Ephemeral Speciation in a New Guinean Honeyeater Complex ( <i>Aves</i> : <i>Melidectes</i> )

Speciation is a fundamental concept in evolutionary biology, and understanding the 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 how future speciation could potentially be 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, in contrast, their mitochondrial genomes do group in accordance with species identity, namely, 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.