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 although chicken eggs came first, just as Papineau and
friends say, this has nothing to do with the first chicken being in them. 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. My primary aim, however, 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
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CHICKENS, EGGS, AND SPECIATION
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 some surprising implications: to wit, 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 nonchickens (and came, by Papineau’s stipulation, from non-chicken eggs). 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 a chicken was hatched from it.
Therefore (3) E was a chicken-egg that preceded the first chicken.
Notice that this argument is not valid. For from the fact that a particular chicken “came from”
or can be traced back to egg E, it does not follow that a chicken was hatched from E. To see
that this does not follow, compare: from the fact that this adult chicken came from egg E, it
does not follow that an adult chicken hatched from E. The argument is valid only on the
assumption that the first chicken was a chicken when it hatched. In this paper, I end up arguing
that this cannot be assumed. But I am getting ahead of myself. I need first to show that there
is something wrong with the notion of a first chicken. In this section, I shall argue that this
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CHICKENS, EGGS, AND SPECIATION
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
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 a chicken. Obviously,
then, he was not the first chicken.2
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CHICKENS, EGGS, AND SPECIATION
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 sexual 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
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
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CHICKENS, EGGS, AND SPECIATION
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.
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.
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II. The Perils of Pheneticism
1. The egg-before-chicken thesis was first advanced by Roy Sorensen (1992). His argument
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 chickengenotype, 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. (Note that he 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.)
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
preceded by borderline chickens and so it is simply indeterminate as to where the pre-chickens
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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 11, 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.5 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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CHICKENS, EGGS, AND SPECIATION
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
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.6
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
(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
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CHICKENS, EGGS, AND SPECIATION
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).7
Pheneticism has no principled distinction between variation within a species and the
differences that separate species. 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, common among biologists, that “macro-mutation”, or
saltational change, corresponds to 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 Mayr’s “false antithesis” is
the correct observation that individuals can differ dramatically from their parents, while
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
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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
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 agedependent 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
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CHICKENS, EGGS, AND SPECIATION
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
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 “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
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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
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 antitypical 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 traditional problem of the “origin of species” is the problem of how to account for
gaps in nature. The account, 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
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CHICKENS, EGGS, AND SPECIATION
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 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 ensemble-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)
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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,
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-mostone 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.8
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 atmost-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
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might sometimes be difficult to diagnose which of two species an individual belongs to. The
claim is rather that the proper strategy for explaining gaps in nature inevitably assumes that
there are no individuals that bridge 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-mostone-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
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correct, or nearly correct. It would be correct if there were no complications of nature leading
to freakish events – horror stories.
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-mostone-species rule.
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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 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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CHICKENS, EGGS, AND SPECIATION
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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CHICKENS, EGGS, AND SPECIATION
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.9
“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.
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
20
CHICKENS, EGGS, AND SPECIATION
probability function will then parallel the within-population slope. In other words, the stepwise 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.
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,
21
CHICKENS, EGGS, AND SPECIATION
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 17 th 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
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
22
CHICKENS, EGGS, AND SPECIATION
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.
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
23
CHICKENS, EGGS, AND SPECIATION
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. Charliegene 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.
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 isolated from
other pre-chicken populations – the first moment, that is, when the possibility of nonhybridizing gene-exchange with these other populations is completely extinguished. 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
24
CHICKENS, EGGS, AND SPECIATION
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 geneflow – 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.
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
25
CHICKENS, EGGS, AND SPECIATION
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.10
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.
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
26
CHICKENS, EGGS, AND SPECIATION
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.
VI. 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, but he is a pre-chicken nevertheless.11 The species
chicken comes into being much later than Charlie – when gene-flow inward or outward from a
particular population descended from Charlie ceases to integrate this population with others in
its former species.
When the population 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 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. For note that the
eggs from which these new chickens hatched were pre-chicken eggs, not chicken eggs – that is,
27
CHICKENS, EGGS, AND SPECIATION
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
eggs: 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. These
eggs hatch only after the population’s change of status occurs. Thus, chickens ultimately hatch
from them, and by the Papineau-team’s assumption, they are chicken-eggs. They are so
despite the fact that they were laid by what were then pre-chickens. Note, however, that these
are not the eggs from which the first chickens hatched. And we have already established that
those eggs, the eggs from which the first chickens hatched, were not chicken eggs. Thus, the
first chicken-eggs preceded the first chickens, but the latter did not hatch from the former.
Now, it is possible that a chick hatched at the exact instant when speciation occurred.
The egg from which this bird hatched is a chicken egg because, as Papineau says, it had a
chicken in it (though it would have been a pre-chicken egg had it hatched a moment earlier). By
this reckoning, it was not only a chicken egg that existed before any bird was a chicken, but also
the first chicken egg from which a first chicken hatched, and indeed the first egg from which a
chicken hatched. So it is possible, but neither necessary nor even likely, that a first chicken
came from the first chicken egg.
Notice that this solution 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
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CHICKENS, EGGS, AND SPECIATION
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 speciesmembership 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 Mohan Matthen
(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.
Cracraft, Joel 1992. Species Concepts and Speciation Analysis. In Ereshefsky 1992b. (Originally
published in Current Ornithology 1, 1983.)
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.
29
CHICKENS, EGGS, AND SPECIATION
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: 4565.
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.
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.
30
CHICKENS, EGGS, AND SPECIATION
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
1
For useful comments, I thank 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. 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
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
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CHICKENS, EGGS, AND SPECIATION
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.
6. 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.
7. 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.
8
I am not suggesting that this view has actually been held in the extreme version
presented here.
9. 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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CHICKENS, EGGS, AND SPECIATION
10. 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.
11. 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.
34