Jerry Fodor complains that
the criticisms of the objections he makes to the theory of natural
selection in his paper Against Darwinism don't address his
argument. If he is wrong then someone should say exactly where he has
gone wrong. Fair enough - I will address his argument directly and
show where I think that he has made a mistake. In summary, my
disagreement with Fodor is that he only takes seriously two ways of
making the distinction between being selected and being selected for
and that, despite his claims to the contrary, there is a third and
satisfactory way of making the required distinction.
The crucial question for
Fodor is how we can distinguish between selection of and selection
for. Darwin claimed that giraffes have long necks so that in times of
drought they can feed on leaves that other, less fortunate, giraffes
cannot reach. This increased their chances of surviving to breed, and
so on. As necks grow longer, so the overall weight of giraffes
increases. Both weight and neck length have been selected, but if we
accept the Darwin's explanation (about which more later), then only
length of neck has been selected for, and the concomitant increase in
weight, although selected, is simply a side effect.
Fodor points out that the
distinction between selection for and selection of is intensional:
had neck length not been accompanied by weight gain (by decreasing
bone density, say) neck length would still have been selected. If,
though, weight increase was selected for and not neck length, then
had weight gain not been accompanied by increase in neck length the
heavier giraffes would still have been selected. The distinction
between selection for and selection of is a modal distinction; to say
that there has been selection for a property P is to say that the
counterfactual 'those entities not having P would not have been
selected' is true and to say that there has been selection of but not
for P is to say that such counterfactuals are false.
The task then is to find a
way of making the distinction between selection of and selection for
that respects the intensional, modal nature of the distinction. Here
is an example, deliberately non-biological, but non-intentional, that
will show how the distinction can be made. Suppose that you are
walking along the beach with a friend on a volcanic island the day
after a violent storm. You come across a cave and you notice that
there are unusually many small pieces of volcanic rock on the cave
floor. You ask yourself: why are there so many small rocks on the
cave floor? You look up and you see a crack in the ceiling of the
cave, sufficiently large to let small rocks fall through. You
therefore form the hypothesis that the reason why there are so many
small rocks is because over a period of time they fell through the
gap. Your friend, when told of your hypothesis, responds by saying
that although that is a possible explanation, there are others. For
example, during the storm the wind may have been blowing in from the
sea, and the small pieces of rock were light enough to have been
blown through the mouth of the cave. These hypotheses don't exhaust
the options, nor are they mutually exclusive. Children might have
been playing in the area the previous day and they might have
collected small rocks and dumped them in the cave at the end of the
day. Some of the stones could have been blown there, and some might
have fallen through the crack in the ceiling. If we were really keen,
we could test the hypotheses. We could ask the children whether they
put any rocks in the cave, we could build a model of the environment
and see whether either the blown-in-the-wind or fall-through-the-gap
hypothesis is plausible.
But regardless of how we
might determine which, if either, of the two initial hypotheses are
correct, each provides a different explanation of why there are small
rocks on the cave floor. If after having examined in detail all the
various alternatives we decided in favour of one hypothesis over
another, we would not feel that the explanation was in some sense
incomplete - we might think that we have explained what really
happened.
This kind of explanation
could reasonably be called a sieve explanation. To sieve flour there
are two things that we need to do: first, we need a sieve, and second
we need to pour the flour through the sieve. Once we have done so, we
have separated the sieved from the lumpy flour and we can then
explain why there is no lumpy flour in the bowl - it has been
sieved. In general, when giving a sieve explanation we claim (i) that
there exists a condition that objects must satisfy that separates
those objects in a larger set that satisfy the condition from those
that don't and (ii) that the conditions must have been applied to the
objects in the larger set. We can now say that those objects that
have been sieved have been selected for the condition. The point of
the earlier example was simply to illustrate that not all sieves are
artefacts.
Sieve explanations are
intensional (but not necessarily intentional) and they support the
relevant counterfactuals. In the cave example, the blown-in-the-wind
hypothesis it is true that had there been other objects suitably
placed in the environment that were too big to fall through the gap
in the cave but light enough to be blown around then some of these
would have been blown into the cave; on the fall-through-the-gap
hypothesis, it is true that had there been other heavier rocks around
that were small enough to fall through the hole, then they would have
dropped onto the floor of the cave. But as volcanic islands tend to
be geologically pretty dreary, with lots of similar rocks around and
not much else, examination of the rocks on the cave floor would not
help in deciding amongst the two hypotheses.
The two explanations
support different counterfactuals and select for different properties
(weight against shape). However, although counterfactuals are
supported, they do not directly invoke laws of nature - being
sievable is not a causal property we would normally ascribe to flour.
But in itself this is not a problem. Laws of nature support
counterfactuals, but not all counterfactuals support laws of nature,
and the original requirement was to find a form of explanation that
supports counterfactuals.
Fodor is not unaware of
this kind of argument, which he ascribes to Sober (Dawkins also talks
of filters and sieves), but he only discusses this proposal in a
footnote and gives it pretty short shrift. He writes (about Sober's
pebble sorting machine):
...what grounds the
counterfactuals in Sober's example is the structure of the
mechanism; given how it works, it lets the round pebbles through but
no others; one's intuitions about which trait is selected for
follow not from what laws of selection per from mechanics. Notice,
for example, that whereas competition plays a central role in the
explanation of every bona fide Darwinian selection, it plays no role
at all in explaining how Sober's machine sorts for round marbles.
Note first that, as I
pointed out earlier, selection for traits does not have to follow
from laws of selection; they just have to support the appropriate
counterfactuals. Second, although Fodor is right in claiming that
competition plays a crucial role in Darwinian selection, this does
not mean that it has to play a crucial role in the explication of the
distinction between selection for and selection of. Sieve
explanations are not local to the theory of natural selection or any
other theory that involves competition.
Although sieve
explanations have a temporal aspect, they are ahistorical. But it is
easy to see, in outline anyway, the role that sieves play in the
theory of natural selection. Assuming once again Darwin's explanation
of why giraffes have long necks, there is a sieve for neck length in
giraffes that filters out those giraffes that cannot reach the leaves
at the top of the trees. Without competition for resources amongst
giraffes the existence of such a sieve is irrelevant to natural
selection, for in such a case how well-fed a giraffe is would have no
differential impact on the number of offspring produced. But if there
is such competition, and if further the sieve is in operation over a
number of generations, and neck length is heritable, then there would
be upward pressure on neck length leading to the fixation of the
phenotype that the theory of natural selection is supposed to
explain.
Adaptationist explanations
are historical explanations that do not depend on laws of selection
in Fodor's sense, but do depend on sieve explanations that support
counterfactuals and are thus able to make the distinction between
selection simpliciter and selection for. They are also highly
context-dependent and post hoc, but as far as I can tell no-one is
disputing this - for example, the familiar claim that if we were to
rewind natural history then we would get a different story, is an
expression of a general recognition of the contingency of the actual
historical account. The context-dependency and post hoc nature of
adaptationist explanations are only problematic if they have to be
based on laws of selection construed as causal laws connecting traits
to fitness, and laws of selection in this sense are unnecessary -
sieve explanations provide an alternative.
The discovery of suitable
sieves is a complex matter - possible explanations are not
necessarily true explanations..Darwin's theory of giraffe neck length
could well be wrong. Craig Holdrege ('The Giraffe's Short Neck' ,
http://www.natureinstitute.org/pub/ic/ic10/giraffe.htm) writes: "The
giraffe's neck carries out a variety of functions - it allows feeding
from high branches, serves as a weapon in males, brings the head to
elevated heights that give the giraffe a large field of view, is used
as a pendulum while galloping, and so on". On Holdrege's view,
Darwin got the explanation wrong, but the difficulty of finding the
right explanation does not mean that there is no explanation to be
had, and indeed Holdrege's alternatives are themselves based on
empirical observation.
Those of us who share
Fodor's dislike of evolutionary psychology might share my concern
that if evolutionary psychology is to stand or fall with the theory
of natural selection in general, then it will stand.