Evolution of dark colour in toucans (Ramphastidae): a case of molecular adaptation?

In the last decades, researchers have been able to determine the molecular basis of some phenotypes, to test for evidence of natural selection upon them, and to demonstrate that the same genes or genetic pathways can be associated with convergent traits. Colour traits are often subject to natural selection because even small changes in these traits can have a large effect on fitness via camouflage, sexual selection or other mechanisms. The melanocortin-1 receptor locus (MC1R) is frequently associated with intraspecific coat colour variation in vertebrates, but it has been far harder to demonstrate that this locus is involved in adaptive interspecific colour differences. Here, we investigate the contribution of the MC1R gene to the colour diversity found in toucans (Ramphastidae). We found divergent selection on MC1R in the clade represented by the genus Ramphastos and that this coincided with the evolution of darker plumage in members of this genus. Using phylogenetically corrected correlations, we show significant and specific relationships between the rate of nonsynonymous change in MC1R (dN) and plumage darkness across Ramphastidae, and also between the rate of functionally significant amino acid changes in MC1R and plumage darkness. Furthermore, three of the seven amino acid changes in MC1R that occurred in the ancestral Ramphastos branch are associated with melanism in other birds. Taken together, our results suggest that the dark colour of Ramphastos toucans was related to nonsynonymous substitutions in MC1R that may have been subject to positive selection or to a relaxation of selective pressure. These results also demonstrate a quantitative relationship between gene and phenotype evolution, representing an example of how MC1R molecular evolution may affect macroevolution of plumage phenotypes.