The phylogenetic relationships of species in the Drosophila obscura species group have been reviewed by O’Grady (1999). The obscura group was initially divided into two lineages by Sturtevant (1942). The affinis subgroup consisted exclusively of New World species, with the obscura subgroup containing species found in both Old and New Worlds. In an early morphological study, Buzzati-Traverso and Scossiroli (1955) concluded that, within the “traditional” obscura subgroup (sensu Sturtevant 1942), there were two distinct lineages of Nearctic species and several lineages of Palearctic species. The obscura group is now divided into five subgroups, two in the New World (affinis and pseudoobscura) and three in the Old World (obscura, subobscura, and microlabis). The affinis and pseudoobscura species groups are sister taxa. The affinities of the Old World lineages remain somewhat unclear, although they are considered basal to the New World forms.
(excerpted from Markow and O’Grady, 2006, Chapter 1)
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