Holbrookia maculata within the Chihuahuan Desert

Population subdivision is a function of local interactions between the environment and an individual population. Therefore, climatic and local geologic events may have stronger effects on a species genetic structure than more global events such as mountain building and continental drift. Holbrookia maculata is a small large-ranging lizard species that lives throughout Midwest North America. Within the Chihuahuan Desert this species is locally abundant, but lives in pockets throughout the desert. Hypotheses for the Chihuahuan Desert have suggested that a single area south of the Rio Conchos, Mexico was used by xerically adapted species as a refugium. Alternatively, it has been hypothesized that a large Pleistocene lake which formed at the end of the Pliocene/beginning of the Pleistocene throughout the Chihuahuan Desert, which may suggest more that one refugia for the Chihuahuan Desert during the Pleistocene. The formation of this lake was a result of local climatic shifts within the Chihuahuan Desert that increased the Upper Rio Grande effluent and the original basin-shaped topology of the central Chihuahuan Desert. Preliminary mtDNA data suggest that multiple refugia were formed in late Pliocene/ early Pleistocene and persisted to the Holocene. We are in the process of collecting nDNA intron regions to further test this hypothesis.

North American sand lizard diversification

The sand lizards genera Holbrookia, Cophosaurus, Callisaurus and Uma are distributed throughout the southwest and hypotheses concerning their early divergence has been contentious. Understanding these early divergences among sand lizards genera and between phylogeographically distinct clades within species may provide insight into the ecological and vicariant processes that impacted the southwest during its early development in the Miocene and Pliocene. All manner of lineage relationships have been considered for Holbrookia, Cophosaurus, Callisaurus and Uma, but Uma has historically been considered the basal lineage to all other sand lizards. To identify these relationships multiple genetic markers will be used, to include mtDNA and several nuclear markers. Preliminary results from sequence data suggest the sand lizards underwent very rapid diversification during the early part of its history and that ecological speciation may have played a role in the diversification of Holbrookia ancestors and Cophosaurus. In addition, there seems to be high levels of ancestral polymorphism both within each of the genera and among species within each of the genera.

Using global climate models to develop a priori predictions for population genetic studies and the designation of conservation areas.

The recent introduction of climate models defining a species fundamental environmental niche has been used to explain diversification within a several tropical species groups. Many of these studies have shown that genetic diversification was a result of allopatric fragmentation due to an interaction between climatological shifts and the inability of a species to use new environments. These models have been used in areas where factors other than precipitation and/or temperature have influenced species distribution and diversification. Plants and animals that exist in the desert southwest are adapted to limited amounts of precipitation and an temperatures in excess of 50 degrees C. Therefore, these species represent a better test of how well these models can predict the geographic range of a species. Secondly, if these models are appropriate for describing species ranges specific hypotheses concerning the population genetic structure of southwest species can be identified a priori and tested with phylogeographic data. A better model for the diversification of southwestern species may be the interaction between local physiographic and climatic processes and a species’ life history. This research is multidisciplinary, integrating phylogeographic, ecological and GIS techniques to discern the mechanisms involved in North American southwest species diversification.