A second highly selective selection from the 2008 conference of the NZ branch of the Australasian Association of Philosophy.
When not to have an Argument: the Different Explanatory Goals of Population Biology and Evo-Devo. Brett Calcott (ANU)
This talk was an attempt to resolve (in so far as it can be resolved) the debate between population biologists and evolutionary developmental biologists. A persuasive talk, but Calcott may have missed something important. I should note that part of Calcott's project is to unify evo-devo explanations with pop-bio explanations by showing how they both fit into a single account of explanation -- the “difference-maker” account of explanation. But the meat of the talk comes in Calcott's account of how explanations differ between the two approaches to biology.
Calcott distinguishes between “population-level” and “individual-level” explanations for evolutionary change. The former uses the genetic make-up of a population, plus the mathematical theory of genetic change, to explain changes in that genetic make-up. The latter uses certain physical differences between two stages in an individual's evolution, plus a knowledge of bioengineering, to explain certain other physical differences between the same two stages.
Calcott elaborates this distinction by saying that the former explanation is one of motive, and the other of means. Imagine an explanation of why a child got the chocolate bars on the top shelf in the kitchen. One could say that the child liked chocolate very much, so he was motivated to get it. Or one could say that he used a stool to help him reach the chocolate – he had the means. Both of these are genuine explanations of the child's behavior; whether we are satisfied with one or the other will depend on what we already know and what we want to find out. We can't ascribe motives to evolution, of course. But it seems reasonable to distinguish between what drives an evolutionary change (gene frequencies and their interactions) and what facilitates it (ie. how adaptive changes in structure to an individual are underwritten by other structural changes).
If this distinction holds, two things are clear. First, Calcott's distinction cuts across another popular distinction that is sometimes used to account for the disagreements between evo-devo and pop-bio – that between “proximate” and “distal” explanations. How can we explain, say, the human scab? We could give a biochemical account of why blood goes hard when exposed to air, and how this helps to heal the wound underneath. We need not look to the past to do this. Or we could try to explain how the tendency to form a scab has evolved – how it has come about that humans have the kind of biochemistry that leads to scabbing. This would certainly involves studying the past. Sometimes evo-devo is said to give “proximate” and pop-bio to give “ultimate” explanations.
Against this, Calcott insists that individual-level explanations, as he describes them, can account for how current phenotypical traits came about. After all, those explanations tell us how the structure of an individual at one point of time leads to a different structure in the same individual at another time. It tells us the mechanical means for this change. If we want a more fine-grained account of change over time, we just give a continuous series of these individual-level explanations.
As Calcott points out, certain “lineage diagrams” do just this. The lineage diagram we all know is of human evolution: a series of imags of our species in profile, running from the hunched and hairy ancestors to the pale and upright modern human. This doesn't explain much. But a more sophisticated diagram might. To illustrate, consider a set of instructions on how to make an oragami figure. These usually consist in a series of five or six drawings of different stages along the way to the final figure. And each drawing has dotted lines and arrows that tell you how to get to the next figure – what combination of small changes are needed in order to add a wing here, an envelope there, a sharp point there. Producing these “instruction series” is one job of biologists, and Calcott has all the right slides to show that they take the job seriously.
Calcott is clear that his distinction between individual-level and population-level explanations defuses the debate between evo-devo and pob-bio, rather than resolving it. He points out that the two forms of explanation he describes can come into conflict. That is, one form can give an explanation of a phenomenon that the other can refute. In any particular case of such a conflict, there can be a dispute about which account should hold more weight. But this is just as reconcilable as any other dispute in science between two competing explanations for a phenomenon. No two correct explanations of the two kinds could ever disagree over the facts of evolution. In this sense, they are “commensurable.” What should not be disputed is that both of them can give insight into how species have evolved.
Now, all of this strikes me as pretty well on the right track. But I'm not convinced that Calcott has covered all the areas of serious disagreement between evo-devos and pop-bios. As I understand them, evo-devos do in fact propose a new population-level process of evolution. The don't just give us, for each evolutionary event, an account of how certain structural changes are grounded in other structural changes. They give us an alternative view of how natural selection occurs. On the old view, genotypes fix phenotypes. And evolution occurs when a new and better genotype is randomly produced and outlasts the other ones. On the new view, genotypes have a very loose hold on phenotypes. Evolution occurs when the environment changes and phenotypes change in response to it, without any genotypical adjustments occuring. Genes only change further down the line. They help to ground the new phenotype, but the new phenotype arose earlier and independently, as an adaptive response to the environment. In a phrase, phenotypes lead genotypes, and not the other way around.
One can argue about whether this new picture is really new, or just a new angle on the old one. But it is a matter of sociological fact (I thought, perhaps wrongly), that the evo-devo picture was, and is, new enough in appearance to cause a big split between the evo-devos and the pop-bios. And this really does seem to be a split about the way in which evolution occurs, not about two different patterns of explanation that can be applied to evolution. Hence Calcott's distinction does not help to heal this particlar split, even if it can heal others.
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