Drifting away from selection

2 minute read

Following up on yesterday's post on annoying misconceptions, I noticed Razib had posted his own candidate:

My problem is not an misconception, it is a pet peeve. As I've noted before, random genetic drift is a catchall explanation for everything.

Well, I thought that was worth a post of its own instead of an update, because it probably annoys me even more than the species divergence thing.

It is a periodic revelation for me how many committed Darwinists don't use or understand natural selection. I think they place natural selection somewhere between Santa Claus and the Tooth Fairy -- useful to explain a few really strange phenomena, but pretty much irrelevant to evolution.

There is, of course, some reason to be cautious about selection. A little selection goes a long way toward explaining almost any pattern of evolution. Selection can make populations stay the same, and it can make them change. It can make them change fast, or it can make them change slowly. And at the genetic level, there are good reasons to suppose that many nucleotide changes don't have a phenotypic effect -- necessary for them to be selected. So it is reasonable a lot of the time to take neutrality (and therefore, genetic drift) as a null hypothesis for change.

Null hypotheses aren't there to be believed, they are there to be tested! Neutrality is a better null hypothesis because the hypothesis of some kind of selection is harder to refute. But neutrality is pretty hard to refute too, at least for the kind of evidence we usually have at hand.

It doesn't help at the molecular level that clearly non-neutral patterns of variation can be explained by extreme demographic changes (and no selection), or selection. Which hypothesis do we choose then? Most people pick neutrality, but not always for good reasons.

Nor does it help that morphological change over long time spans tends to average to very small amounts per unit time. That pattern of change is thoroughly consistent with genetic drift, but equally consistent with slowly changing stabilizing selection. And depending on the density of fossil sampling, it is often consistent with occasional pulses of strong directional selection.

Testing the difference between these hypotheses statistically is murderously hard with fossil samples. So what do we accept provisionally?

Myself, I'm a natural selection man. Accident is overrated.

You know, there's something depressing about collecting a bunch of annoying misconceptions -- and I suppose reading all of them must be sort of annoying itself. Of course, there's always the hope that writing about them might have some effect ...