Paleodistributions and Comparative Molecular Phylogeography of Leafcutter Ants (Atta spp.) Provide New Insight into the Origins of Amazonian Diversity

Scott E. Solomon is with UT Austin, Smithsonian Institution, and São Paulo State University; Mauricio Bacci Jr is with São Paulo State University; Joaquim Martins Jr is with São Paulo State University; Giovanna Gonçalves Vinha is with São Paulo State University; Ulrich G. Mueller is with UT Austin.The evolutionary basis for high species diversity in tropical regions of the world remains unresolved. Much research has focused on the biogeography of speciation in the Amazon Basin, which harbors the greatest diversity of terrestrial life. The leading hypotheses on allopatric diversification of Amazonian taxa are the Pleistocene refugia, marine incursion, and riverine barrier hypotheses. Recent advances in the fields of phylogeography and species-distribution modeling permit a modern re-evaluation of these hypotheses. Our approach combines comparative, molecular phylogeographic analyses using mitochondrial DNA sequence data with paleodistribution modeling of species ranges at the last glacial maximum (LGM) to test these hypotheses for three co-distributed species of leafcutter ants (Atta spp.). The cumulative results of all tests reject every prediction of the riverine barrier hypothesis, but are unable to reject several predictions of the Pleistocene refugia and marine incursion hypotheses. Coalescent dating analyses suggest that population structure formed recently (Pleistocene-Pliocene), but are unable to reject the possibility that Miocene events may be responsible for structuring populations in two of the three species examined. The available data therefore suggest that either marine incursions in the Miocene or climate changes during the Pleistocene—or both—have shaped the population structure of the three species examined. Our results also reconceptualize the traditional Pleistocene refugia hypothesis, and offer a novel framework for future research into the area.S.E.S. was funded by a DDIG grant from the National Science Foundation (DEB 0407772), the IGERT program in computational phylogenetics at The University of Texas at Austin, graduate fellowships from the Section of Integrative Biology at The University of Texas at Austin, and a grant from the Amazon Conservation Association and the NSF International Reseach Fellowship Program (IRFP #07012333). MBJ was funded by FAPESP (06/00185-7), CAPES (Aux-UT-165/2005) and CNPq (310826/2006-3 and 479990/2006-9). J.M.J. was funded by CAPES (Brazil). G.G.V was funded by FAPESP (05/54250-1). Additional funding was provided by NSF IRCEB Grant DEB-0110073 to U.G.M.Biological Sciences, School o