A millennium of stasis in avian ornamentation? Implications for sexual selection theory

Abstract

Sexual selection is widely accepted as an explanation for the evolution of ornamental traits in animals. Theory predicts that ornament evolution via sexual selection is triggered when a population is moved from an equilibrium state by changes in environmental conditions or population parameters and that once initiated, the rate of change can be rapid in comparison to change induced by natural selection. To assess these ideas, we considered whether there are examples of substantive changes to ornamental traits in any species of bird over recent human history. We proposed and tested a new means to assess contemporary evolution in the ornamental traits in birds, covering a period of more than a thousand years before present. We predicted that cases of rapid change in avian ornaments would be captured in the pictorial record across the centuries for which bird plumage has been documented. We found no substantial change in the ornamental traits of any species of bird, and we found few instances of even small changes in the ornamentation of bird species as depicted in the historical record. Our study is the first to systematically evaluate changes in ornamental traits within extant species on a time scale of centuries, and our findings have important implications for the mechanisms that generate the diversity of ornaments in birds.