Diversification Processes in Lizards and Snakes from the Middle São Francisco River Dune Region, Brazil

Abstract

In the middle course of the São Francisco River (SFR), inserted in the semiarid domain of the Caatinga, there is an extensive field of palaeodunes which represents an important testimony of former, drier climates in northeastern Brazil. The highly diverse sand-dwelling fauna of this dune region comprises several endemic reptiles with geographically structured distributions. Based on geomorphological, paleoclimatic, and distribution data of pairs of psammophilous reptiles, a paleolacustrine vicariant diversification scenario was proposed to explain the origin of this endemic fauna. In this model, after an endorheic phase during the last glacial maximum, when the SFR flowed into an interior lake, the SFR made its way out to the ocean through the dunes, separating ancestral populations of several groups of lizards and snakes on opposite margins. Phylogenetic data from endemic species are only partially concordant with this model, and highlight the contribution of both local and regional events as drivers of diversification. Estimates of divergence times suggest that speciation was not simultaneous, and that some events occurred prior to Pleistocene. Species that are widespread in the SFR dune region lack genetic structure between the two margins, contrasting with the pattern observed for endemic, habitat-specialist lineages. Both Pliocene and Pleistocene events appear to have promoted diversification in SFR dune endemic reptiles, and ecological interactions may have contributed to species turnover. The SFR may have acted both as a vicariant barrier and as a boundary delimiting secondary contact, reinforcing ongoing processes of speciation.