Industrial melanism in the Vermilion Flycatcher (Pyrocephalus rubinus): the first case in a vertebrate?

Industrial melanism is adaptive darkening of pigmentation in response to anthropogenic industrial pollution. This phenomenon which was described from the famous case of the peppered moth is unknown outside of the insect order Lepidoptera. We examine a potential case of industrial melanism in a vertebrate, the Vermilion Flycatcher (Pyrocephalus rubinus). Vermilion Flycatchers are strikingly bright red and sexually dimorphic throughout their range, from the southwestern United States to Argentina. However, in the heavily polluted city of Lima, Peru, over half of Vermilion Flycatcher individuals have sooty brown plumage. The brown morph appears limited to the extent of the urban area. We found that variation in the MC1R gene perfectly predicts plumage morph. The dominant allele confers a solid sooty brown plumage that is notably darker and lacking any red color in homozygotes. Heterozygous individuals have intermediate phenotypes which are sooty brown with a few pink or red feathers on the face, flanks, and vent. Homozygous recessive individuals have bright red plumage that is typical of the species. We used this new understanding of the genetic basis of the plumage morphs to quantify genotype frequencies in and around the city of Lima to compare to predicted genotype frequencies under Hardy-Weinberg Equilibrium. We found striking differences in allele frequencies between the urban center and adjacent agricultural areas. The most parsimonious explanation for this pattern is strong diversifying selection associated with industrial pollution.