Evolution of the Drosophila obscura species group inferred from the Gpdh and Sod genes.

The Drosophila obscura group consists of several dozen Nearctic and Palearctic species, with a phylogeny that remains largely unresolved in spite of numerous morphological, cytogenetic, and molecular investigations. We have partially sequenced two genes, Gpdh (about 1000 bp) and Sod (about 700 bp) in 12-13 species and 1 subspecies in order to settle the issues. Difficulties in resolving the phylogeny emanate from the rapid sequence of successive radiations. Nevertheless, the following conclusions are warranted: (1) The Palearctic species include two monophyletic subgroups, subobscura (which may be the most ancient clade of the whole group) and obscura, plus two other species with unresolved phylogenetic positions, D. bifasciata and D. subsilvestris; (2) the Nearctic species form a monophyletic group consisting of two monophyletic sister clades, the affinis subgroup and the pseudoobscura subgroup. The Palearctic radiation may have resulted from adaptation to expanding temperate forests in the Old World. A second radiation occurred during the colonization of the deciduous forests of the New World by the descendants of a single lineage that soon split into the affinis and pseudoobscura subgroups.