Description
The geographic distribution of terrestrial fauna on oceanic islands has long intrigued biologists, and remains a salient topic in the field, especially for the understudied islands of the Pacific region. These landmasses are geologically young as well as geographically isolated from continental sources of biota, yet contain an incredible array of biological diversity, particularly for squamates, which represent the dominant terrestrial organisms in many archipelagos. A prevailing hypothesis for how Pacific island organisms have obtained their extant distribution is that of a stepping stone model, in which flora and fauna originated from Papua New Guinea in the western Pacific and gradually dispersed eastward. Under this mainland-to-island colonization hypothesis, biodiversity results from in-situ diversification or multiple colonization events. This study utilizes a multi-locus DNA sequence dataset from lizards representing seven of the nine species within the Emoia cyanura species group (Family: Scincidae), to elucidate the group's evolutionary relationships and test the stepping stone hypothesis of dispersal for the two species (E. cyanura and E. impar) that have a widespread distribution across Pacific islands. Additionally, previous studies of these two species have suggested that they may actually consist of multiple morphologically cryptic lineages (i.e., each may represent a species complex). Using coalescent-based analyses of the genetic data and principle components analysis of morphological characters, the results provide evidence for at least three evolutionary lineages contained within E. impar, and one new species related to E. cyanura. In contrast to the expectations of a stepping stone process of dispersal, E. cyanura and E. impar each independently exhibit the genetic signature of a rapid radiation out of the western end of their ranges, during the early to mid Pleistocene. Human-mediated colonization, in both historic and modern eras, has also invariably contributed to the very low degree of genetic variability observed among most of the sampled populations. This work and concomitant findings expand our understanding of biota endemic to the insular Pacific, which stand to face drastic alterations in the wake of global climate change.
