Phylogeny, historical biogeography and body size evolution in Pacific Island Crocodile skinks Tribolonotus (Squamata; Scincidae)

Competition heavily influences the structure of island communities, particularly in species-rich areas. If ecologically similar lineages come into contact following dispersal, selection may favor rapid evolutionary change; if constraints prevent such change, lineage extinction may result. One mechanism for relieving competition among newly sympatric species is the evolution of body size differences, such as through character displacement or size assortment. The Crocodile skinks of the genus Tribolonotus exhibit a threefold variation in body size, and several species occur in sympatry. We use 2252 bp of DNA sequence data spanning two mitochondrial (cyt b and ND2) and three nuclear (C-mos, Rhodopsin and Phosducin) gene regions to reconstruct the phylogeny of Tribolonotus, use it to examine the biogeography of the genus, and test for size assortment or character displacement. We find evidence that Tribolonotus originated on either Greater Bougainville or in New Guinea, and subsequently colonized surrounding islands via multiple colonization events. Our ancestral state reconstructions support multiple instances of parallel and independent change in body size within Tribolonotus. Additionally, we find no evidence for size assortment and conflicting evidence for character displacement, which we argue suggests that character displacement, combined with ecological differences between New Guinean species (T. gracilis and T. novaeguineae), best explains the evolution of body size in the genus Tribolonotus.