Phylogeny of Pacific starlings (genus Aplonis) reveals cryptic diversity and diverse biogeographic patterns

Pacific island taxa have long informed our understanding of speciation and biogeography. However, until genomic data became commonplace, many explosive radiations have remained unresolved. One underappreciated radiation is of the genus Aplonis, a clade of starlings distributed across the Pacific from continental Sundaland to the far-flung Cook Islands of Polynesia. Of note are the high levels of secondary sympatry in Aplonis relative to other geographic radiations in the region. Here we attempt to resolve relationships in this group by sampling all extant and three extinct taxa. We sequenced ultraconserved elements from 141 ingroup samples (approximately 2 per subspecies), of which 104 were derived from historic toepad samples. We inferred a strongly supported phylogeny and used it to reconstruct the biogeographic history of the genus, which largely followed two one-way colonization routes via a stepping-stone pattern from west to east. However, there was evidence for three independent, long-distance colonizations of Micronesia, and two cases of back-colonization from more distant to more nearby islands. Furthermore, based on patterns of sympatry and rules of monophyly, we found evidence for three species-level splits, including two in the widespread and polyphyletic Asian Glossy Starling (Aplonis panayensis). Finally, the subspecies-level sampling we performed allowed us to better understand the diversification dynamics in this group. We recovered a stable accumulation of lineages over time when considering species or superspecies-level groups, but continued accumulation of geographic variation (i.e., subspecies) towards the present.