Introgression catalyzing plumage evolution and speciation in an Asian songbird complex.

Hybridization and introgression can promote speciation and phenotypic diversity. We investigated genomic divergence between two closely related Asian songbirds, Naumann's Thrush (Turdus naumanni, Nau) and Dusky Thrush (T. eunomus, Eun), and their evolutionary relationship with the geographically adjacent Red-throated Thrush (T. ruficollis, Ruf). Utilizing a de novo chromosome-level genome and whole-genome resequencing data from 66 individuals, we characterized differentiation and introgression among the three species. Genomic divergence between Nau and Eun was restricted to a few genomic islands that overlap with regions introgressed into Nau from Ruf. Notably, these regions harbor three melanin-related genes under strong selection in Nau (ASIP, OCA2, and HERC2), which may account for the plumage similarity between Nau and Ruf. These findings suggest that the morphological and genetic divergence between Nau and Eun may have been driven by adaptive introgression of melanin-related genes from Ruf into Nau. This study reveals how introgression can generate rapid genomic and phenotypic divergence, indicating a potentially ubiquitous yet overlooked speciation mechanism.