Which evolutionary force is more important: natural or sexual selection?
Charles Darwin and Alfred Russel Wallace independently hit upon the idea of evolution through natural selection. They did, however, disagree on some aspects of the theory. For instance, Wallace stated that human evolution is vastly different from the way animals evolve. He even ventured into spiritualism to explain the human mind. Darwin, on the other hand, saw humans as just another branch on the primate tree. Another main disagreement concerns the evolution of animal coloration. Darwin considered dull colors as the default and called upon sexual selection to explain conspicuous colors, while Wallace viewed showy colors as the default and argued that natural selection favors dull colors to improve camouflage. Two recent studies indicate that both men had a valid point (at least when it comes to the evolution of animal coloration).
Fifty Shades of Brown
The Furnariida, a group of more than 600 species of Neotropical birds, can best be described as dull. The title of an Evolution paper by Rafael Marcondes and Robb Brumfield captures the plumage diversity in this bird group nicely: “Fifty shades of brown”. But what is driving the evolution of these different shades of brown? To answer this question, Marcondes and Brumfield compared color data from 3096 museum specimens and reconstructed the evolutionary history of the Furnariida.
The macro-evolutionary analyses revealed that birds tend to be darker in darker habitats. This findings indicates that natural selection for camouflage is the main evolutionary force driving plumage coloration in this bird group. There was no difference in the speed of plumage evolution between males and females, suggesting that sexual selection (which usually speeds up the evolution of male traits) is not involved here.
The evolutionary pattern in the Furnariida could not be more different from that in the colorful Tyrannida. Christopher Cooney and his colleagues explored the evolution of plumage coloration in this large radiation of songbirds. Their study, published in the journal Nature Communications, shows faster evolution of plumage coloration in males compared to females. This pattern points to sexual selection as the main driver.
In addition, evolution tended to be fastest when red and yellow plumage was involved. These colors are based on carotenoids, the yellow, orange, and red organic pigments that are produced by plants. Birds acquire these pigments through their diet. Previous work has shown that carotenoid-based colors are good signals of individual quality. So, females can use these colors to select the best male.
Although both studies found different evolutionary forces driving plumage coloration, they did converge on a common theme, namely variation in evolutionary rates of different plumage patches. In the Furnariida, evolution was more rapid in ventral plumage coloration compared to dorsal feathers. This differences could be due to social selection in which the birds use these patches in sexual signalling. Similarly, in the Tyrannida, plumage patches involved in signalling (e.g., crown, throat, breast) showed faster rates of evolution.
A logical next step will thus be to study the evolution of distinct plumage patches. It seems that both Darwin and Wallace had a point. The evolution of animal coloration is probably the outcome of sexual and natural selection on particular plumage patches.
Cooney, C.R., Varley, Z.K., Nouri, L.O., Moody, C.J.A., Jardine, M.D. & Thomas, G.H. (2019) Sexual selection predicts the rate and direction of colour divergence in a large avian radiation. Nature Communications, 10:1773.
Marcondes, R.S. & Brumfield, R.T. (2019) Fifty shades of brown: Macroevolution of plumage brightness in the furnariida, a large clade of drab neotropical passerines. Evolution, 73(4):704-719.
Weaver, R.J., Santos, E.S.A., Tucker, A.M., Wilson, A.E. & Hill, G.E. (2018) Carotenoid metabolism strengthens the link between feather coloration and individual quality. Nature Communications, 9:73.