A Comprehensive Study of Genic Variation in Natural Populations of Drosophila melanogaster. I. Estimates of Gene Flow from Rare Alleles.

In order to assess the evolutionary significance of molecular variation in natural populations of Drosophila melanogaster, we have started a comprehensive genetic variation study program employing a relatively large number of gene-protein loci and an array of populations obtained from various geographic locations throughout the world. In this first report we provide estimates of gene flow based on the spatial distributions of rare alleles at 117 gene loci in 15 worldwide populations of D. melanogaster . Estimates of Nm (number of migrants exchanged per generation among populations) range from 1.09 in East-Asian populations (Taiwan, Vietnam and Australia) to 2.66 in West-Coast populations of North America. These estimates, among geographic populations separated by hundreds or even thousands of miles, suggest that gene flow among neighboring populations of D. melanogaster is quite extensive. This means that, for selectively neutral genes, we should expect little differentiation among neighboring populations. A survey of eight West-Coast populations of D. melanogaster (geographically comparable to Drosophila pseudoobscura) showed that in spite of extensive gene flow, populations of D. melanogaster show much more geographic differentiation than comparable populations of D. pseudoobscura. From this we conclude that migration in combination with natural selection rather than migration alone is responsible for the geographic uniformity of molecular polymorphisms in D. pseudoobscura.