Swallows with sexually dimorphic tails have higher speciation rates

Sexual selection can promote speciation in theory, but macroevolutionary studies have reported mixed results. A possible explanation for this inconsistency is the difficulty of choosing appropriate 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 proxy 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. Extinction rate was estimated to be small with no detectable difference between them, and thus hirundines with sexually dimorphic tail length, but not those with sexually monomorphic tail length, have a positive net diversification rate. When sexual dimorphism in tail length was replaced with sexual dimorphism in wing length or sexual plumage dichromatism, neither explained speciation and extinction rates. 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. Taken together, the current study demonstrated the importance of choosing appropriate proxies (or “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.