Genetic and ecological analysis of a species' range

The geographic ranges of species' are ultimately limited by the inability of peripheral populations to adapt to local conditions and continue range expansion. This thesis examines the genetic mechanisms that maintain a species range, by exploring range-wide patterns of genetic and adaptive variation in the broadly distributed lizard, the five-lined skink, Plestidon fasciatus. First, I used cytochrome b mitochondrial DNA sequence to describe the phylogeographic patterns of the species. This knowledge provides a historical context in which further observations of genetic and ecological variation can be embedded.; To test two evolutionary hypotheses for range limits, I genotyped 649 individuals from across the range of P. fasciatus for six microsatellite markers. First, I tested the hypothesis that range limits are limited by reduced levels of genetic diversity in geographically peripheral populations. Results showed that peripheral populations that are not bordered by an obvious physical boundary (e.g. the ocean) have reduced levels of intrapopulation genetic diversity relative to populations located in the centre of the species' range and peripheral populations that are bordered by an obvious physical boundary (e.g. the Atlantic Ocean). Thus, range limits in P. fasciatus may be limited by a lack of genetic diversity in peripheral populations, insofar as neutral microsatellite markers reflect overall genetic diversity. I also tested the hypothesis that peripheral populations receive asymmetrical gene flow from populations in the centre of the range, and that this gene flow may "swamp" local adaptation in peripheral populations with potentially maladapted genes, thereby precluding range expansion. Results did not support this hypothesis, but rather showed that asymmetrical gene flow occurs from the periphery to the centre of the species' range.; Finally, I investigated range-wide variation in an adaptive trait across the range of P. fasciatus. Male body size and male-biased sexual size dimorphism increased with latitude across the range, and did not correspond to range-wide genetic patterns. By identifying patterns of genetic structuring and adaptive variation within a species, we can better understand the evolutionary mechanisms involved in the maintenance of range limits. This thesis describes such patterns of intraspecific variation in the five-lined skink, and highlights the need for continued empirical research into evolutionary hypotheses for range limits.