Gene frequency comparisons between taxa: support for the natural selection of protein polymorphisms.

The hypothesis has been advanced that the pervasive protein variation found in natural populations of many organisms is adaptively neutral, and thus not subject to natural selection. This neutrality hypothesis predicts that at polymorphic gene loci different configurations of allelic frequencies will occur in different species. Results of an extensive study of protein variation in several species of Drosophila show that any two species have very similar allelic frequencies at a substantial proportion of all gene loci, while at many other loci the species have very different sets of alleles. Genetic distances have been calculated between pairs of subspecies, morphologically similar species, and morphologically different species. The distribution of genetic distances is strikingly different from the predictions of the neutrality theory. Protein variation appears to be maintained by natural selection.