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Back in black: concealed skin colour and skin colour polymorphism promotes diversification in birds

Nicolaï, Michaël1

Published Oct 20, 2023 on Dryad. https://doi.org/10.5061/dryad.8pk0p2ntc

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Abstract

Evolutionary biologists have long been interested in understanding the factors that promote diversification in organisms, often focusing on distinct and/or conspicuous phenotypes with direct effects on natural or sexual selection such as body size and plumage colouration. However, multiple traits that potentially influence net diversification are not conspicuous and/or might be concealed. One such trait, the dark, melanin-rich skin concealed beneath the feathers, evolved more than 100 times during avian evolution, frequently in association with white feathers on the crown and UV-rich environments, suggesting that it is a UV-photoprotective adaptation. Furthermore, multiple species are polymorphic, having both light and dark skin. Such polymorphisms might aid in species occupying different UV radiation environments with dark skin enabling their presence in high UV-rich areas, and light skin to cope with potential negative effects of melanised skin when this becomes redundant. As such these polymorphisms are predicted in species with large latitudinal variation in their distribution. Furthermore, via this polymorphism, and by alleviating evolutionary constraints on feather colour, the evolution of dark skin may promote net diversification. Here, using an expanded dataset on bird skin colouration of 3033 species (with all families and 99% of bird genera), we found that more than 19% of species had dark skin. Contrary to our prediction, dark-skinned birds have smaller distribution ranges. Furthermore, we show that the evolution of dark skin colours and polymorphism in skin colouration, promotes net diversification. These results suggest that even hidden or concealed traits can influence large-scale evolutionary events such as diversification in birds.