Sexual tail dimorphism explains speciation rate in swallows and martins

Sexual selection can promote speciation in theory, but macroevolutionary studies have reported mixed results. A possible explanation for this inconsistency is the use of inappropriate proxies of sexual selection. Here, by focusing on swallows and allies (Aves: Hirundinidae), I examined whether or not a classic example of sexually selected trait, long outermost tail feathers, explains speciation rate in this clade. Long tails have been repeatedly shown to be intersexually selected in manipulative experiments, which is further corroborated by a series of macroevolutionary studies, thereby validating their use as a target of sexual selection. I found that hirundines with sexually dimorphic tail length have a significantly higher speciation rate than those with sexually monomorphic tail length. Furthermore, evolutionary changes in the extent of sexual tail dimorphism (and those in the extent of male outermost tail exaggeration) was significantly positively associated with speciation rate. Sexual plumage dichromatism and wing dimorphism are irrelevant to speciation rate. Together, the current study demonstrated the importance of using valid targets of sexual selection in studies on the macroevolutionary patterns of speciation. Using a classic, unidimensional sexual trait (i.e., long tail), I demonstrated a strong macroevolutionary support that divergence of intersexually selected traits promotes speciation.