Some thoughts on Genetic Drift and Selection

We've been discussing the question of how we can distinguish genetic drift from selection. Part of this has been our discussion of Beatty's very interesting paper.

Beatty argues only that the role of probability and randomness in evolution (or in evolutionary theory) is less clear than we might at first think. I would like to go farther. I think there are at least two genuinely philosophical problems we might find, when we consider the kind of cases he describes, or that we discussed together in class.

(1) The first problem is, when is something an adaptation, and when is it not an adaptation?

We want to reason something like this (I'll adopt genocentric terminology, but we could run the argument the same way with some other kind of trait identified):

1. There came to be a preponderance of gene g in the population to which the ancestors of organism o belong.
2. If over time there came to be a preponderance of gene g in the population to which the ancestors of o belong, then gene g provided a selection benefit to ancestors of o
3. If gene g provided a selection benefit to ancestors of o, then gene g of organism o is an adaptation for o.
4. By steps 1 and 2, we conclude that gene g provided a selection benefit to ancestors of o.
5. By steps 3 and 4, we conclude that gene g of organism o is an adaptation for o.

That's crude, in part because I wanted to shoehorn the argument into propositional logic, but it's not far off the mark. Now, the problem is with premise 2. As we noted, there could be cases where a preponderance of gene g in the population to which the ancestors of o belong arises because of an event like a lightening strike. Thus, if some trait becomes prevalent because an individual with an alternate trait is struck by lightening, we will say its preponderance is an adaptation.

There is no contradiction here. We could simply say that the trait is an adaptation. But if we make that move, then there is no difference between an adaptation and random genetic drift.

The question is: is there such a difference? And, if so, how can we make sense of it?

You raised in class some suggestions. For example, some of us proposed that we could say that the gene g was irrelevant to lightening strikes, as was the competing allele to g (say, g*) which lightening kicked out of the gene pool. But now note: with an explanation like that, we've added something very strong to our account. We've added something like an assessment of what a gene is good for, what it could do, and thus of what is and is not relevant to questions of it being an adaptation.

The same would seem to be true of the statistical outs we might pursue. If we say, gene g* would have been most prevalent if the population were infinitely large, the natural question is: why is that? (Another question is: how would we know? But I'm interested in the metaphysical, not the epistemic, question here.) It would seem we should answer this question with an account of what gene g* and gene g can do, what they're good for, and so on, and why neither would help regarding lightening strikes but one might be helpful regarding some more prevalent environmental challenges, and so on. And, again, that refers to something like a functional description of g and g*, and not just to their prevalence in the population.

(2) The second problem is, can we make sense of something having a function in terms of it being selected?

We'll talk more about this notion of purpose or function later. But, briefly, many philosophers hope that we can say that the purpose of the heart is to pump blood; and what this means is that the heart was selected for pumping blood (that's too quick, but it conveys the idea).

You can see immediately our problem. Again being very rough, our problem is something like this; we want to make the following kind of argument. Assume that gene g causes events of kinds { e₁...e_m...e_n } in the organisms of in the population O that have the gene; let o be one of the organisms of kind O.

1. The ancestors of the organism o had gene g and g caused events of kind e_m and because of these events these ancestors had more progeny than other organisms in the population with alternative genes (alleles).
2. The purpose of gene g in organism o is to cause events of kind e_m if the ancestors of the organism o had gene g and g caused events of kind e_m and because of these events these ancestors had more progeny than other organisms in the population with alternative genes (alleles).
3. Therefore, the purpose of gene g in organism o is to cause events of kind e_m.

So, the idea is, if there were a gene for having a heart (this, again, is simplistic), that gene would cause many kinds of events, including for example making the noise of a heartbeat. But pumping blood is the purpose of the heart because that provided a selection benefit.

But how do we cash out the "because" in premise 1? That's actually very tricky, since it seems to require us to see what the benefit of the gene is, independent of whether it was selected or not. But, an ideally quick and least costly answer would be: variations of gene g that did not cause that kind of event became less prevalent in the population (there is the problem that there may not be such variations of g, but setting that aside...). That is, the most parsimonious explanation would get rid of the "because" by saying that this is simply the thing that happened, in distinction from various alternatives.

But if there is genetic drift, we cannot make this move. We cannot assume that because a gene is present it provided a selection benefit. In that case, it really does look like there is not much hope in cashing out our talk about functions by this kind of account of inherited functions, without having some robust theory of why a trait is beneficial, and what about the trait is not beneficial, independent of what actually became more prevalent in the population (or, as we might say, independent of what was "selected"). And that would seem to put us right back at square one when we try to explain what we mean by biological function.

[2/26/2012]