Limitations in Evolutionary Genetics

PZ Myers is reposting some of his greatest hits from the old Pharyngula website to his new digs at ScienceBlogs. In one post he gets into the deficiencies of modern evolutionary theory using West-Eberhard’s book as a guide. I agree with most of the thing he says (and I’ll get into how I agree with him below), but first I need to scratch a pet-peeve itch:

“You can see this in any textbook of population genetics: the effect of selection is to impose a gradual shift in the mode of a pattern of continuous variation. Stabilizing selection chops off both tails of the distribution, directional selection works against one or the other tail, and disruptive selection favors the extremes.”

I’m not sure a population genetics text would dwell on (or even mention) terms like “stabilizing selection” or “disruptive selection”. I think the more appropriate terms would be purifying selection and underdominance. I’m not even clear as to what stabilizing and disruptive selection mean in a popgen framework (I think they are more geared toward describing phenotypic evolution). Once again, I agree with what Myers is saying, I just get think if you’re gonna rag on my discipline, please use our terminology (you know, how we get pissed off when creationists make fun of Darwinism).

Myers also deals with the discreet nature of genetics and the continuous nature of phenotypic variation:

“We often see studies of graded phenotypes where we blithely accept that these are driven by underlying sets of graded distributions of genes, such as the studies of Darwin’s finches by the Grants, yet those genes are unidentified. Conversely, the characters that taxonomists use to distinguish species are usually qualitatively distinct are at least abruptly discontinuous. There is a gap in our thinking about these things, a gap that really requires developmental biology. Wouldn’t it be useful to know the molecular mechanisms that regulate beak size in Darwin’s finches?”

I think the finch example is too poorly studied to be a model in this field. Those who know me won’t be surprised when I suggest a better organism: Drosophila. Researchers are already examining quantitative variation in Drosophila, studying things like the genetics of bristle number variation. Or, if you’re sick of hearing about Drosophila, any model organism on which we have some background information on its genetics will do.

I think the reason genetics has attached itself to evolutionary theory is that it’s so much easier to study genetics than developmental biology. We can measure and quantify genetic variability because it’s already coded for us. It’s up to developmental biologists/physiologist/anatomists to catch up and figure out how to objectively measure the phenotypic differences within and between species. And it sounds like a lot of work (I don’t envy you guys at all!). I’ve discussed this before, so I won’t dwell on it.

Evolutionary geneticists can just sequence a some homologous DNA and study that thing to hell (just look at all of the great work that has come out of the Drosophila Adh locus). Developmental biologists must spend years (or decades) examining the cellular biology, morphology, and genetics of a particular system before they can outline a model for that system’s evolution. As Myers points out, even genetics and development didn’t get integrated until Nusslein-Volhard and other’s work on, you guessed it, Drosophila development. I think evolutionary biology needs to be updated to include developmental biology; it just requires so much more work than evolutionary genetics.


  1. #1 PZ Myers
    January 17, 2006

    Unfortunately, those terms about selection are in very common use in introductory biology textbooks — I know Purves’ Life uses them, and I recall that at least the editions of Campbell I used a few years ago did so too.

    I agree that Drosophila is a better model than a finch. The gap is there, though: I’ve read the Grants’ work, and everything always jumps from descriptions of differences in phenotype, all the way to differences in nucleotide sequences…with no mention even of all of the processes in between those two levels.

  2. #2 Ricardo Azevedo
    January 17, 2006

    Directional selection and stabilizing selection can be formulated precisely in the context of quantitative traits. For example, see Sean Rice’s book “Evolutionary Theory: Mathematical and Conceptual Foundations” (2004). In fact Rice goes further than anyone I know in extending population genetics to incorporate developmental thinking into evolutionary theory.

  3. #3 RPM
    January 17, 2006

    I agree with you Ricardo, that stabilizing selection can be thought of in terms of quantitative traits. But I was thinking more in terms of the traditional one locus population genetics models that you usually see in an intro bio or intro evolution course. You could also argue that disruptive selection can be thought of in terms of quantitative traits. I was thinking that it’s so difficult to go from the simple population genetics models to more biologically meaningful holistic concepts. Maybe that got lost in the post somewhere.

    As for directional selection, I never disagreed that it is an acceptable popgen term.