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 morph ological characters in Drosophila melanogaster. Isofemale lines were e stablished from flies collected along a transect extending from Winnip eg, Manitoba (Canada) to Tampa Bay, Florida (U.S.A.). Offspring from d ifferent lines within each location were then cultured under standardi zed conditions and used to examine phenotypic variation in seven morph ological characters along the dine. In addition, allozyme variation at seven polymorphic loci was examined for the same set of clinal popula tions. Scutellum length and wing length show the strongest clinal tren ds. Clinal variation is nonmonotonic, with larger flies in the middle latitudes and smaller flies in the north and south. This result contra sts with other studies which have shown monotonic dines. Patterns of p opulation subdivision were different for the different characters. Thi s implies that there are different selective forces acting on the diff erent morphological characters. Based on a comparison of morphological and molecular population subdivision for adjacent populations it is i nferred that natural selection is operating to maintain a high level o f population subdivision for wing width and the first principal compon ent between one of the sets of populations. A combined approach using molecules and morphology may provide an alternative to retrospective s election analysis for detecting selection in nature.