Comparative phylogeography of some aridland bird species

We compared mitochondrial DNA sequences for six species distributed across the aridlands of North America to document phylogeographic patterns acid as sess levels of congruence. The Curve-billed Thrasher (Toxostoma curvirostre ) and Canyon Towhee (Pipilo fuscus) show genetic divisions between the Sono ran and Chihuahuan Deserts, whereas the Cactus Wren (Campylorhynchus brunne icapillus), Black-tailed Gnatcatcher (Polioptila melanura), and Verdin (Aur iparus flaviceps) do not. Most likely, species without phylogeographic stru cture only recently colonized their entire current range. Therefore, althou gh these species are today part of a widespread avifauna, species' distribu tions were historically different from today. In Baja California, the Cactu s Wren and the Verdin show phylogeographic breaks at 28 degrees -30 degrees N, consistent with a division previously described in the LeConte's Thrashe r (Toxostoma lecontei) and in some members of the herpetofauna. These genet ic divisions were likely caused by isolation resulting from a mid-peninsula r seaway that existed one million years ago. Hence, these species appear to have been broadly sympatric for at least one million years. In contrast, t he California Gnatcatcher (Polioptila californica) lacks such a phylogeogra phic division, and apparently only recently expanded into the northern part of its current range. Thus, not all species in Baja California have had si milar histories, although further sampling might reveal a general pattern. Comparative phylogeography therefore provides an indirect method of evaluat ing the long-term stability of faunas via assessment of levels of phylogeog raphic congruence, and can show whether particular species are likely to ha ve had a long period of co-association.