Mitochondrial DNA variation among the Drosophila athabasca semispecies and Drosophila affinis

Morphology, allozymes, and levels of postreproductive isolation indicate that the semispecies of Drosophila athabasca are as recently diverged as typical populations within a Drosophila species. However, levels of behavioral isolation, divergence in male mating song, and divergence in X-chromosome inversions suggest a much more ancient divergence. In this article, we have examined mitochondrial DNA restriction site variation within and among the three semispecies. These data support the hypothesis that the three semispecies of Drosophila athabasca are at least as recently diverged as are typical populations within other species of Drosophila. Male mating song, behavioral isolation, and X-chromosome differences thus appear to have evolved very rapidly. In addition, hypothesized phylogenetic relationships of these three semispecies, based on our mtDNA data, are identical to those based on allozymes and chromosomal inversions. According to this phylogeny, recency of ancestry is not well correlated with levels of behavioral isolation, whereas patterns of sympatry and allopatry are. These patterns strongly implicate the action of selection in the rapid evolution of behavioral isolation and X-chromosome gene arrangement.