| Summary: | The distributions of most Holarctic freshwater fish species were severely altered and restricted during the many glaciation events that have occurred throughout the Pleistocene. Isolation of groups of fish into distinct glacial refugia provided the opportunity for genetic divergence during these periods of allopatry through genetic drift and novel selection pressures. In this thesis, I examined the signature of such isolation and postglacial range expansion in the Arctic grayling (Thymallus arcticus) by assaying mitochondrial and microsatellite (nuclear) DNA variation throughout the species' range in North America. I also examined local population structure in the Peace River, British Columbia, because local demographics are integral to a species' phylogeographic structure. I found a dramatic decline in genetic diversity from Alaska to the southeast, which suggests Arctic grayling survived the last ice age in Beringia then bottlenecks and founder events reduced diversity during southward postglacial range expansion. Genetic similarities among regions suggest that Arctic grayling survived glaciation in three refugia north of the ice sheets and in one region south of the ice sheets. A north Beringian lineage dispersed south during the Wisconsinan glaciation and founded populations in an upper Missouri River glacial refuge. These upper Missouri grayling dispersed north postglacially, and founded populations in Saskatchewan and eastern British Columbia. A south Beringian lineage dispersed south from the Yukon River Valley as far as the Peace and Stikine rivers in British Columbia. A third lineage from the Nahanni Valley in the Northwest Territories was more locally distributed in the Mackenzie drainage between Great Slave Lake and the lower Liard River. Population subdivision in the Peace River strongly suggests that Arctic grayling home to their natal stream to spawn. Such local population subdivision and low genetic diversity throughout the species range suggest that Arctic grayling habitat is partitioned among small isolated effective population sizes. Genetic diversity is distributed among lineages on a large geographic scale, and among populations on a local geographic scale. Consequently, to preserve the evolutionary potential of Arctic grayling, several populations within a watershed, several watersheds within a lineage, and several lineages within their geographic range must be prioritized for conservation. Science, Faculty of Zoology, Department of Graduate
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