Monday, December 10, 2012

Origins of the North American Desert Fauna

The Rosy Boa, Lichanura trivirgata.
Phylogeographic studies of the southwestern deserts of North America have suggested diverse historical processes, with two hypotheses posed as most important for shaping genetic structure: climate fluctuations in the Pleistocene and pre-Pleistocene vicariance. Support for the climate fluctuation hypotheses emerged from a historical understanding of Pleistocene vegetation patterns within North American deserts and from the perception that desert floras were relatively young. Environmental reconstructions, based largely on analysis of packrat middens, have suggested that present distributions of the Mojave and northern Sonoran Desert biomes were largely comprised of mesic woodlands during the Last Glacial Maximum (LGM: 18,000−20,000 years ago. Furthermore, these deserts only recently reached their current extents in the present interglacial period Presumably, recent biotic responses of desert habitats to climatic change have also occurred during the repeated glacial/interglacial cycling throughout the Pleistocene (especially over the last 700,000 years. As such, several studies have posited that arid-adapted species spanning multiple deserts would have been limited to isolated refugia within the Mojave and Sonoran Deserts during glacial periods, assuming that niche requirements remained the same over time. Repeated displacement and fragmentation of habitats during climatic fluctuations should have resulted in genetic lineage diversification between refugial populations. Similarly, evidence of demographic expansion within lineages should coincide with the end of the LGM, as desert habitats expanded to maximum distributions.

Wood et al. (2012) investigate the phylogeographic history of the Mojave and Sonoran Deserts using a comparative approach. They examining spatial and temporal patterns of DNA sequence variation in 12 species, and evaluate the concordance of phylogeographic breaks and lineage diversification for both Pleistocene climate fluctuation hypotheses and pre-Pleistocene vicariance hypotheses. And, they used visualization methods to assess the spatial pattern of genetic diversity and divergence across species and identify regional evolutionary hotspots.

In seven of the 12 species, lineage divergence substantially predated the Pleistocene. Historical population expansion was found in eight species, but expansion events postdated the Last Glacial Maximum in only four lineages. For all species assessed, six hotspots of high genetic divergence and diversity were concentrated in the Colorado Desert, along the Colorado River and in the Mojave/Sonoran ecotone. At least some proportion of the land within each recovered hotspot was categorized as protected, yet four of the six also overlapped with major areas of human development.

Eight of the 12 species used were members of the herpetofauna (the red spotted toad, Anaxyrus punctatus the shovel-nosed snake, Chionactus occipitalis; the collared lizard, Crotaphytus bicinctores; the sidewinder, Crotalus cerastes; the rosy boa, Licanura trivirgata; and the horned lizard, Phryanosoma platyrhinos; the spiny lizard, Scleroporus magister; and the night lizard, Xantusia vigilis).

Most of the species examined in this study diversified into distinct Mojave and Sonoran lineages prior to the LGM – supporting the older diversification hypotheses. Several evolutionary hotspots were recovered but are not strategically paired with areas of protected land. Long-term preservation of species-level biodiversity would entail selecting areas for protection in Mojave and Sonoran deeserts to retain divergent genetic diversity and ensure connectedness across environmental gradients.

The entire article is available on-line.

1 comment:

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