Environmental conditions are among the main drivers of variable natural selection on phenotypic traits. Often, this variation in selection results in allele frequency shifts in the genes underlying these traits. Understanding the genetic underpinnings of environmental adaptation helps address fundamental questions in evolutionary biology, such as how ecological divergence occurs, how natural selection acts on standing variation, and how species will adapt to new environments. My doctoral dissertation combined museum collections, remote sensing data, field research, and genomic tools in a non-model system to test how selective landscapes form in natural populations in response to environmental change.More specifically, I explored environmental adaptation from both a phenotypic and genotypic perspective across space and time in the Black-throated Blue Warbler (Setophaga caerulescens). In chapter 1, I examined phenotypic variation of plumage across the range of this species to test for associations between phenotypic, genotypic, and environmental variation. I found that black plumage varies with Gloger’s Rule: individuals are darker in warmer, wetter conditions, indicating this trait is shaped by local adaptation. I found that nano-scale differences in the number of melanin granules in feathers drive differences in appearance of the mantle of male Black-throated Blue Warblers. Finally, I found evidence that variation in plumage color is driven by many genes of small effect. In chapter 2, I expanded on the connection between genotype, phenotype, and the environment with a Genotype-Environment Association (GEA) analysis to identify loci putatively involved in local adaptation to the environment. I found an overrepresentation of melanogenesis genes associated with environmental variables, including the same genes involved in plumage variation. This suggests that Black-throated Blue Warblers have experienced selection on traits related to melanin production that serves an adaptive function. In chapter 3, I examined the efficacy of predictive tools for forecasting how species will respond to environmental change. I directly examined the change in allele frequencies across a 30-year time period and found that measures of genetic diversity are the strongest predictors of population-level change. Adaptive Potential (heterozygosity at climate-linked loci) performed best at predicting how allele frequencies change through time, such that populations with higher heterozygosity at climate-linked loci were most likely to undergo the greatest allele frequency shifts, with some evidence that this is driven by selection on these loci. Evolutionary Potential (heterozygosity at all loci) was the best predictor of population trends, such that populations with the highest starting heterozygosity were most likely to have increasing abundance over the 30-year period. Collectively this work establishes that standing genetic variation, particularly at melanogenesis and climate-linked loci, is a key substrate for adaptation in natural populations, with direct implications for how we assess the vulnerability and resilience of species facing rapid environmental change.