On the DNA level, most fixations of new alleles are due to genetic drift. But what's the role of neutral processes in phenotypic evolution? I ask because Larry Moran is claiming (in the comments here) thar most phenotypic evolution is also due to drift. I find that hard to believe-- one of his examples, PTC tasting in humans, is certainly not drifting neutrally, and the expression levels of most genes (a "low-level" phenotype one might expect to be allowed to drift) seem to be under strict control (I reviewed some of the evidence for that here).

Good evidence for lack of selective constraint could come from mutation accumulation (MA) lines-- lines bred with a tiny effective population size for a number of generations. The small effective population size allows nearly all alleles (except the most deleterious ones) to drift randomly. One could compare a given phenotype in a number of these lines, then compare to wild isolates-- if the variance in the MA lines is about equal to that in the wild lines, you might conclude that selection is not operating on the phenotype in the wild.

Has this been done? Is anyone aware of experimental evidence for lack of constraint on a phenotype?

Labels: Evolution