Plumage color evolution in birds has been the focus of theoretical and empirical research on sexual selection since Darwin. Many of the yellow, orange, and red hues seen in bird plumage are the result of carotenoid pigmentation. While a great number of recent studies have examined the functions of carotenoid-based plumage coloration in a single species, few have examined the evolutionary history of this trait in a comparative phylogenetic context. Using the New World blackbirds as a model clade, I focus on two questions that a comparative phylogenetic approach can uniquely address. First, what is the history of evolutionary change in carotenoid color that led to the colors seen in extant blackbird taxa? Second, by what proximate mechanisms have carotenoid pigments evolved? In Chapter 1, I present an ancestral state reconstruction of carotenoid-based plumage coloration across the Icterid phylogeny, based on reflectance measurements of museum skins. My results show robust evidence that red coloration was gained repeatedly from a yellow common ancestor. In Chapter 2, I used pigment biochemistry of meadowlark (Sturnella) and Cacique (Cacicus) feathers to test whether independent gains of red coloration are the result of parallel or convergent metabolic mechanisms. Meadowlarks have evolved red coloration using a different set of carotenoids than caciques, but the caciques have evolved the same set of carotenoids twice. This suggests that red coloration evolved by convergent evolution among different blackbird clades, but evolved by parallel evolution within the caciques. Lastly, in Chapter 3 I examine the relationship between color and carotenoid pigmentation in orioles, a blackbird clade in which orange has been gained at least twice independently from a yellow common ancestor. I found red-producing keto-carotenoids only in orange species and never in yellow species. This result is a striking contrast to our expectation for a continuous gradient of a carotenoid pigment concentration. These results suggest that repeated gains of C4-oxygenation ability best explain evolutionary changes in orange coloration in orioles. To summarize, I showed using phylogenetic comparative methods that blackbirds have repeatedly evolved towards redder carotenoid coloration. Using HPLC biochemistry, I showed that each of these gains of orange and red coloration is likely the result of a gain of C4-oxygenation ability. The prevalence of gains of orange and red coloration suggests that there may be a directional bias towards evolving longer-wavelength carotenoid plumage. The research presented in these chapters provides the phylogenetic framework necessary for future studies to examine the functional causes underlying the repeated evolution of carotenoid-based coloration.
|Advisor:||Omland, Kevin E.|
|Commitee:||Cronin, Thomas W., Leips, Jeff, Mendelson, Tamra C., Price, J. Jordan|
|School:||University of Maryland, Baltimore County|
|School Location:||United States -- Maryland|
|Source:||DAI-B 74/09(E), Dissertation Abstracts International|
|Subjects:||Evolution and Development, Systematic|
|Keywords:||Bird coloration, Carotenoids, Convergent evolution, Icteridae, Pigmentation|
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