Reticulate and hybrid speciation is promoted by environmental instability in an Indo-Pacific species complex of whistlers (Aves: Pachycephala)

Genomic studies have revealed introgressive hybridization as a common phenomenon across the tree of life, particularly among young radiations. As incipient speciation tends to be induced by vicariance events, it is assumed that introgressive hybridization is more frequent in young radiations in which allopatrically distributed species have a high probability of coming into secondary contact. In this study we utilize whole genomic data to investigate spatio-temporal introgression patterns in a songbird radiation that has colonized a highly dynamic island region in the Indo-Pacific. Some taxa within this radiation have colonized remote oceanic islands whereas others occur on landmasses and islands in the Sahul region that were periodically connected during Pleistocene periods of lower sea levels. Our results show that introgressive hybridization has been pervasive within this young radiation, despite prominent plumage differences between taxa. Geographical proximity has been an important factor for hybridization and we further find that species occupying islands in the environmentally unstable Sahul region exhibit particularly high signatures of introgressive hybridization. Yet, one species appears to have been shielded against hybridization, perhaps due to specific ecological specializations. Finally, we identify a hybrid species on an island where two oceanic radiations meet. Our results also caution against relying solely on analyses that only detect asymmetric introgression when examining systems with complex introgression histories. Collectively, our results support a growing body of literature that suggests that reticulate speciation is much more common than previously thought. This has implications for our understanding of species formation and their persistence through time.