Avian sexual dichromatism in relation to phylogeny and ecology

The extent and diversity of sexual dichromatism in birds is thought to be due to the intensity of current sexual selection on the plumage ornamentation of males and females. This view leads to an expectation of concordance between ecological conditions and sexual dichromatism. Yet, because expression of dichromatism is the result of not only current selection, but also historical patterns of development, function, and selection, the concordance between ecology and current sexual dichromatism is not straightforward. Recent studies have revealed a number of trends in the evolution of avian sexual ornamentation that seem contrary to what is expected if current sexual selection is the primary force shaping dichromatism. For example, change in sexual dichromatism is often the result of evolutionary changes in female rather than male ornamentation. Moreover, sexual dichromatism is often an ancestral rather than a derived state; current expression of dichromatism is frequently the result of selection for lesser ornamentation in one sex and not for ornament elaboration. Loss and gain of sexual ornamentation sometimes precedes changes in preference for sexual ornamentation, and sexual ornaments can have high evolutionary lability despite their developmental and functional complexity. These findings emphasize that phylogenetic reconstructions must play a central role in attempts to understand the function and evolution of sexual dichromatism. With a historical perspective, one can test the relative importance of direct selection, indirect selection, and drift in relation to changes of sexual dichromatism. If sexual selection is invoked, the mechanisms of sexual selection can be explored by examining the concordance between the elaboration of ornamentation and the preferences for ornamentation across species and by tracing phylogenetic trajectories of sexual ornaments. Finally, placing physiological, genetic, and developmental mechanisms of sexual ornamentation into such a phylogenetic framework will enable greater inference about the past evolution and current function of sexual dichromatism in birds.