Abstract

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.