Integrating phylogenetic and taxonomic evidence illuminates complex biogeographic patterns along Huxley's modification of Wallace's Line

Aim Nearly 150 years ago, T. H. Huxley modified Wallace's Line, including the island of Palawan as a component of the Asian biogeographic realm and separating it from the oceanic Philippines. Although Huxley recognized some characteristics of a transition between the regions, Palawan has since been regarded primarily as a peripheral component of the Sunda Shelf. However, several recent phylogenetic studies of Southeast Asian lineages document populations on Palawan to be closely related to taxa from the oceanic Philippines, apparently contradicting the biogeographic association of Palawan with the Sunda Shelf. In the light of recent evidence, we evaluate taxonomic and phylogenetic data in an attempt to identify the origin(s) of Palawan's terrestrial vertebrate fauna. Location The Sunda Shelf and the Philippines. Methods We review distributional and phylogenetic data for populations of terrestrial vertebrates from Palawan. Using taxonomic data, we compare the number of Palawan taxa (species and genera) shared with the Sunda Shelf and oceanic Philippines. Among widespread lineages, we use phylogenetic data to identify the number of Palawan taxa with sister relationships to populations or species from the Sunda Shelf or oceanic Philippines. Results Although many terrestrial vertebrate taxa are shared between Palawan and the Sunda Shelf, an increasing number of species and populations are now recognized as close relatives of lineages from the oceanic Philippines. Among the 39 putative lineages included in molecular phylogenetic studies with sampling from the Sunda Shelf, Palawan and the oceanic Philippines, 17 of them reveal sister relationships between lineages from Palawan and the oceanic Philippines. Main conclusions Rather than a simple nested subset of Sunda Shelf populations, Palawan is best viewed as having played multiple biogeographic roles, including a young and old extension of the Sunda Shelf, a springboard to diversification in the oceanic Philippines, and a biogeographic component of the Philippine archipelago. Palawan has a long, complex geological history, which may explain this variation in pattern. Huxley originally noted transitional elements in Palawan's fauna; we therefore suggest that his modification of Wallace's Line should be recognized as a filter zone, reflecting both his original intent and available taxonomic and molecular evidence.