Reticulate and Hybrid Speciation is Promoted by Environmental Instability in an Indo-Pacific Species Complex of Whistlers (Aves: Pachycephala).

Genomic studies have revealed introgressive hybridisation 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 hybridisation is more frequent in young radiations in which allopatrically distributed species have a high probability of coming into secondary contact. In this study, we use whole genomic data to investigate spatio-temporal introgression patterns in a songbird radiation that has colonised a highly dynamic island region in the Indo-Pacific. Some taxa within this radiation have colonised 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 hybridisation has been pervasive within this young radiation, despite prominent plumage differences between taxa. Geographical proximity has been an important factor for hybridisation and we further find that species occupying islands in the environmentally unstable Sahul region exhibit particularly high signatures of introgressive hybridisation. Yet, one species appears to have been shielded from hybridisation, perhaps due to specific ecological specialisations. 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 more common than previously thought. This has implications for our understanding of species formation and their persistence through time.