Species Richness Covaries with Mating System in Birds

Many studies have sought to identify traits that influence the relative number of species in related taxa. We examined whether species richness was associated with social mating system in birds. Taxa with promiscuous mating systems tended to be more species-rich than their nonpromiscuous sister taxa. This association was statistically significant when examined with tests that take into account the magnitudes of paired contrasts. The results do not arise from covariation between mating system and body size. We discuss these findings in the context of the hypothesis that sexual selection promotes speciation.