Gradual transitions in genetics and songs between coastal and inland populations of Setophaga townsendi

Setophaga townsendi is a species of wood warbler (family Parulidae) in northwestern North America that has geographic structure in the mitochondrial and nuclear genomes: while interior populations have differentiated mitonuclear ancestry from the sister species S. occidentalis, coastal populations have a mix of inland and S. occidentalis mitonuclear ancestries. This coastal to inland transition in genomic ancestry raises the possibility of similar geographic structure in phenotypic traits, especially those involved in mate choice. Using qualitative and multivariate approaches, we investigated whether there is a sharp transition between coastal and inland populations in both song and in nuclear DNA. We find there is a shallow geographic cline in Type I song but not in Type II song. Nuclear DNA shows a gradient between coast and inland. There is little correlation between variation in song and the isolation-by-distance pattern in the nuclear DNA. Learned songbird song is shaped by both genetic and cultural processes. There has been a debate on whether song learning promotes or slows down population differentiation. By comparing the within-species variation in song and genetic structures, we can expand our understanding of the dynamic interplay between mating signals and population differentiation.