Research

(1) Speciation and the Evolution of Reproductive Isolation
Speciation is one of the most important processes in evolution, but the mechanisms underlying reproductive isolation remain poorly understood in natural populations.  One proposed mechanism of speciation is divergent sexual selection, whereby divergence in female preferences and male signals results in behavioral isolation.  Despite this hypothesis’ appeal, evidence for it has been inconclusive.  My colleagues at the University of Texas and I recently found evidence supporting this hypothesis in the Amazonian frog Physalaemus petersi.  Using a combination of molecular genetic data, phylogenetic and coalescent analyses, and behavioral experiments, we found that sexual selection has promoted divergence in male mating calls and female preferences, resulting in strong behavioral isolation and initiating speciation (Proc R Soc B Biol Sci, 2007, 274:399; featured by Nature, 2006, 444:4).  These results provide some of the best evidence to date that sexual selection can cause speciation in the wild.  The next steps will involve investigating:  (1) whether other isolating mechanisms in addition to sexual selection, such as ecological divergence or postzygotic isolation, are involved in generating reproductive isolation; and (2) the genetic basis of behavioral isolation.  Physalaemus petersi should be an excellent species for addressing these questions because the methodology for assaying female preferences with phonotaxis experiments and for breeding and rearing frogs in the lab has already been developed.  I have also already developed genetic resources for P. petersi, including several microsatellite libraries and a molecular phylogeny.

(2) Phylogeography and Molecular Systematics
The processes that generate the high species richness of the Amazon Basin have fascinated evolutionary biologists and naturalists since the nineteenth century.  We used a phylogeographic approach to test two biogeographic hypotheses for diversification in the frog Physalaemus petersi across the Amazon Basin, the riverine barrier and the elevational gradients hypotheses.  We were able to reject the elevational gradients hypothesis, but found support for the riverine barrier hypothesis across the Río Madre de Dios in southeastern Peru (Mol Phylogenet Evol, 2007, 44:825).  We also found evidence for divergence over small spatial scales which has important implications for the latitudinal diversity gradient. This was one of only a handful of phylogeographic studies to test a priori hypotheses for diversification in the Amazon Basin. I plan on using a comparative approach to test the generality of our findings for taxa with different life histories.
     I strongly believe in the importance of general collections of tissues, specimens, natural history data, and call recordings.  Thanks to our collections, my colleagues and I have discovered 7 new frog species in Ecuador and Peru based on a combination of molecular, morphological, and call data (e.g., Zool J Linn Soc, 2006, 147:489).  Systematic and museum work is an essential service, and forms the foundation for future evolutionary and ecological research as well as conservation planning.

(3) Effects of Landscape Structure on Genetic Variation and Evolutionary Trajectories
Landscape genetics is an emerging field that integrates population genetics and landscape ecology to understand how landscape features such as mountains, rivers, and environmental heterogeneity affect spatial patterns of genetic variation.  The field is related to phylogeography, but focuses on evolutionary processes over smaller spatial and temporal scales. Understanding how landscapes mold genetic variation is also important in conservation for defining demographically independent management units and understanding movement patterns.  In a microsatellite analysis of 790 Columbia spotted frogs (Rana luteiventris) from three regions in Montana and Idaho, we found that mountain ridges and elevational gradients strongly restricted gene flow (Mol Ecol, 2005, 14:483).  We also estimated dispersal in two replicate basins using an intensive capture-recapture study of over 10,000 uniquely marked frogs that confirmed high dispersal rates among low-elevation populations (Biol Lett, 2005, 1:13).  I am now expanding this research to investigate the effects of these same landscape features on patterns of adaptive genetic variation, particularly across steep elevational gradients over which Columbia spotted frogs and other species experience strikingly divergent selection pressures over small spatial scales.

(4) Conservation of Amphibians and Other Threatened Vertebrates
I find that the boundaries between “basic” evolutionary ecology and “applied” conservation research are often blurry.  For example, motivated by amphibian decline research that only examined early life history stages, we used ecological sensitivity analysis to test the relative importance of different vital rates (birth and death rates) for amphibian population growth, but found that these parameters were often unavailable even for the best studied amphibians (Conserv Biol, 2002, 16:728).  This inspired me to begin a large-scale study of the basic evolutionary ecology and natural history of amphibians in Ecuador; this project is now in its seventh year.  I am also working with colleagues at Oregon State to test the importance of local adaptation, phenotypic plasticity, and emerging disease in bullfrog invasions.
     New genomic analyses can also provide critical information about the historical demography and connectivity of threatened species that is difficult to acquire from field data alone.  My colleagues at the USGS Forest and Rangeland Ecosystem Science Center and I recently finished a series of papers on the population genomics of the iconic northern spotted owl (Strix occidentalis caurina) to identify spotted owl-barred owl hybrids; test for genetic bottlenecks; define subspecies borders and patterns of introgression; and understand the effects of landscape features on gene flow (Evol Appl, 2008, in press; Mol Ecol Notes, 2007, 7:284; 2 manuscripts in review or revision).  We also recently completed a project on conservation genetics of the threatened Pacific Coast population of snowy plovers (Charadrius alexandrinus; Conserv Gen, 2007, 8:1287), and are now finishing a study aimed at defining species boundaries and evolutionarily significant units in Oregon spotted frogs (Rana pretiosa) and Columbia spotted frogs (Rana luteiventris), both candidates for listing under the U.S. Endangered Species Act.