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Genetic analyses can facilitate effective and timely conservation and management actions. Arctic-migratory species in particular are in need of conservation genetic insights as they are experiencing substantial population pressures due to the impact of climate change (and other anthropogenic effects) on processes that affect their survival and reproduction. Therefore, identification of genetic mechanisms driving population divergence and variation in reproductive fitness in such species is critical. The goal of this thesis is to examine reproductive isolation among breeding populations of an Arctic-migratory passerine, the snow bunting (Plectrophenax nivalis) and determine factors that drive local adaptation and variation in reproductive success in this species. Using neutral and functional genetic markers, I show substantial population isolation among six globally distributed snow bunting breeding populations that is primarily driven by high levels of genetic drift and stabilizing selection, but with divergent selection acting at key functional loci. While there were no significant predictors of within-pair reproductive success, I identify key male quality traits such as body mass, testosterone levels, and breast plumage as important drivers of extra-pair reproductive success, which ultimately contribute to realized fitness in snow buntings. My work highlights the population-specific responses that reinforce the importance of genetic variability of individuals and their subsequent reproductive outcomes. The information contained in this thesis, combined with the methodological approaches, will help direct conservation efforts at the among- and within-population levels to maintain genetic diversity and adaptive potential as rapid environmental change continues to threaten Arctic-migratory species.