Alan O. Bergland, Ray Tobler, Josefa Gonzalez, Paul Schmidt, Dmitri Petrov
Populations arrayed along broad latitudinal gradients often show patterns of clinal variation in phenotype and genotype. Such population differentiation can be generated and maintained by a combination of demographic events and adaptive evolutionary processes. Here, we investigate the evolutionary forces that generated and maintain clinal variation genome-wide among populations of Drosophila melanogaster sampled in North America and Australia. We contrast patterns of clinal variation in these continents with patterns of differentiation among ancestral European and African populations. We show that recently derived North America and Australia populations were likely founded by both European and African lineages and that this admixture event generated genome-wide patterns of parallel clinal variation. The pervasive effects of admixture meant that only a handful of loci could be attributed to the operation of spatially varying selection using an FST outlier approach. Our results provide novel insight into a well-studied system of clinal differentiation and provide a context for future studies seeking to identify loci contributing to local adaptation in D. melanogaster.