Download Why do individuals 4 and 5 have G rather than B

Appeared in: Studies in the History and Philosophy of Biological and Biomedical Sciences; 41
(2010): 61–66.
PENULTIMATE DRAFT
What Can Natural Selection Explain?
Ulrich E. Stegmann
Department of Philosophy, University of Aberdeen, Old Brewery, High Street, Aberdeen, AB24
3UB
Abstract
One approach to assess the explanatory power of natural selection is to ask what type of facts it
can explain. The standard list of explananda includes facts like trait frequencies or the survival
of particular organisms. Here I argue that this list is incomplete: natural selection can also
explain a specific kind of individual-level fact that involves traits. The ability of selection to
explain this sort of fact (‘trait facts’) vindicates the explanatory commitments of empirical
studies on microevolution. Trait facts must be distinguished from a closely related kind of fact,
i.e. the fact that a particular individual x has one trait rather than another. Whether or not
selection can explain the latter type of fact is highly controversial. According to the so-called
‘Negative View’ it cannot be explained by selection. I defend the Negative View against
Nanay’s (2005) objection.
Key Words
Explananda of natural selection, explanation of traits, adaptation, Sober-Neander controversy,
the Negative View.
1. Introduction
What kinds of facts can natural selection explain? Here is a standard list of things that at least
some authors believe selection can explain:
1. The dynamics of trait frequencies in populations across time, i.e. their change or
stagnation (Sober, 1984);
1
2. The composition of a population at a particular point in time (Sober, 1995, p. 384),
e.g. the fact that 90% of the population are G-individuals and 10% are B-individuals
(individuals with trait G or B, respectively);
3. The origin of traits in a population, in addition to their spread and maintenance as
acknowledge in (1) (Forber, 2005).
4. An individual’s survival, its reproductive success (Sober, 1984, p. 152) and its
existence (Sober, 1995, p. 388);
5. The fact that a particular individual has trait G rather than trait B.
It is uncontroversial that selection can explain facts (1) and (2). It is also generally
accepted that selection can explain (4), at least as long as selection is regarded as a causal
process1. However, a long-standing controversy surrounds (5). Many authors deny that selection
explains the traits of individuals (Sober, 1984; 1995; Walsh, 1998; Lewens, 2001; Pust, 2001;
2004), a position known as the ‘Negative View’2. Others insist that selection can explain this
fact in special circumstances (e.g., Neander, 1995; Matthen, 1999; Forber, 2005; Nanay, 2005).
More recently, Forber (2005) has added (3) to the list, arguing convincingly that under some
circumstances selection explains the origin of traits.
It is easy to loose sight of the significance of this debate, so much so that some may
wonder whether it has any. So let me say why I think it is important, but also where its
relevance may have been overstated. First, the list of explananda is not a haphazard list of
empirical facts for which we have good evidence that selection explains them. Debating such a
list might well be tedious and pointless. Rather, the list, and the debate surrounding it, concerns
the kinds of facts selection can explain. Identifying the kinds of facts selection can explain is
imperative if the goal is to understand the explanatory power of natural selection. And surely,
understanding the explanatory power of such a fundamental notion is a worthy goal for both
philosophers of science and biologists. The debate is essential for this project.
Second, scrutinizing the Negative View in particular is justified because it is deeply
counter-intuitive. It suggests that strong selection for, say, long necks in giraffes over many
generations does nothing whatsoever to explain why a particular giraffe has a long neck, despite
its manifest causal impact in the past. That the Negative View is so counter-intuitive reveals that
there is more to natural selection than meets the eye, which makes the debate all the more
pressing.
In the end I believe the Negative View is correct. But it would be a mistake to take this as
vindicating the view that evolutionary biology is somehow not interested in particular
individuals. Some branches of this discipline are not, but others are. Behaviour ecology, in
2
particular, takes profound interest in the properties of individual organisms, as is vividly
documented in recent work on, for instance, ‘personality’ traits (Réale et al., 2007) and the
social significance of long-term memory of elephant matriarchs (McComb et al., 2001; Foley et
al., 2008). Indeed, while the Negative View is correct, evolutionary biology is nonetheless
committed to explaining a certain kind of fact concerning traits at the individual level, or so I
will argue.
The significance of the debate has been overstated in other respects. It has been argued
that the Negative View implies two far-reaching consequences, i.e. that selection cannot explain
adaptations (e.g., Neander, 1995; Nanay, 2005) and that, on a grander philosophical scale, it
dissolves the basis of teleosemantic theories of mental content (Walsh, 1998; Nanay, 2005). I
believe it implies neither.
Admittedly, there is something right in claiming that selection cannot explain adaptations
on the Negative View. According to the Negative View, selection cannot explain why a
particular individual has trait G rather than B; so, when G is an adaptation, it cannot explain
why the individual has adaptation G, rather than B. However, the claim that selection cannot
explain adaptations tout court follows only on the further assumption that explaining
adaptations consists in explaining why a particular individual has adaptation G rather than B.
That assumption is questionable. ‘Explaining adaptations’ may reasonably be interpreted as
involving the explanation of population-level facts regarding adaptations, especially the spread
and eventual prevalence of an adaptation (Sober, 1984) or the composition of populations at a
given point in time (Sober, 1995).3
The view to the contrary may spring from a familiar way of construing adaptations.
Adaptations are often taken to be traits whose possession by particular individuals is due to
selection (cf. ‘this giraffe has a long neck because of past selection for long necks’). 4
Consequently, if selection cannot explain why individuals have a trait, then that trait is not an
adaptation. From this vantage point the implication of the Negative View appears even worse
than the critics allege. It is not so much that selection cannot explain adaptations, but rather that
there are no adaptations in the first place, because selection lacks that crucial bit of explanatory
power. But, of course, we may and should revise our concept of adaptation to fit the explanatory
status selection actually has.5 With such a refinement in place, there is no more reason to
demand that the only genuine explanations of adaptations be explanations of why an individual
has adaptation G.
Does the Negative View undermine the very project of accounting for mental content in
terms of evolutionary function? Consider what the various versions of teleosemantics require.
3
What they require, among other things, is that token mental states, or the token mechanisms
producing them, have evolutionary functions. But as just argued, token states or mechanisms
can be adaptations without selection being able to explain why particular individuals have them.
Hence, since teleosemantics is not tied to the truth or falsity of the Negative View, the debate
does not have quite the ramifications some think it has.
Finally, a word about an issue that has complicated the debate about the explanatory status
of natural selection: how the identity of individual organisms is determined. One prominent way
of delineating individuals is origin essentialism, which is the view that an individual’s identity
is determined by parentage or, in other words, that a given individual could not have been
generated by different parents. Since the Negative View relies on origin essentialism (Matthen,
1999; Pust, 2001), the debate has partly turned on whether in evolutionary biology this view is
plausible (Lewens, 2001; Pust, 2001; 2004) or not (Matthen, 1999; 2003; Forber, 2005). But the
debate about the explananda of selection is broader than this. As will be shown here, important
issues remain to be settled even on the assumption of origin essentialism, not least because a
more recent attack on the Negative View does not target this assumption (Nanay, 2005). I
therefore adopt origin essentialism for the purposes of this paper.6
Here is the plan. In the first section I argue that selection can explain an individual-level
fact that has been overlooked thus far. In other words, I believe the present list of explananda is
incomplete. One consequence, explored in the second section, is that the Negative View can be
shown to account for certain fine-grained explanatory commitments entailed by some studies of
microevolution. Another consequence is to appreciate that the new fact can itself assume a
unique explanatory role under some conditions. The last section defends the Negative View
against Nanay’s (2005) objection.
2. Selection explains ‘trait facts’
Consider a hypothetical uniparental organism (fig. 1)7. Individuals are born consecutively
into lineages of this organism, so that there will be one second, one third, one twenty-fourth
member, and so on. Definite descriptions like ‘the lineage’s second member’, abbreviated in
figure 1 as numerals (e.g. 2), are used to denote whichever particular individual happens to be
the relevant member of a given lineage. The numerals are therefore non-rigid designators.
Finally, lineages of this organism are individuated by their founding individuals, e.g. 1 in figure
1. ‘1’ denotes the same individual in figures 1A and 1B. The two figures depict alternative
variants of the same lineage, which differ in their trajectories after the second generation. Figure
4
1A depicts the actual lineage, in which only 3 reproduces (in generation II). Figure 1B depicts
the lineage’s trajectory if, contrary to fact, 2 had reproduced instead of 3.
[Space for figure 1]
Quite obviously, distinct individuals can become any given member depending on the
circumstances (with the exception of 1). For example, in the actual circumstances only 3
reproduces and the lineage continues with an offspring of 3 (fig. 1A). The lineage’s next
(fourth) member is therefore an offspring of 3. By contrast, if only 2 had reproduced, then the
fourth member would have been an offspring of 2 and, hence, a different individual (fig. 1B).
Which organism reproduces has consequences for how the traits G and B are distributed
(the parents of our hypothetical organisms simply pass on to their offspring whichever trait they
happen to have; 3 is the exception insofar as a mutation caused it to develop G). Since in the
actual lineage (fig. 1A) one of 3’s offspring becomes the fourth member, and since all of 3’s
offspring have G, 4 will be a G-individual. If, however, 2 had reproduced instead of 3, the
fourth member would have been a B-individual, because 2’s offspring would have had B (fig.
1B). In other words, the fourth member may be a G- or a B-individual depending on the
circumstances.
[Space for figure 2]
In the actual circumstances, the property of being the fourth member is exemplified by a
G-individual (rigidly denoted as a1 in fig. 2B). But if circumstances had been different, then the
fourth member would have been a B-individual (rigidly denoted as a2 in fig. 2B). Facts like the
fourth member’s being a G-individual, rather than a B-individual, are called ‘trait facts’ in what
follows. For comparison, figure 2A illustrates what might be termed a Sober-fact: an individual
has trait G rather than B.
We can now ask: why is the fourth member in figure 1A a G- rather than a B-individual?
The answer is that all of 3’s offspring have G and 4 happens to be one of 3’s offspring. Hence 4
is a G-individual. Now, one of the explanatory factors may easily depend on natural selection.
More specifically, the fact that the fourth member is one of 3’s offspring may depend on
selection. We only need to assume that generation II experiences strong selection for G, so
strong that only G-individuals can reproduce. Since in that generation only 3 has G, only 3 will
reproduce. The next (fourth) member of the lineage will therefore be an offspring of 3. So, in
5
virtue of explaining why the fourth member is an offspring of 3, natural selection contributes to
the explanation of why the fourth member is a G-individual.
What if selection were less than absolute, allowing 2 to generate a few offspring alongside
3? If the fourth member would still be one of 3’s offspring, rather than one of 2’s, then this
would simply be an effect of 3 giving birth a bit earlier than 2. Since selection has no influence
on the order of births, selection would not determine that the fourth member stems from 3
(rather than 2). So for selection to explain why the fourth member is an offspring of 3, it may
seem as if we must endorse the unrealistically strong assumption that selection is an all-ornothing affair.8
However, this worry is overstated. First, all-or-nothing selection may occur, even if rarely.
So there is a set of circumstances in which selection explains trait facts. Saying that selection
can explain trait facts is not to say it always does. Second, even when selection is not all-ornothing it can explain the probability of a certain member stemming from a particular parent.
For example, if due to selection 3 contributed 90 % of generation III, then selection would
account for the probability being 0.9 that the next (fourth) member stems from 3 (rather than 2).
And given that all of 3’s offspring have G, selection would also make it likely that 4 would be a
G-individual, rather than a B-individual. Further, selection is responsible for the probability
being 1.0 that 4 is a G-individual in the deterministic case initially presented.
A different worry concerns the relation between the referent of ‘the fourth member’ and
what Matthen (1999; 2003) calls “receptacles”. At least on one interpretation, Matthen’s
receptacles are best construed as abstract entities that can be ‘filled’ by different genomes,
which in turn may be identified with distinct individuals (Lewens, 2001, p. 594). It may
therefore seem as if there is not much of a difference between receptacles and genealogical
positions, since both are like ‘slots’ that can be filled with different individuals.
But the similarity is only superficial. Matthen (1999; 2003) rejects origin essentialism,
instead embracing the view that different parents may generate the very same individual. Such
individuals are receptacles. That is, receptacles are individuals “whose identity does not depend
on parentage” (Matthen, 2003, p. 306), and who instead have a “very thin essence” (p. 307). By
contrast, ‘the fourth member’ is neither a name for an individual with wide identity conditions,
nor for a particular ‘slot’ or genealogical position that exists independently of the individual
filling it. It rather denotes an ordinary individual, i.e. as individuated by its parentage, in virtue
of its having the property of being the fourth member. Consequently, whereas for Matthen
selection explains Sober-facts involving thinly specified individuals, I suggest it explains trait
facts involving ordinary individuals.
6
3. Implications
Once it is accepted that selection explains trait facts it can be seen that the Negative View
accounts for the fine-grained explanatory commitments associated with some empirical studies
in microevolution. This is the first implication I will explore in this section.
Studies of microevolution in the wild have documented cases in which the crossgeneration change in the mean value of a phenotypic trait is due to directional selection for the
trait in question. For example, selection for larger weight in a species of Darwin finches resulted
in birds of the post-selection generation being heavier, on average, than birds of the preselection cohort (Grant and Grant, 1995, and references therein). Similarly, members of a guppy
species matured at a later age after 18 generations of selection for increased age at maturity
(Reznick et al., 1997).
In some studies, notably Peter and Rosemary Grant’s long-term project on Darwin
finches, organisms were marked individually. This enabled tracing parent-offspring relations
and documenting high heritabilities (e.g., larger-billed individuals produced larger-billed
offspring). It was also shown that individuals with, say, larger bills survived and reproduced
because of selection (they were better at opening hard seeds, which became the only food
available during a severe draught). Taken together, these data imply that selection can in
principle explain why a specific member of a lineage is a heavier or large-billed individual,
rather than a lighter or small-billed individual. In other words, the level of detailed information
available in some microevolutionary studies implies the possibility that selection can explain
trait facts.
This degree of explanatory resolution is not readily accounted for by the explanatory
powers of selection as presently conceived by the Negative View. On the one hand, proponents
of the Negative View accept that selection may explain facts concerning individuals. But these
facts have nothing to do with traits: they concern an individual’s survival, its reproductive
success (Sober, 1984, p. 152) and its existence (Sober, 1995, p. 388). On the other hand, it is
acknowledged that selection can explain facts regarding traits. But these facts do not pertain to
individuals: what selection explains about traits are population-level facts, e.g. that a population
is entirely composed of X-individuals rather than of Y-individuals (Sober, 1995, p. 384). Thus,
the Negative View has not yet been explicitly construed as allowing selection to explain
individual-level facts concerning traits.
In conclusion, although selection cannot explain why an individual has A instead of B on
the Negative View, it nevertheless allows selection to explain a different individual-level fact
7
concerning traits. This restores some degree of explanatory power at the level of individuals,
which selection seemed to have lost.
I have made liberal use of the distinction between individual-level and population-level
facts, but more needs to be said about it. Intuitively, the distinction is clear enough: trait facts
are individual-level facts9 because a proposition like ‘the fourth member is a G-individual’
asserts something about the individual that satisfies the definite description. It expresses a fact
concerning that individual, just as ‘the current president of France has dark hair’ asserts
something about the man Nikolas Sarkozy. However, when attempting to specify what
underpins the distinction, complications arise. The distinction does not appear to rely on a
difference in the relevant truthmakers.10 So I suggest appealing to the subjects of predication
instead: if the subjects are individuals, the fact is individual-level; but if they are populations,
then the fact is population-level. Thus, Lonesome George’s surviving to reproductive age is an
individual-level fact because ‘—has survived to reproductive age’ is predicated of an individual.
Similarly, ‘—being taller than—‘ is predicated of individuals, as is ‘—is a G-individual’. Both
rigid and non-rigid designators may be used in subject position to denote individuals. By
contrast, ‘—contains 70% females’ is predicated of a population, not of individuals. Of course,
the population is composed of individuals and they contribute to making the proposition true,
but they are not the subject of predication; ‘—contains 70% females’ is not predicated of any
individual.11
So far I argued that acknowledging trait facts helps restoring some of the fine-grained
explanatory force of selection. The second implication to emphasize is that under some
conditions trait facts themselves can have unique causal and explanatory relevance. Such
conditions could be met, for example, in birds with obligate siblicide. Some eagles, pelicans,
penguins, vultures, cranes, and owls lay two eggs per clutch, although only the first-hatched
nestling survives. The older nestling, being larger and heavier, outcompetes the younger over
access to food, eventually killing it (e.g., Edwards and Collopy, 1983; Anderson, 1990). Now,
suppose the older nestling, member 50 of the lineage, expresses the advantageous novel trait G
due to a rare mutation and kills its younger sibling, 51. G spreads in subsequent generations
because it is advantageous and its first bearer, 50, survives to reproduce. 12 In this scenario, the
evolutionary consequences uniquely depend on the trait fact that 50 is a G-individual. If 51
(rather than 50) had been the G-individual, the new trait would not have spread because 51 was
destined to die and no other individual in the population had G. Furthermore, even though it
may now be Polly who has G, G’s spread does not depend on Polly’s having G. For Polly might
well have been 51, in which case her having G would have been inconsequential (because she
8
would have been killed), and any G-individual other than Polly would have ensured G’s spread
provided it had been 50 instead of her. Trait facts can therefore make causal and explanatory
contributions that neither population-level nor Sober-facts can make.13
4. Defending the Negative View
In the previous section I argued that the standard list of explananda of natural selection is
incomplete. We should add trait facts. In this section I defend the view that Sober-facts are not
proper explananda for selection.
As mentioned in the introduction, whether selection can explain Sober-facts has been
discussed for many years. Some authors insist that selection can explain this sort of fact at least
in special circumstances, i.e. when selection is cumulative (e.g., Neander, 1995), when
reproduction is sexual (Matthen, 1999; 2003), or when resources are limited (Nanay, 2005).
Neander’s and Matthen’s arguments have already been discussed extensively elsewhere and
found wanting14. I will therefore be concerned with Nanay’s argument.
Nanay (2005) asks us to consider a case in which an individual x has trait A, there is
selection for A in x’s population, and the population also contains B-individuals. His argument
then aims to show that x’s having A ultimately depends on selection. Here is the argument
(paraphrased from Nanay, 2005, p. 1105-06)15:
1) X’s having A depends counterfactually on x’s mother having A and surviving to
reproductive age.
2) The mother’s survival depends counterfactually on the death of B-individuals.
3) The death of B-individuals depends counterfactually on selection for A.
Hence, x’s having A depends counterfactually on selection for A. Selection therefore partly
explains x’s having A.
Setting aside other issues16, my worry here is with the first premise. I believe that the first
premise is ambiguous because it does not specify the relevant contrast class, and that proper
disambiguation breaks the chain of counterfactual dependence on selection.
The phrase ‘x’s having A counterfactually depends on…’ from the first premise asserts a
counterfactual dependence. But it does not specify whether what depends is (1) x’s, rather than
y’s, having A, or (2) x’s having, rather than lacking, A, or (3) x’s having A, rather than some
other trait B. The debate about whether selection can explain Sober-facts only concerns contrast
(3) (Pust, 2001; the significance of contrast classes to explanatory projects in general has been
emphasized for instance by Van Fraassen, 1980)17. So, in order for Nanay’s argument to work
against Sober, the first premise must be disambiguated accordingly:
9
1*. X’s having A, rather than B, depends counterfactually on x’s mother having A and
surviving to reproductive age18.
The first thing to note is that premise 1* makes x’s having A rather than B (or x’s having
A, for short) counterfactually dependent on a conjunction of two conditions. Both must obtain
for x to have A instead of B. The truth of 1* therefore implies that if one of the conditions did
not obtain, then it would not be the case that x had A, rather than B. We can assess 1* on the
basis of this implication.
Unfortunately, it is not entirely clear what the truth conditions are for counterfactuals
whose antecedent is a proposition of the form ‘A rather than B’. This in itself weakens Nanay’s
argument somewhat. It seems the best we can do, short of fully working out the semantics of
these counterfactuals, is to impose the following constraint: the negation of ‘A rather than B’ is
equivalent to ‘B rather than A’. This constraint holds quite generally for such counterfactuals.
Consider the following example: my remaining dry (rather than getting wet) during rain
counterfactually depends on my using an umbrella (rather than not using it). I would get wet if I
did not use it. So, the counterfactual asserts that if the relevant condition was not satisfied (if I
did not use my umbrella), then the contrasting case of the antecedent would be realized (I would
get wet). It seems this is the whole point of asserting that my remaining dry counterfactually
depends on my using the umbrella.
With this constraint in place, we can now assess 1* by asking whether x would have B
instead of A if either one of the two conditions did not hold. One of the conditions is that x’s
mother survives to reproductive age. So is it true that if the mother had died before reaching
reproductive age, then x would have B instead of A? Clearly not. If the mother had died, the
very same x would not have been born (assuming origin essentialism) and x would not have B
or any other trait; a fortiori, x would not have B instead of A.19 Premise 1* therefore falsely
asserts a counterfactual dependence both on x’s mother having A and on her survival.
Now, the condition that allegedly links x’s having A to selection (via the chain of
counterfactuals expressed in premises 2 and 3) is the mother’s survival. But since x’s having A
does not counterfactually depend on the mother’s survival, it does not depend on selection
either. These considerations therefore sever the purported chain of counterfactual dependencies.
Further, since the mother’s survival is the only condition linked to selection, it does not help to
point out that x’s having A does depend on the other condition, i.e. on x’s mother having A.20
One worry still needs to be addressed. X needs to have trait A in order to have A rather
than B. If x did not have A in the first place, there is no sense in which it could have A instead of
another trait. Furthermore, for x to have A, x’s mother must survive to reproductive age; x would
10
not be born otherwise. Apparently then, if the mother had died, x would not have A. Does this
not show that x’s having A counterfactually depends on the mother’s survival, after all?
All this shows is that the mother’s survival and x’s existence are preconditions for x’s
having A, albeit preconditions not unique to x’s having A. They are also preconditions for the
opposite fact, x’s having B rather than A: if x’s mother had died, x would not have existed and it
would not have any traits (e.g., x would not have B as opposed to A). The mother’s survival and
x’s existence are therefore preconditions common to x’s having either A or B. They do not
decide between the two alternatives.21
Acknowledgments
Bence Nanay, Phyllis McKay, Mark Textor, Eliott Sober, Kim Sterelny, and Dennis
Walsh commented very helpfully on earlier versions of this paper. Audiences at the Stirling
Workshop in Philosophy of Biology and the First Graduate Conference on the Philosophy of the
Natural Sciences (both in May 2005) provided valuable feedback. Financial support from the
British Society for the Philosophy of Science and the British Academy is gratefully
acknowledged.
11
Figures and legends
4 (G)
2 (B)
5 (G)
3 (G)
III
5 (B)
4 (B)
2 (B)
3 (G)
II
Selection
1 (B)
1 (B)
A
B
I
Figure 1. Different trajectories of a lineage of a uniparental organism. Numbers denote
(non-rigidly) individual organisms; B and G denote distinct types of traits. (A) Selection favours
G in generation II such that only 3 reproduces. (B) Trajectory of the same lineage if selection
had favoured B instead of G, such that only 2 would have reproduced.
12
G
B
a1
A
G
4
a1
B
a2
B
Figure 2. Comparing Sober-facts with trait facts. Squares represent particular individuals,
circles represent properties; solid lines represent actually obtaining features, whereas dotted
lines represent non-actual features. (A) A Sober-fact: individual a1 has trait G rather than trait B.
(B) A trait fact: the lineage’s fourth member is a G-individual (here: a1), rather than Bindividual (a2).
13
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Footnotes
1
The causal understanding of selection has recently come under attack (Matthen and Ariew,
2002; Walsh, 2007).
2
This is in line with Pust’s (2001; 2004) use of the ‘Negative View’, but not with Neander’s
(1995), who uses this expression for the view that selection cannot explain how genetic plans
for traits originated.
3
Selection explains, for example, why beaver populations are composed of flat-tailed
individuals (rather than of rod-shaped ones): flat tails are an adaptation for, presumably,
effective swimming. Selection might even be said to explain why species or populations ‘have’
certain adaptations, if ‘having adaptation X’ is understood in the derived sense of ‘being
composed of X-individuals’ (the European beaver ‘has’ a flat tail in the sense that all current
Castor fiber populations are composed of flat-tailed individuals).
4
Here is a definition of adaptation easily interpretable along these lines: “Characteristic c is an
adaptation for doing task t in a population if and only if members of the population now have c
because, ancestrally, there was selection for having c and c conferred a fitness advantage
because it performed task t.” (Sober, 2000), p. 85. On a natural (though uncharitable) reading
15
of this definition, it is one of the conditions for trait c’s being an adaptation that selection
explains of individual population members why they have c. The reading is uncharitable
because the phrase “members of the population now have c because, ancestrally, there was
selection for having c” can, and should, be construed as stating that selection is responsible for
the current population’s being composed entirely of c-individuals (Sober, pers. comm.).
5
Consider Sober’s (1984), p. 208, earlier definition: “A is an adaptation for task T in population
P if and only if A became prevalent in P because there was selection for A, where the selective
advantage of A was due to the fact that A helped perform task T.” According to this definition,
selection causes traits to become prevalent in populations; it does not cause members of the
population to have them.
6
Furthermore, origin essentialism has underpinned much of the discussion to date, and it is still
an intuitive and plausible view despites its difficulties. In particular, it can withstand Matthen’s
(1999; 2002) objections (Pust, 2001; 2004). Moreover, it is far from obvious whether
surrendering origin essentialism offers a less demanding or even “metaphysically neutral”
(Matthen, 2003), p. 303, account of natural selection, especially if this involves commitment to
entities such as ‘receptacles’.
7
This is the example Sober (1995) used to illustrate the Negative View. Figure 1a contains the
first three generations from Sober’s original figure, which comprises five generations and a total
of seventeen individuals (his figure 1 on p. 386, Sober, 1995). Note that Sober uses the
numerals as rigid designators.
8
An objection of this sort was raised by Phyllis McKay.
9
Although trait facts are individual-level facts, they may entail population-level facts. Suppose
in a population of 100 individuals, members 56, 57, and 93 (and only they) had G, rather than B.
It then follows that the population would have the property of consisting of 3 G-individuals and
97 B-individuals. Note that the reverse does not hold. It does not follow from population-level
facts like population size and trait frequencies which members of the population have which
traits (unless trait frequency is 100 %).
10
Traditionally, facts are the state of affairs that make propositions true. Accordingly, one
might suggest that when a proposition is made true by just one individual it expresses an
individual-level fact, whereas if its truthmaker involves several individuals, then it expresses a
population-level fact. This works well with some propositions. For example, ‘Lonesome George
has survived to reproductive age’ is made true only by Lonesome George, whereas ‘This
population contains 70% females’ is made true by a group of individuals. However, the appeal
16
to truthmakers fails when relational properties are involved. For all we know, Jane’s being taller
than Jill is a fact concerning, at least, Jane. It is thus an individual-level fact. But the proposition
is made true by both Jane and Jill, not by Jane alone.
11
This also helps to make sense of propositions like ‘Boris belongs to population P’, where ‘—
belongs to—‘ is predicated of both an individual and a population. We have no qualms
regarding this proposition as expressing a fact regarding Boris the individual (that he belongs to
population P) and, at the same time, a fact concerning P the population (that P contains Boris).
12
This is a case of cumulative selection for G. Note that not even cumulative selection explains
why a distant descendant of 50 has G. As Sober (1995), p. 392-393, argued against Neander
(1995), in each generation selection does not explain why the offspring has the trait is does;
repeated rounds of selection do not change this.
13
Here I assume that the explanatory interest is in giving a fine-grained explanation of why a
descendant population is composed of G-individuals. If the explanatory interest were different,
it may well be explanatorily irrelevant that the G-ancestor was the 50th member. It may suffice
to establish that at least some ancestor or other had G.
14
Neander’s arguments have been discussed by Sober (1995), Walsh (1998), Pust (2001), and
Forber (2005). For replies to Matthen’s work see Pust (2001; 2004) and Lewens (2001), as well
as Matthen’s (2003) response.
15
For reasons of simplicity I paraphrase Nanay’s probabilistic counterfactuals in deterministic
terms.
16
Apart from the contentious issue of the transitivity of counterfactuals, to which Nanay has a
response, one may have doubts about the second premise. Nanay suggests that the scarcity of
resources implies that individuals can become more successful in exploiting a resource only at
the expense of other individuals. Hence the idea that x’s mother could survive only if the death
of a B-individual renders a resource accessible for x’s mother (Nanay puts this in probabilistic
terms). But not all competition over limited resources takes this form (Sober, 1984), p. 17. So
the argument’s second premise may not hold even if it is rendered probabilistically.
17
Walsh (1998) used a scope ambiguity in propositions like ‘selection can explain why giraffes
have long necks’ to make a similar point.
18
Note that the restated first premise is consistent with origin essentialism. According to origin
essentialism, one and the same individual may have traits other than it actually has (Pust, 2004).
19
Perhaps it is meaningless to say that x lacks B when x not even exists. After all, for x to lack B
seems to require that x instantiates the second-order property of lacking B, and x must exist in
17
order to instantiate that property. But the relevant counterfactual can easily be reformulated: if
x’s mother had died, x would exist such that x would have B instead of A. Again, this
counterfactual is false.
20
Suppose that x’s mother had B instead of A and that she survived (only one of the conditions
is changed in order to assess counterfactual dependence). Since she survived, she would
generate x and pass on trait B to her offspring. So under these circumstances, x would have B
instead of A.
21
The mother’s having A is very different in this respect. It is a factor on which x’s having A
depends, but x’s having B does not. X’s having B counterfactually depends on the mother’s
having B (not on her having A).
Note that modifying the first premise to ‘x’s existence depends counterfactually on x’s mother
having A and (thus) being capable of reproduction’ does not change the result for the reason
outlined in the main text. The conclusion would be that x’s existence, but not x’s having A,
depends on selection for A.
18