Nonclinality of molecular variation implicates selection in maintaining a morphological cline of Drosophila melanogaster

One general approach for assessing whether phenotypic variation is due to s election is to test its correlation with presumably neutral molecular varia tion. Neutral variation is determined by population history, the most likel y alternative explanation of spatial genetic structure, whereas phenotypic variation may be influenced by the spatial pattern or selection pressure. S everal methods for comparing the spatial apportionment of molecular and mor phological variation have been used. Here, we present an analysis of varian ce framework that compares the magnitudes of latitudinal effects for molecu lar and morphological variation along a body size dine in Australian Drosop hila populations. Explicit incorporation of the relevant. environmental gra dient can result in a simple and powerful test of selection. For the Austra lian dine, our analysis provides strong internal evidence that the dine is due to selection.