Molecules versus morphology: the detection of selection acting on morphological characters along a cline in Drosophila melanogaster

This work examines the nature of north-south clinal variation in morphological characters in Drosophila melanogaster. Isofemale lines were established from flies collected along a transect extending from Winnipeg, Manitoba (Canada) to Tampa Bay, Florida (U.S.A.). Offspring from different lines within each location were then cultured under standardized conditions and used to examine phenotypic variation in seven morphological characters along the cline. In addition, allozyme variation at seven polymorphic loci was examined for the same set of clinal populations. Scutellum length and wing length show the strongest clinal trends. Clinal variation is nonmonotonic, with larger flies in the middle latitudes and smaller flies in the north and south. This result contrasts with other studies which have shown monotonic clines. Patterns of population subdivision were different for the different characters. This implies that there are different selective forces acting on the different morphological characters. Based on a comparison of morphological and molecular population subdivision for adjacent populations it is inferred that natural selection is operating to maintain a high level of population subdivision for wing width and the first principal component between one of the sets of populations. A combined approach using molecules and morphology may provide an alternative to retrospective selection analysis for detecting selection in nature.