MOLECULAR SYSTEMATICS OF SHORT-HORNED LIZARDS - BIOGEOGRAPHY AND TAXONOMY OF A WIDESPREAD SPECIES COMPLEX

We surveyed mitochondrial DNA (mtDNA) sequence variation in short-home d lizards (Phrynosoma douglasi) from throughout western North America and used these data to estimate an intraspecific phylogeny and to asse ss biogeographic scenarios underlying the geographic structure of line ages in this species. We sequenced 783 base pairs from 38 populations of P. douglasi and three putative outgroups (P. ditmarsi, P. orbicular e, P. platyrhinos). We detected high levels of nucleotide variation am ong populations and a spatial distribution of mtDNA lineages compatibl e with major geographic regions. The phylogenetic hypotheses best supp orted by the data suggest that P. douglasi, as currently described, is paraphyletic with respect to P. ditmarsi. Populations of P. douglasi from the Pacific Northwest (ID,CA, OR, WA) form a monophyletic group t hat is sister to the subsequent radiation of P. ditmarsi and other P. douglasi clades. These results suggest that divergences within this wi despread species are fairly old. We focused on the genetic structure o f populations of P. douglasi from a geographic perspective and interpr eted the inh aspecific phylogeny in light of geologic and climatic cha nges in western North America during the last 20 million years. The ge nerally high levels of genetic variation found in these population com parisons are in accord with high levels of morphological variation in this species group; however, only in the Pacific Northwest region is t here spatial congruence between these phylogenetic results and subspec ific ranges based on previous morphological studies. We compared the e volutionary units delineated in this study with previously described s ubspecies of P. douglasi and evaluated the support (from morphology an d mtDNA) for each population lineage in the phylogeny and the implicat ions for the taxonomy of this group.