in nature nor is it about the

value of reproductive and ecological integration in explaining them – both are universally

conceded. Thus, the dispute is not about the explanatory level. Rather, it is about the

extensional level: about the degree to which a certain explanatory strategy entails the at-most-

one-species rule.

At this point, ontology can be called in to help. Let us posit populations – groups of

organisms that are ecologically and reproductively integrated as our explanatory strategy

demands. (For the moment, I shall treat species as populations, and make the distinction

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between populations and species in the following section.) This should offend nobody, because

we have made no stipulation about the permissible overlap between populations. This leaves

the way open for us to say that the integrative properties of populations explain gaps in nature.

This way of putting things allows us to be flexible with regard to the overlap of populations.

Some think that there is a lot; some think that there is not very much. The diplomat’s way

around this dispute is to observe that both sides agree that there is at most as much as will

preserve the integrative character of populations – the disagreement is just about where this

Goldilocks point is located. Since we have stated the integration-condition without committing

ourselves to non-overlap, we are off the Equivalence Class Requirement hook. But note that

there is an ontological cost: we have just stipulated the existence of populations over and

above individuals. As we shall see, however, this is a cost cheerfully borne by some species-

theorists, for example Elizabeth Vrba (1995), who posits species and populations as complex

systems.

With this point in mind, we can now return to the problem of the first chicken. As we

have seen this is a problem that raises questions about intermediates – about whether Charlie

is a chicken or a pre-chicken or both. For this reason, the at-most-one-species rule is the point

of attack for many philosophers and biologists who want to respond to this problem – they ask

which species given transitional individuals belong to. The most common response is to

abandon the idea that speciation occurs suddenly, i.e., to suggest that temporal species

boundaries during speciation are fuzzy – and Sorensen tries to cut through the problem of how

fuzziness affects the at-most-one species rule. But if my analysis is correct, we would be wise to

skirt around the question of overlap. Another point of attack is available to us. We should ask

how populations figure in the emergence of chickens.

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V. Enter the Population

1. Speciation is generally thought of as a change in the features of individual organisms. And,

of course, this is not completely wrong: as we shall see, speciation is both the cause and the

consequence of individual-level change. I have, however, been emphasizing the role of

ensembles and population structures in evolution. Species, and even more so populations, are

evolved and evolving complex systems (Vrba 1995). Gene exchange and competition within

these ensembles, and their radiation into specialized ecological niches, are major factors in

evolution; they are factors that involve the ensembles as coherent and unitary causal actors.

Moreover, the differentiation of species from one another is not, as I argued earlier, primarily

the emergence of new individual characteristics, but the emergence of new population

structures to adapt to specialized needs.

The role of the ensemble is often overlooked in definitions of species. Yet, in view of the

argument given so far, it is natural to understand species not as classes of organisms that share

certain intrinsic characteristics, but rather as ensembles of organisms with characteristics that

enable them as ensembles to follow separate evolutionary trajectories. To facilitate such an

approach, I shall propose that populations mediate species-membership – species are

collections of populations. I shall propose, further, that individual organisms are members of

species because they are members of populations that belong to species. I shall attempt to

build this change of perspective into the definition of speciation. I claim that speciation is

something that happens to an ensemble – to wit, that it diverges from others, with the

consequence that two complex systems emerge, each subject separately to reproductive and

ecological integration, where previously there was only one such system.

2. Let me begin therefore by defining population:

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A population consists at any given time of organisms all of whose non sex-related

genes have relatively free access to all others in the group for purposes of such

gene-recombination by means of normal sexual reproduction.11

“Relatively free access” seems like a vague term. However, it can be used to construct

a more precise definition. Consider two organisms in the same population. The probability of

their genes recombining rises steadily as the elapsed number of generations rises. Consider, by

contrast, two organisms from different populations. In order for their genes to recombine, one

of them has to migrate to the other population. The probability of this occurring is much lower

than that of recombination within a population. However, the probability that some organism

from one population will migrate to another rises slowly with the passage of time until it

reaches that of recombination in one generation within a population. At that point,

recombination between the migrating organism’s genes and others in its host population will

be the same as that of any other within-population pair – and the same applies to any other

organism whose genes (or rather copies thereof) are resident in the migrating organism’s

genotype. For instance, the migrating organism’s parents’ genes can now recombine with

those of the host population, though the parents did not themselves migrate.

11. Two clauses of this statement should be carefully noted. The first is that all of the non

sex-related genes of one organism are available to be recombined with those of the other.

Since humans have pre-human genes in their genotypes, there are presumably ancestral genes

sitting in each of our genotypes waiting for recombination with genes drawn from other

humans. This, however, does not mean that the ancestor in question is drawn into the human

species, because not all of its genes are available to us, nor all of ours to it. Secondly, note the

qualification about normal sexual reproduction. This is meant to exclude recombination in

vitro, or by natural processes involving viruses etc.

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Consider therefore the pair-wise recombination probability P(x,y,n) – the probability that

organism x’s genes will recombine with organism y’s genes in n generations by normal sexual

reproduction. For some pairs of organisms, x and y, P(x,y,n) rises smoothly as the number of

elapsed generations n increases from 0, and asymptotically approaches unity. For other pairs,

P(x,y,n) rises much more slowly for several generations, but then rises at the same rate as the

low-n rate for the former pairs. The former pairs belong to the same population; the latter

belong to different populations. To repeat, for the latter, the initial low probability is ascribed

to the low probability of migration; after this probability has reached some threshold value,

however, it becomes relatively unimportant and the value of our pair-wise recombination

probability function will then parallel the within-population slope. In other words, the step-

wise rise in probability after a largish number of generations defines pairs of organisms that

belong to different populations. Conversely, two organisms belong to the same population if

the recombination probability rises smoothly right from the start. (It should be said that this is

not meant to be a diagnostic criterion, since these probability values are not easily estimated.)

The smooth increase of recombination-probability over time defines a population at a

time. But populations persist for a period of time. We can trace their temporal career as

follows. There are causal factors that enable and ensure that a collection of organisms possess

access to one another at a time. Most importantly, of course, they must possess a common

fertilization system, to use Hugh Paterson’s term. But also, they need to inhabit a contiguous

geographical locale and enjoy other circumstantial commonalities. These conditions persist for

a period of time. The temporal extent of a population is defined by the persistence of these

conditions or others that replace and supplement them. The membership of a population over

time consists, in other words, of all those organisms that have lived under the said conditions as

long as they last. For even if two organisms are widely separated in time, their genes can

recombine through the very same channels, provided that the conditions in question stay in

place.

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This imparts a historical characteristic to populations. A bird that resides in Madagascar

might make a suitable member of a population found in South America if it were transported

there. However, the historical accident of where it lives makes it inaccessible to its potential

South American mates – for the time being, at least. (I assume that there is a non-zero, if

sometimes very low, probability, that some member of any given population will eventually

reach any place on the globe.) For its part, the South American population will display many

family connections: the offspring of any given pair will likely remain within the population,

spreading the parents’ genes around. Which population a given organism happens to belong to

depends on the historical circumstance of where it was born, not which organisms it resembles.

Despite their persistence over many generations, populations are relatively transitory

entities. Here is an example. Local human populations were once more isolated than they are

now: most of the humans in the British Isles in 1800 were descended from humans who lived

and found their mates there in the century previous – though not all, since some migrated from

and to France, Spain, and even further afield, thus introducing themselves into new populations

and taking themselves out of old ones. Some travelled back and forth, and participated in more

than one group, leaving descendants who may similarly have participated in more than one, or

may have confined themselves to just one. Even taking such leakages into account, however,

the human inhabitants of the British Isles in the 17th century constituted a temporal slice of a

population: the likelihood of their mating outside was significantly lower than inside.

Nowadays, however, because of global migration, the causal foundations of British

reproductive isolation have been undermined. The globalizing process that has made Chicken

Tikka Masala the British national dish has also destroyed the human population of the British

Isles by merging it with other once-local populations. The new merged population has replaced

ancestral local populations. (I hope it will be understood that I mean no political comment –

either mournful or gleeful – when I say that the British population has been “destroyed”. I

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mean only that a once distinct entity no longer exists as a distinct entity, but has become

instead a part of a larger one.)

Populations are vehicles of natural selection. Primarily, this is so because the selective

advantages that a gene brings can spread through a population only through recombination,

and recombination occurs for the most part with within-population mating pairs. But also, the

conditions that ensure availability for purposes of gene-recombination simultaneously ensure

proximity for purposes of competition; in other words, they bring ecological integration with

them. Thus, the emergence of a favourable mutant in an Asian insect population will lead to a

change only in that population. It will not lead to any change in a population of conspecifics in

Australia because the advantageous Asian variants neither compete with nor recombine with

Australian genes unless there is some leakage from one population to the other. Such leakage

does occur, of course: this, as we shall see in a moment, is what ensures that the two

populations belong to the same species. The point is that when an Asian animal migrates to

Australia, it will normally become a member of the Australian population and start off a new

selective train of events there – new, because this train of events is under the influence of a

hitherto unprecedented competition.

A species is a collection of populations with a stable (but evolvable) population structure

maintained by gene flow and ecological equivalence within and between populations. In the

preceding section, I argued that the standard explanatory strategy for explaining “gaps in

nature” is best articulated by positing that populations as entities over and above, and not

merely collections of, individual organisms. Here, I am proposing that species be defined in

terms of populations, not organisms.

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VI. Speciation: A Population-Based Account

The core of my proposal is this: organisms belong to species because they belong to a

population that belongs to that species. As we shall see, mediating the organism-species

relation in this way leads to some surprising results. And it casts new light on the status of

Charlie, the supposed first chicken.

I argued in section I that even though Charlie was more like a chicken than a pre-chicken,

he is not a chicken. He is a reproductively integrated member of a population of pre-chickens,

and this makes him a pre-chicken. Consider, however, P, the population of which Charlie is a

member. Suppose that, being advantageous, the Charlie-gene starts to spread within P.

Suppose further that during the course of selection, bearers of the Charlie-gene become

progressively less likely to mate with non-bearers. This could happen in several ways. Charlie-

gene bearers may just prefer others of the same kind. Or further mutations and consequent

behavioural modifications might achieve the result. Or these birds may behave this way for the

simple reason that the increasing frequency of Charlie-gene bearers gives them less and less

choice.

In time, one of two things may happen as a consequence of this mating pattern.

First, it might happen that a certain sub-population of P, consisting of those that

carry the Charlie-gene, becomes reproductively isolated from the rest of P. Here, a

new “isolating mechanism” comes into play, cutting some members of P off from

others. In virtue of this new barrier, we say that the sub-population becomes a new

population – call it C for Charlie – separate and distinct from the other part of P.

Alternatively, P itself might become progressively more and more isolated from

other pre-chicken populations as the Charlie-gene bearing birds come to dominate.

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Either way, we get a population – P itself or C – that is newly isolated from other

pre-chicken populations.

Let’s idealize a bit and pretend that the Equivalence Class Requirement is still in force.

Consider then the first moment M when a population has become completely separate from

other pre-chicken populations – the first moment, that is, when the possibility of non-

hybridizing gene-exchange with these other populations is completely extinguished, as well as

all other integrating conditions. Such an event would occur, for instance, when the last

surviving bridge-individual between C and the rest of P dies or ceases to be fertile. This

individual’s genes were able to recombine with those of all other organisms in P, whether or

not these others carried the Charlie-gene; further, the products of such recombination are able

to do the same. When this last bridge-individual ceases to be one, the last channel of

communication that unites P is destroyed. Henceforth, members of C cannot recombine their

genes with non-members. (Notice that the Charlie-gene itself need not be responsible for this.

It might be that some other gene identifies Charlie-gene bearers – or a subset – to one another,

or gives them a common distinguishing fertilization system.)

I claimed earlier that if this event should take place because a subset of P gets isolated

from the rest of P, then a new population C is thereby created. Mainly for the sake of

convenience, I stipulate further that if the isolation-event should occur because the entire

population P becomes isolated, P is thereby recreated as a new population, P'. That is, even if

the membership of P stays intact, it will be, by my stipulation, a new population. Thus, the

event that I am envisaging entails the creation of a new population.

The point that I want to make is that this is a speciation event. It is the creation of a new

population completely isolated from all others. Since such a population constitutes a species –

a collection of one population with a stable population structure maintained (in part) by gene-

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flow – this is the creation of a new species. Here, in other words, is the idealized version of my

thesis:

Speciation occurs when a population comes to be reproductively isolated because

the last individual that formerly bridged that population to others died, or because

this individual ceased to be fertile (or when other integrating factors cease to

operate).

It is worth noting that the sequence of events sketched above corresponds quite closely

to the story usually told regarding polyploidy. Polyploidy consists of a duplication of one or

more chromosomes, so that the offspring has a higher chromosome number than the parents.

Let us call this event Y and the offspring thus created O. O’s chromosome incompatibility often

constitutes a barrier to mating successfully with other organisms. (Recall my earlier stipulation

that since I am dealing with the complications of biparental reproduction, I am not worried

about self-fertilization.) If this barrier is absolute, then of course, this is the end of the story: O

is barred from having offspring. Suppose, however, that O is simply less fertile when she mates

with those of her conspecifics who happen to have the ancestral, i.e. lower, chromosome

number. What then occurs is that O has descendants. Some of these will have the same

chromosome number as O; others will have the ancestral chromosome number. Suppose that

those with the higher number are more fertile when they mate with others of the same type,

and similarly those of the lower number. Under these circumstances, it will be advantageous

for them to evolve isolating mechanisms that prevent each from mating with the other type. As

these isolating mechanisms are evolving, there will be a transitional phase when some

members of each type are unable to mate with those of the other type, while others are able to

do so. Suppose that those of the latter “bridging” type decline in frequency, and at M the last

one dies. It is my contention that M is when the descendants of O become reproductively

isolated.12

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This, as I said, is the standard story of speciation by polyploidy. Most biologists claim that

polyploidy results in a sudden speciation event. I agree with this. However, the claim that most

biologists make is that the speciation event is the polyploidy event that starts the whole

process off. This is where I disagree. I am suggesting that it is the culminating isolation event. I

am claiming also that there is always such a culminating event when there is speciation.

Another point that is worth making here is that my account requires that we abandon the

notion of monophyly and paraphyly adopted by most cladists. That requirement is as follows:

Individual monophyly For all biological taxa (including species), there is a single

individual, such that the taxon consists of all and only those individuals that

descended from that individual. (The individual paraphyly requirement is derived

by dropping the “all and” from the monophyly requirement.)

Now, the account given in section V above violates the individual monophyly and paraphyly

requirements, because it is possible (a) for members of a single population not to share a

common ancestor, and (b) for siblings to be separated such that one belongs to a population

that breaks away from a species and the other belongs to a population that stays with the

ancestral grouping. In this case, not all of the descendants of the parent belong to the one

population. Now, my account envisages a descendancy relationship on populations. Thus, I

would replace the above requirement with the following:

Population monophyly For all biological taxa (including species), there is a single

population, such that the taxon consists of all and only those populations that

descended from that population. (Population paraphyly is also defined by dropping

‘all and’.)

12. The merger of species is a possibility on my account: what is needed is that hybrids

should so evolve as to create stable gene-flow between previously isolated populations. I leave

it to biologists to say why this is unlikely – if it is.

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To conclude, let me now drop the Equivalence Class Requirement. I proposed earlier that

a population is an entity within which there is reproductive and ecological integration, and that

a species is a collection of such entities which is also integrated because of gene-flow between

the populations. On this account, speciation occurs when a population separates from the

collection and goes its own way. The gene-flow between this population and other populations

in its former species may continue, but it is not enough to maintain integration. This gives us a

non-idealized account:

Speciation occurs when a population comes to have integration conditions separate

from the integrated class of populations to which it formerly belonged.

This is a gradualist, rather than punctate, account of speciation. Much like the pheneticist

account given by Sorensen, it is subject to vagueness. But the vagueness occurs with reference

to an explanatory property of populations, not a pheneticist property of individuals.

VII. Which Came First, the Chicken or the Egg?

Back, finally, to the chicken and the egg.

Papineau and his colleagues assume that speciation occurs when the first “deviation” is

born. (This parallels the standard treatment of polyploidy.) From this, and the principle that “it

is a chicken egg if it has a chicken in it”, they reason that the egg comes first. In my view,

however, speciation occurs much later. Charlie, the deviant pre-chicken, resembles chickens

more than he resembles pre-chickens: he is a pre-chicken nevertheless.13 The species chicken

comes into being much later than Charlie – when some population with Charlie-genes in it

separates from its ancestor population.

When the population separates and becomes a chicken-population, its members become

chickens. (A new principle: it is a chicken if it belongs to a chicken population.) Indeed, all of its

30


members simultaneously become chickens. Which came first, then – the chicken or the egg?

At first sight, it is the chicken. And not the chicken either: a whole lot of chickens.

But what about the eggs from which these first chickens hatched? They were pre-chicken

eggs by Papineau’s criterion, not chicken eggs – that is, they had pre-chickens in them. True,

these pre-chickens later became chickens, and so it is true to say that things inside the eggs

were later chickens. But I understand Papineau’s criterion to mean: “it is a chicken egg if the

organism inside is a chicken at the time when it is in the egg, or at the moment of hatching.”

On this understanding, the first chickens came from pre-chicken egg because the first chickens

were pre-chickens when they hatched.

The first chickens did not come from chicken eggs, then. But it does not follow that these

chickens pre-existed chicken-eggs. For just before the first chicken population became a

chicken population, there were a number of unhatched pre-chicken eggs in existence. At the

moment of speciation, these eggs turn into chicken-eggs, because at that moment, they have

chickens in them. They are so despite the fact that they were laid by what were then pre-

chickens.

However, there is a small complication here. Papineau and others treat two criteria as

equivalent: having a chicken inside, and hatching a chicken. These criteria are equivalent if one

13. According to Joel Cracraft (1992), Charlie is a chicken because of the isolating events

that took place later: he is the original member of the chicken clade. I am sceptical: cladistic

analysis is very useful, no doubt, in establishing the boundaries of higher taxa, but it is

retrospective. According to this concept, Charlie is a chicken only because of events that took

place long after he died – events that need not have occurred. Precisely because it is

retrospective in this way, the phylogenetic species-concept does not offer us an explanatory

concept of species sameness and difference. That is, it does not rest on causal influences

operating on the early descendants of Charlie.

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assumes that an organism cannot change its species. But under the relational conception, this

assumption is defeated. Thus, the two criteria are no longer equivalent. The eggs that were

around at the moment of speciation will always have been eggs that will hatch chickens. On

this understanding, then, some chicken-eggs preceded all chickens. Note, however, that these

are not the eggs from which the first chickens hatched. We have already established that the

eggs from which the first chickens hatched were not chicken eggs.

I have emphasized that my approach contradicts a widely held philosophical position,

namely that organisms necessarily belong to the species to which they belong. I am suggesting,

indeed, that some organisms actually change their species during their lifetimes, i.e., at the

moment when the population to which they belong becomes isolated. For any pheneticist

treatment of species, this is a fatal blow: if the same organism can belong first to one species,

then to another, and this without intrinsic change, pheneticism has to be wrong.

These perhaps startling conclusions trace to the relationality of my notion of species-

membership and of speciation itself. What species an organism belongs to depends, on my

analysis, not solely on intrinsic characteristics of the organism but also on how the population

to which it belongs interacts with other populations. This is of a piece with the Population

Structure Theory view of biological taxa introduced by Marc Ereshefsky and myself (2005).

References

Boyd, Richard 1991. Realism, Anti-foundationalism, and the Enthusiasm for Natural

Kinds. Philosophical Studies 61: 127-148.

Boyd, Richard 1999 “Kinds, Complexity and Multiple Realization: Comments on Millikan's

“Historical Kinds and the Special Sciences”” Philosophical Studies 95:67-98.

Coyne, Jerry A. and H. Allen Orr. 2004 Speciation Sunderland MA: Sinauer.

32


Cracraft, Joel 1992. Species Concepts and Speciation Analysis. In Ereshefsky 1992b. (Originally

published in Current Ornithology 1, 1983.)

Devitt, Michael (forthcoming). Resurrecting Biological Essentialism. Philosophy of Science.

Diamond, Jared M. 1992 Horrible Plant Species. Nature 360: 627-8.

Ereshefsky, Marc

1992a. Eliminative Pluralism. Philosophy of Science 59: 671-690.

1992b The Units of Evolution: Essays on the Nature of Species Cambridge, MA: MIT

Press.

Ereshefsky, Marc and Mohan Matthen 2005 Taxonomy, Polymorphism, and History: An

introduction to Population Structure Theory. Philosophy of Science 72: 1-21.

Griffiths, Paul 1999 Squaring the Circle: Natural Kinds with Historical Essences. In Wilson 1999a.

Hauser, Marc D., Chomsky, Noam, and Fitch, W. Tecumseh 2002. The Faculty of Language:

What Is It, Who Has It, and How Did It Evolve? Science 298: 1569-1579.

Lambert, David M. and Hamish Spencer (eds) 1995. Speciation and the Recognition Concept

Baltimore: Johns Hopkins University Press.

Millikan, Ruth 1999. Historical Kinds and the “Special Sciences”. Philosophical Studies 95: 45-

65.

Paterson, H. E. H. 1992. The Recognition Concept of Species. In Ereshefsky (1992b): 139-158.

(Originally published in E. Vrba [ed.] Species and Speciation Pretoria: Transvaal Museum

Monograph No. 4, 1985.)

Sober, Elliott 1988 Reconstructing the Past: Parsimony, Evolution and Inference. Cambridge,

MA: MIT Press.

33


Sokal, Robert and Crovello, Theodore 1992. The Biological Species Concept: A Critical

Evaluation. In Ereshefsky (1992b): 27-55. (Originally published in American Naturalist

104, 1970.)

Sorensen, Roy A. 1992. The Chicken Came Before the Egg. Mind 101: 541-2.

Templeton, Alan R. 1992. The Meaning of Species and Speciation: A Genetic Perspective. In

Ereshefskyb (1992): 159-183. (Originally published in D. Otte and J. A. Endler (eds)

Speciation and its Consequences [Sunderland MA: Sinauer, 1989].)

van Valen, Leigh 1971. Adaptive Zones and the Orders of Mammals. Evolution 25: 420-8.

Vrba, Elisabeth S. 1995. Species as Habitat-Specific, Complex Systems. In Lambert and Spencer

1995: 3-44.

White, Michael J. D. 1978. Modes of Speciation San Francisco: W. H. Freeman.

Wilson, Robert A. (ed.) 1999a. Species: New Interdisciplinary Essays. Cambridge, MA: MIT

Press.

Wilson, Robert A. 1999b. Realism, Essence, and Kind: Resuscitating Species Essentialism? In

Wilson (1999a).

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NOTES

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Documents

Chicken, Eggs, and Speciation1