Sexual selection is often considered a driver of diversification of a lineage. I was introduced to the concept in Jared Diamond’s The Third Chimpanzee, where he suggested that racial differences in appearance might be due to sexual preference, following a suggestion originally made by Charles Darwin. Though sexual selection emerges now and then as a deus ex machina in discussion sections of papers, in general it hasn’t panned out addressing this topic.
But a new paper using shorebirds offers results which oppose this sort of inference, in that sexual selection may be a homogenizing force. Basically the authors used the fact that shorebird lineages have related monogamous and polygamous species. They looked at species richness and genetic diversity using STRUCTURE and microsatellites.
Polygamy slows down population divergence in shorebirds:
Examining microsatellite data from 79 populations in 10 plover species (Genus: Charadrius) we found that polygamous species display significantly less genetic structure and weaker isolation-by-distance effects than monogamous species. Consistent with this result, a comparative analysis including 136 shorebird species showed significantly fewer subspecies for polygamous than for monogamous species. By contrast, migratory behavior neither predicted genetic differentiation nor subspecies richness. Taken together, our results suggest that dispersal associated with polygamy may facilitate gene flow and limit population divergence. Therefore, intense sexual selection, as occurs in polygamous species, may act as a brake rather than an engine of speciation in shorebirds.
A reminder that lots of theorizing may lead you nowhere fast, but a quick empirical check can be very humbling. I’m not sure as to the generality of this result, and ultimately it probably has to do with reproductive variance. But it is a starting point.
Addendum: Overall Geoffrey Miller’s The Mating Mind is probably wrong in most of the details, though perhaps on the most general level there may be something there (I’m wondering particularly in regards to mutational load). But it’s a decent introduction to sexual selection theory in human context, and has a lot of interesting ideas. And Miller is actually a good writer as far as scientists go.