Reticulated Adaptive Radiation and Phylogenomic Diversity Loss in the Hawaiian Honeycreepers.

The Hawaiian honeycreepers, one of the world's most iconic adaptive radiations, are facing a human-mediated extinction crisis. More than 60 Hawaiian honeycreepers (Fringillidae: Carduelinae: Drepanidini) evolved from a single rosefinch-like ancestor that colonized the Hawaiian Islands. Only 17 Hawaiian honeycreeper taxa are known to be extant, of which only the common 'amakihi (Chlorodrepanis virens) and 'apapane (Himatione sanguinea) are classified as "Least Concern." Using mitogenome, whole-genome, and hybridization-capture data, we present a comprehensive phylogeny of the historically known Hawaiian honeycreeper radiation. We also place two taxa known only from paleontological specimens (Vangulifer neophasis and cf. Xestospiza conica) in their phylogenomic context. We demonstrate that most Hawaiian honeycreeper lineages diversified in a "Big Bang" of adaptive radiation correlating with the formation of O'ahu and that interspecies introgression likely played a role in this process. Using whole-genome data, we confirm putative interspecies admixture [N. A. S. Przelomska et al., Biol. Lett. 21, 20250265 (2025)] between the 'ō'ū (Psittirostra psittacea) and Lāna'i hookbill (Dysmorodrepanis munroi) and document ongoing gene flow across the 'amakihi species complex. Furthermore, we reject the synonymization of Vestiaria ('i'iwi) and Drepanis (Hawai'i and black mamo). Instead, the mamo species form an extinct clade with no close extant relatives, demonstrating a greater loss of phylogenomic diversity than previously understood.