Spatial decoupling of song and plumage generates novel phenotypes between 2 avian subspecies

Sexual signals can be important mechanisms of reproductive isolation among incipient species, but the relative effectiveness and geographic distribution of different signals may vary, particularly when those signals have different modes of inheritance (e.g., cultural vs. genetic). Indeed, novel phenotypes may develop when traits with different modes of inheritance or different selection pressures become spatially decoupled from one another in zones of secondary contact, and such novel phenotypes may facilitate or hinder reproductive isolation. We assessed divergence of a culturally transmitted signal (song) and conspecific responses to that signal across a zone of reduced gene flow where plumage characters show asymmetrical introgression among subspecies of the red-backed fairy-wren (Malurus melanocephalus). We found that, in contrast to plumage, geographic variation in many song characteristics correlated well with geographic genetic structure. Additionally, playback experiments revealed that birds responded most strongly to songs from within the same genetic region irrespective of plumage type. These results indicate that song has been resistant to introgression, yet song traits and preferences have not hindered introgression of plumage traits. We suggest that this is because different modes of inheritance (here, cultural vs. genetic) facilitate decoupling of song and plumage, resulting in a large region where males exhibit a novel combination of sexual traits.