Geographic variation in Drosophila: From molecules to morphology and back

The examination of spatial variation can act as a substitute for temporal variation in population studies. Of particular use in the effort to understand the selective forces that govern spatial variation are the sibling species Drosophila melanogaster and D. simulans. Recent work at the level of both molecules and morphology has uncovered a great deal of spatial variation within and between these two species. Here we summarize these data, with reference to what they tell us about the history and nature of selection operating in these species.