POTENTIAL PALEO-DRIVERS OF ANTI-TROPICAL DISTRIBUTIONS ACROSS THE FISH TREE OF LIFE

Anti-tropical species are uniquely distributed north and south of the tropics, but lack tropical populations. Several hypotheses regarding the mechanisms for such disjunct distributions have been suggested, however, molecular evidence suggests Plio-Pleistocene genetic divergence times for most taxa. This reduces the potential explanations to two main hypotheses: dispersal across the tropics during cooler glacial periods and biotic exclusion from the tropics. These two hypotheses predict very different biogeographic patterns of abiotic habitat suitability in the tropics: the former predicts that present-day tropical habitats are not suitable, while the latter predicts that they are. Here we take an ecological niche modeling (ENM) approach to testing these hypotheses using 32 species of teleost fishes from across the fish tree of life. The Maxent ENM algorithm was used to model northern and southern populations independently to determine degree of niche differentiation in modern populations and test for current suitable habitat in the tropics. These predictions, as well as models trained with the full species distribution, were projected onto LGM conditions to investigate the potential for a glacial dispersal corridor across the tropics. Modern and LGM abiotic conditions were derived from the Paleo-MARSPEC database (Sbrocco 2014). Our results indicate that both dispersal and biotic interactions may impact current species distributions, in species-specific ways. Whereas the pattern of anti-tropical distributions shows moderate phylogenetic clustering, species showing potential glacial dispersal vs. biotic exclusion are not. This suggests that neither mechanism is phylogenetically conserved. Notably, in many species modern northern and southern populations indicate at least some degree of niche divergence, which may reflect post-glacial adaptation and/or insipient speciation. Overall, this study highlights the species-specific nature of biogeographic patterns in their response to abiotic and biotic change since the last glacial maximum.