Sexual selection and diversification: reexamining the correlation between dichromatism and speciation rate in birds

Theory predicts that sexual selection can serve as an important driver of speciation, but phylogenetic comparative analyses have failed to demonstrate a consistent effect of sexual selection on species richness at macroevolutionary scales. Sexual dichromatism in birds is an example of a phenotypic trait that is hypothesized to reflect the intensity of sexual selection, yet previous studies have reached ambiguous conclusions regarding its role in promoting species diversification. Here, we revisit this problem by pairing published spectrophotometer estimates of plumage dichromatism in the bird-visible range with a newly developed method for modeling speciation rates on phylogenetic trees that explicitly accounts for diversification rate variation through time and among clades. We find little evidence linking dichromatism to speciation across birds, using several measures of dichromatism and macroevolutionary diversification. These results suggest that sexual dichromatism plays a limited role in determining speciation rates at macroevolutionary scales in birds.