Phylogeography of the tree lizard, Urosaurus ornatus: responses of populations to past climate change.

Isolation due to both geological barriers and range contractions during the Pleistocene glacial maxima has been an important cause of diversification of arid-adapted species in the North American deserts. Tree lizards, Urosaurus ornatus, are distributed across much of the southwestern arid regions and can tolerate a wide range of environments. Thus, they may have avoided large-scale shifts in distribution caused by Pleistocene climate change and any subsequent evolutionary impacts. Cytochrome b sequences were sampled from U. ornatus across the northern part of their range to test if current structure of these populations resulted from post-Pleistocene range expansion and habitat fragmentation, or prior geological isolation. Phylogenetic analyses found geographical structuring of populations consistent with a model of long-term geographical isolation corresponding to each of the desert regions. The two post-Pleistocene hypotheses were not well supported as estimated times of divergence predated the retreat of the last continental ice sheet. Populations in different regions were impacted by different processes. Southern populations of U. ornatus appear to have remained largely independent of more derived northern and eastern populations during Pleistocene climate change, while populations in regions containing more derived populations showed evidence of more recent range expansion (Colorado Plateau). As populations of U. ornatus attest to, the complex and dynamic history of the southwestern USA has left a deep-rooted and multifaceted imprint on genetic and phylogeographical structure of the species living there.