Back in black: melanin-rich skin colour associated with increased net diversification rates in birds

Evolutionary biologists have long been interested in understanding the factorsthat promote diversification in organisms, often focussing on distinct and/orconspicuous phenotypes with direct effects on natural or sexual selectionsuch as body size and plumage coloration. However, multiple traits that poten-tially influence net diversification are not conspicuous and/or might beconcealed. One such trait, the dark, melanin-rich skin concealed beneath thefeathers, evolved more than 100 times during avian evolution, frequently inassociation with white feathers on the crown and UV-rich environments,suggesting that it is a UV-photoprotective adaptation. Furthermore, multiplespecies are polymorphic, having both light and dark skin potentially aidingoccupation in different UV radiation environments. As such these polymorph-isms are predicted to occur in species with large latitudinal variation intheir distribution. Furthermore, by alleviating evolutionary constraints onfeather colour, the evolution of dark skin may promote net diversification.Here, using an expanded dataset on bird skin coloration of 3033 species wefound that more than 19% of species had dark skin. In contrast to our predic-tion, dark skinned birds have smaller distribution ranges. Furthermore, bothdark skin and polymorphism in skin coloration promote net diversification.These results suggest that even concealed traits can influence large scaleevolutionary events such as diversification in birds.

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