Systematics, introgression, and adaptation in Western Rattlesnakes: a model system for studying gene flow, selection, and speciation

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Despite substantial research, the roles of natural selection in the formation of species and in preventing hybridization between species remain poorly understood. In a rapidly changing world, there is an urgent need to understand the importance of these processes in species formation and the impact of these processes on how scientists identify and name species. This research program will study genetic, venom protein, and anatomical data to test how natural selection shapes and maintains species, and it will use these results to test several approaches for appropriately identifying species in nature. The researchers will focus on a widespread and medically important group of rattlesnake species (Western Rattlesnakes and their close relatives) as a model system to study species formation. Many previous studies have disagreed about how many species should be recognized within this group of snakes, and different populations can produce diverse symptoms from snake bite due to differences in venom biochemistry. This study will resolve these issues by developing a new system for naming species, provide new insight into the process of species formation, develop new methods for identifying species, and refine the appropriate medical treatment of snakebite in North America. The proposed research includes methodological and theoretical scientific innovation, undergraduate and graduate student training, establishment of collaborative networks, and public outreach. Public outreach will be conducted at the Dallas and Denver Zoos, thereby reaching millions of public visitors per year.The overarching goals of this research program are to understand the interaction of admixture and selection in speciation, and to leverage an empirical system to test how these processes may influence coalescent-based species delimitation methods. This research program will focus on the Western Rattlesnake species group (Crotalus viridis species complex and its sister taxon, C. scutulatus; collectively Cvos hereafter) as a study system. This recently diverged species complex has offered major challenges for systematics, yet provides an ideal model system for studying speciation and species delimitation. This integrated study will combine genomic and phenotypic data to delimit species, inform species delimitation approaches, and provide new genome-scale insight into the process of speciation. This research program will also test hypotheses about the repeatability of patterns selection in the processes of speciation and resistance to gene flow and hybridization. The specific aims of this proposal are to: (1) integrate genome-wide sampling and ecological niche modeling to infer population structure and test hypotheses of gene flow among lineages of Cvos; (2) analyze selection in contact zones between putative taxa by integrating niche modeling, genotypic, and phenotypic data to test hypotheses regarding divergence and selection in the speciation process; and (3) develop and test new approaches for delimiting species that more appropriately handle admixture and selection, and apply these methods to test taxonomic hypotheses for the Cvos complex. Successful completion of the project will provide important data to understanding the role of admixture and selection in speciation and vastly improve our analytical methods for delimiting species in the presence of interbreeding across organismal groups. Additionally, an added benefit of the project is the information that will be obtained regarding venom complexity and evolution, which could contribute to better treatment of envenomation from snake bites.