Water barriers and intra-island isolation contribute to diversification in the insular Aethopyga sunbirds (Aves: Nectariniidae)
Correspondence: Peter A. Hosner, 1345 Jayhawk Blvd, Department of Ecology and Evolutionary Biology and Biodiversity Institute, University of Kansas, Lawrence, KS 66045, USA.
Colonization and subsequent isolation across deep-water barriers is thought to be the primary driver of diversification in insular birds. Shallow-water barriers and intra-island isolation are less well-documented drivers of avian diversification. We examined the relative roles of different geographical barriers in the diversification of Aethopyga sunbirds, a widespread Southeast Asian genus that has its greatest diversity in the Philippine Archipelago.
Philippines, Southeast Asia, Wallacea.
We reconstructed the phylogenetic relationships among Aethopyga sunbirds with mitochondrial and nuclear DNA sequences. Phylogeny was inferred using concatenated and coalescent frameworks, implemented in maximum likelihood and Bayesian analyses. We used maximum likelihood ancestral state reconstructions to examine the ancestral distribution and colonization history of Aethopyga. To determine whether the diversification at each node occurred within a continent, across a shallow-water barrier, across a deep-water barrier or within an island, we used a series of statements based on the phylogeny, current distribution of species and bathymetric reconstructions.
Ancestral state reconstructions inferred that the core Aethopyga ancestor was continental, and that the diversity of Aethopyga on oceanic islands is the result of three or four independent colonization events. Dispersal and subsequent isolation across deep-water barriers was the most common mode of diversification in insular Aethopyga, although intra-island isolation contributed to diversity, producing a small montane radiation within Mindanao. Analyses inferred only a single unequivocal event of diversification across a shallow-water barrier. Deep molecular divergences between phenotypically distinct subspecies suggested that the taxonomy of Aethopyga is overly conservative and obscures biogeographical patterns. We recommend elevating five subspecies, all of which are endemic to the Philippines, to full species.
In addition to corroborating the importance of isolation across deep-water barriers, these data also underscore a potential role for additional isolating mechanisms in the generation of biodiversity on oceanic islands. A clade of four montane Aethopyga taxa evolved in situ within the Mindanao sky-island system, an exception to the paradigm that birds do not diversify within the geographical confines of oceanic islands.