Spatial Structure in Drosophila buzzatii Populations: Simple and Directional Spatial Autocorrelation

The spatial structure of 12 allele frequencies was examined for 57 populations of the cactophilic fly Drosophila buzzatii from eastern Australia. Techniques include spatial-autocorrelation analysis and the newly developed directional spatial autocorrelation. Although 11 allele frequencies differ among localities, only 6 show spatial structure along one dimension. Directional correlograms support clines or major geographic trends in different directions for 5 allele frequencies. Spatial correlograms were also computed for genetic distances. The overall results-genetic heterogeneity among localities, weak correlation between allele-frequency surfaces, moderate spatial structure, and moderate parallelism of correlograms-permit elimination of genetic drift, selection against a single environmental gradient, or the effects of a single migrational event, and they suggest that selection operates on different spatial scales ranging from a continental one to a strictly local one. These results are interpreted in terms of the known history and biology of these organisms in Australia and are compared with spatial analyses of allele frequencies in other organisms.