How do complex, colorful traits evolve?

How complex traits composed of multiple independent parts evolve is largely unknown. We use electron microscopy, spectrophotometry and phylogenetic analyses to show that the components of complex nanostructures producing bright colors in duck feathers evolve at different rates. In turn, this leads to independent evolution of aspects of the colors themselves, potentially helping to explain the broad diversity of colors in this group.

HFSP Young Investigator Grant holder Matthew Shawkey and colleagues
authored on Tue, 31 March 2015

Many traits of living organisms are composed of multiple parts that work together to perform a particular function.  For example, bird wings require precise arrangements of feather, bone and muscle to enable flight. A long-standing area of interest in evolutionary biology is how these complex traits evolve- do their individual components evolve together or separately? And how does this independence affect their evolutionary trajectories? Research on traits critical to survival has shown that greater independence can lead to greater diversification, but whether the same is true of traits that affect reproduction has not been tested.

Figure: Color diversity in dabbling duck wing feathers. Light microscope images of iridescent feather barbs from the secondary feathers of dabbling duck species. Row 1: Anas rubripes, Anas platyrhynchos, Anas zonorhyncha, Anas diazi, Anas fulvigula, Anas sparsa, Amazonetta brasiliensis, Anas crecca, Anas carolinensis, Anas luzonica, Anas superciliosa, Anas melleri. Row 2:Anas rhynchotis, Anas undulata, Anas penelope, Anas poecilorhyncha, Anas flavirostris, Anas bahamensis, Anas versicolor, Anas puna, Anas platalea, Anas laysensis, Anas hottentota, Anas querquedula. Row 3:Anas capensis, Anas cyanoptera, Anas gibberifrons, Anas gracilis, Anas discors, Anas americana, Anas acuta, Anas castanea, Anas aucklandica, Anas erythrorhyncha, Speculanas specularis, Lophonetta specularioides. Image credit: Chad Eliason.

Animal colors are used in a wide variety of contexts, but the brightest and shiniest seem to serve mostly to attract mates. These colors can be produced by pigments or by optical effects from tissues that are organized at the nanometer scale. These so-called structural colors, like bird wings, require precise coordination of numerous individual components to function correctly. In the case of iridescent plumage patches on duck wings, these components are thin nanometer-scaled organelles called melanosomes arranged in a honeycomb pattern throughout a keratin matrix.

We used electron microscopy to examine the nanostructures, spectrophotometry to measure color, and phylogenetic techniques to reconstruct the evolutionary history of most dabbling duck species. We show that both aspects of the complex nanostructures that produce these colors, and the color variables themselves that they control, are decoupled from one another and thus evolve at different rates. For example, distance between melanosomes evolves more quickly than other morhologocial traits while color brightness evolves faster than hue or saturation. Over time, this decoupling may enable rapid diversification, potentially explaining why some groups of birds are more colorful than others. The insights and methodological advances made in this work will carry over into our HFSP team’s examination of the structural basis of egg colors and its behavioral and evolutionary consequences.

Reference

Modular evolution facilitated by a complex nanostructure. Eliason CM & Shawkey MD. Evolution. 2015 Feb;69(2):357-67. doi: 10.1111/evo.12575. Epub 2015 Jan 16.

Pubmed link