Date of Award
Doctor of Philosophy
Ecology and Evolutionary Biology
Gordon M. Burghardt
Christine R. B. Boake, Arthur C. Echternacht, Randall L. Small
Islands have long been of interest to biologists, as they are often home to unusual species or populations of species that are characterized by unique behavior, morphology, and gene pools. My research explores the mechanisms driving and maintaining phenotypic variation in the common gartersnake, Thamnophis sirtalis, populations of the Beaver Archipelago. Specifically, I focused on antipredator-related traits, foraging/feeding-related traits, and reproductive life-history traits, as all are known to vary with differences in predator composition and resource availability, which vary among the islands of the Beaver Archipelago and between the islands and the surrounding mainland. Since the exact origin of Beaver Archipelago fauna is debated, I began this study by using mtDNA sequences to determine the underlying genetics of the island populations and their most likely source. The results of this part of my dissertation indicate that the island populations are derived from both the lower and upper peninsula of Michigan and that the upper peninsula is more closely related to island populations than the lower peninsula populations, contradicting previous hypotheses that suggested the lower peninsula was the most likely point of origin for the islands and that the upper peninsular populations played little or no role in the system. This knowledge proved to be invaluable in understanding observed variation in phenotypic traits, all of which appear to be the result of some combination of their underlying genetics, as related to the mtDNA data, and plastic responses to selection with little if any evolutionary change resulting from selection. In terms of antipredator-related traits, populations that occur with fewer predators appear to be less reactive than those that occur with a greater number of predators as the result of plasticity, although there was no clearcut island/mainland trend. Ventral scale counts for island populations correspond best with the underlying genetics of the populations resulting from historical/colonization processes and do not appear to have changed as the result of current predator pressures. In terms of feeding/foraging-related traits, geographic variation in the head morphology of island snakes appears to be primarily the result of phenotypic plasticity with some evidence of genetic influence from historical/colonization processes. Behavioral responses to prey chemical extracts varied little between populations, with observed differences potentially being due to evolutionary change resulting from current selection pressures. Finally, reproductive life-history traits also appear to be mostly the result of underlying genetics, but some influence of plasticity and microevolution may also play a role. Combined, the genetic and phenotypic traits studied provide researchers with one of the more complete studies of variation in the traits of insular faunas to date and should provide a framework for future studies centered in island systems.
Playk, John S. Jr., "Historical Processes, Evolutionary Change, and Phenotypic Plasticity: Geographic Variation in Behavior, Morphology, and Life-History Traits of Common Gartersnake, Thamnophis sirtalis, Populations. " PhD diss., University of Tennessee, 2006.