MOLECULAR ANALYSIS OF THE ALLELES OF ALCOHOL DEHYDROGENASE ALONG A CLINE IN DROSOPHILA MELANOGASTER. I. MAINE, NORTH CAROLINA, AND FLORIDA.

Clinal variation in natural populations is often assumed to be due to the operation of natural selection. However, for many clines there exist plausible neutralist explanations which suggest that aspects of population structure maintain differences among subpopulations for particular traits. We used a restriction-mapping technique to investigate the contributions of population subdivision and selection to the maintenance of the allozyme polymorphism at the alcohol dehydrogenase (Adh) locus of Drosophila melanogaster. Digestions of genomic DNAs from 270 lines of flies by seven enzymes reveal 15-20% of all possible nucleotide substitutions and virtually all of the insertion/deletion variation in a 2.7-kilobase region containing the Adh structural locus. Analysis of large samples from each of three populations along the east coast of the United States provides evidence of founder effects in the most northerly population. Although there are signs of population differentiation among the samples, similarities between two of the populations indicate that migration among populations is extensive and strengthen the argument that natural selection plays a role in maintaining the cline.