Speciation is a fundamental concept in evolutionary biology and understanding mechanisms driving speciation remains the foremost research topic within this field. Hybridisation is often involved in speciation and can influence its rates, potentially accelerating, decelerating, or even reversing the process. This study investigates the evolutionary history of the New Guinean bird genus Melidectes, consisting of six species that inhabit various montane regions at different elevations. While most Melidectes species have allopatric distributions, two species overlap in the central mountain range and hybridise. However, plumage differences and elevational adaptations are assumed to maintain the species' boundaries. Utilising specimens from natural history collections and comprehensive genomic analyses, including a de novo genome assembly, we characterise allopatric speciation patterns within the genus and highlight potential future speciation driven by climate change. Contrary to previous hypotheses, our findings suggest that in the two distributionally overlapping species, phenotypic differences do not prevent gene flow. We find limited acoustic differentiation and extensive admixture across most of their distributions. Divergence and admixture levels conform poorly to the current taxonomy and follow a geographical pattern in which the most isolated populations at the ends of the distributions are most divergent and show least admixture. However, their mitochondrial genomes are grouping more in accordance with suggested species identities into two deeply divergent lineages. We propose that this system demonstrates the ephemeral nature of speciation, in which two incipient species have started mixing extensively as they came into secondary contact resulting in nearly complete fusion into a single lineage.

Genomic studies have shown that introgressive hybridization is 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 species radiations in which allopatrically distributed species have a high probability of comming into secondary contact. In this study we draw on 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 at times connected during Pleistocene periods of low sea level. 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. 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 how species form and how species are maintained through time.