Plumage genes and little else distinguish the genomes of hybridizing warblers

When related taxa hybridize extensively, their genomes may become increasingly homogenized over time. This mixing via hybridization creates conservation challenges when it reduces genetic or phenotypic diversity and when it endangers previously distinct species via genetic swamping [ 1 ]. However, hybridization also facilitates admixture mapping of traits that distinguish each species and the associated genes that maintain distinctiveness despite ongoing gene flow [ 2 ]. We address these dual aspects of hybridization in the golden-winged/blue-winged warbler complex, two phenotypically divergent warblers that are indistinguishable using traditional molecular markers and that draw substantial conservation attention [ 3–5 ]. Whole-genome comparisons show that differentiation is extremely low: only six small genomic regions exhibit strong differences. Four of these divergence peaks occur in proximity to genes known to be involved in feather development or pigmentation: agouti signaling prot...