The potential for geographic range shifts of species is a pressing issue in ecology given the rapid rate of anthropogenic climate change. The shifts will change total species richness and biodiversity patterns because species likely differ in their capacity to shift under climate change. This concern has sparked efforts to project changes in species' geographic distributions. Several of the underlying assumptions of the ecological theories driving these projections, however, have not been rigorously tested. Current models of species' ranges assume uniformity with respect to climatic impacts on fitness for all individuals of a species, which ignores local selection and historical genetic differences. Following this, these models assume that warmer temperature at the poleward edge of a species' range will increase fitness ('peripheral enhancement'), causing population increases and greater poleward colonization. My dissertation examines this assumption by using two butterfly species, Erynnis propertius and Papillo zelicaon, that co-occur and have contrasting levels of host specialization and dispersal ability. The aim of my research is to determine if populations are uniform or differentiated with respect to their responses to both temperature and host plant. I used a series of common garden experiments in the field and lab where I variede climate and host plant to see if locally adapted forms within species are present. I did not find evidence of local adaption in P. zelicaon, but populations across the species' range performed poorly temperature altered growth and survivorship in this species. Growth and survivorship of E. propertius larvae increased in warmer conditions. However, local adaptation during the overwintering period counteracted the increases found during the growing period. Further, southern population of E. propertius are locally adapted to their natal host plants, so colonization poleward may be further limited. This more nuanced consideration of species could lead to different expectations for the biological consequences of climate change if species can not shift their ranges as previously projected. We can not properly mitigate the effects of climate change on biodiversity until our projection methods capture realistic dynamics of species' ranges.