Perpendicular axes of incipient speciation generated by mitochondrial introgression

Differential introgression of mitochondrial versus nuclear DNA generates discordant patterns of geographic variation and can promote speciation. We examined a potential case of mitochondrial introgression leading to two perpendicular axes of differentiation. The Eastern Yellow Robin, a widespread Australian bird, shows a deep mitochondrial split that is perpendicular to north-south nuclear DNA and plumage colour differentiation. We proposed a scenario to explain this pattern: (1) the two nuclear and mitochondrial genomes differentiated in concert during north-south population divergence; (2) later, their histories disconnected after two mitochondrial introgression events resulting in a deep