Natural selection sculpts phenotypes to enable adaptation to local conditions. Biologists have long been interested in the phenotypic and molecular response to selection, but we still know little about the genetic basis of adaptation. With continuing improvements in molecular technologies, we now have the tools to investigate the genes that underlie ecologically important traits. Assessing the geographic distribution of allelic variation at these loci can point to the agents of selection, if different alleles are advantageous in contrasting environmental conditions [1]–[3]. In theory, predictable patterns of clinal variation in allele frequencies can arise from divergent selection across environmental gradients [1], although demographic factors may complicate identification of adaptive changes [4], [5].