DNA Sequence Variation at the period Locus Within and Among Species of the Drosophila melanogaster Complex Richard

A 1.9-kilobase region of the period locus was sequenced in six individuals of Drosophila melanogaster and from six individuals of each of three sibling species: Drosophila simulans, Drosophila sechellia and Drosophila mauritiana. Extensive genealogical analysis of 174 polymorphic sites reveals a complex history. It appears that D. simulans, as a large population still segregating very old lineages, gave rise to the island species D. mauritiana and D. sechellia. Rather than considering these speciation events as having produced “sister” taxa, it seems more appropriate to consider D. simulans a parent species to D. sechellia and D. mauritiana. The order, in time, of these two phylogenetic events remains unclear. D. mauritiana supports a large number of polymorphisms, many of which are shared with D. simulans, and so appears to have begun and persisted as a large population. In contrast, D. sechellia has very little variation and seems to have experienced a severe population bottleneck. Alternatively, the low variation in D. sechellia could be due to recent directional selection and genetic hitchhiking at or near the per locus. E XPERIMENTAL approaches to the study of evolution are traditionally divided into those that address microevolutionary topics (i.e., forces that determine patterns of genetic variation within species) and those that deal with macroevolutionary issues (i.e., speciation processes and other determinants of phylogenetic patterns). Within recently formed species, however, DNA sequence variation may reflect not only processes acting since divergence, but also processes that acted prior to and during speciation. For example, if new species arise via founder events of very few individuals (e.g., MAYR 1954), rapid genetic drift is expected to lead to low levels of neutral or nearly neutral genetic variation. Alternatively, if speciation events are not preferentially associated with times of very small population size, then recently diverged species may share many polymorphisms. T h e species of the Drosophila melanogaster complex a r e closely related and afford an excellent opportunity for genetic studies of recent speciation events. Two of the species, D. melanogaster and Drosophila simulans, are cosmopolitan, while the other two, Drosophila mauritiana and Drosophila sechellia, are endemic to single island groups (LACHAISE et al. 1988). Phylogenetic studies on a wide array of morphological and genetic characters have revealed only that D. melanogaster is a sister taxon to the other species. D. simulans, D. sechellia and D. mauritiana are very similar to one E:MBL/GenBank Data Libraries under the accession numbers L07811T h e sequence data presented in this article have been submitted to the