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The origin and maintenance of genetic variation in small populations : coastal cutthroat trout (Oncorhynchus clarkii clarki) as a model system Costello, Allan

Abstract

Genetic variation is widely recognized as a major component of species biodiversity, contributing greatly to the maintenance of adaptive potential in populations and their evolutionary response to change. Despite years of study and their obvious importance to evolution and conservation biology, however, the biological processes responsible for the maintenance of genetic variation in the wild are still poorly understood. Throughout this thesis, I have attempted to describe the historical and contemporary forces contributing to the origins and maintenance of genetic variation in coastal cutthroat trout (Oncorhynchus clarkii clarki), a salmonid characterized by small population sizes and one that is of growing conservation concern in western North America. I found evidence for the existence of three, and possibly four glacial refugia for coastal cutthroat trout and a complex pattern of secondary contact between refugial groups in the northern portion of their range. Over subregional scales, both long term and contemporary estimates of gene flow between adjacent populations appear on the order of one migrant per generation, the theoretically optimal level to reduce the loss of genetic diversity in small populations. I also found evidence that interspecific gene flow between coastal cutthroat and rainbow trout (O. mykiss) may be widespread in populations throughout the study area. Finally, through an intensive demographic and genetic study of a single representative population, I found that the breeding system of coastal cutthroat trout may itself compensate for the small, fluctuating number of spawners typical of the subspecies. Mating patterns were found to be quite complex ranging from monogamy to polygynandry, with mating occurring between different cohorts and life history types. My results go further, suggesting that genetic compensation, in the form of an increase in levels of polyandry and a reduction in the variance associated with female reproductive success, partially countered fluctuations in population size. I discuss the key findings of each chapter as they pertain to the maintenance of genetic variation in small populations, the future conservation of coastal cutthroat trout, and, more generally, in terms of applied salmonid management.

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