GENETIC DISTANCE AND EVOLUTIONARY RELATIONSHIPS IN THE DROSOPHILA OBSCURA GROUP.

Abstract

The Drosophila obscura species group extends throughout the temperate zone of the Northern Hemisphere. Within it two subgroups are distinguished, affinis and obscura, with different numbers of representatives in the Old and New Worlds. Eight species of the affinis subgroup have been found in the Nearctic (D. affinis, D. algonquin, D. athabasca, D. azteca, D. narraganset, D. seminole, D. tolteca and D. dobzhanskii) and only one in the Palearctic (D. helvetica), while in the obscura subgroup 12 species are now known in the Old World (D. alpina, D. ambigua, D. bifasciata, D. eskoi, D. guanche, D. imaii, D. madeirensis, D. obscura, D. seguyi, D. subobscura, D. subsilvestris, and D. tristis) as compared with only five in the New World (D. frolovae, D. lowei, D. miranda, D. persimilis and D. pseudoobscura). This group, especially some species of the obscura subgroup, is one of the most comprehensively studied at all levels. Morphological, cytological, ethological and molecular, and phylogenetic relationships have been proposed within several clusters of species within the group (BuzzatiTraverso and Scossiroli, 1952; Clayton and Wheeler, 1975; Dobzhansky and Powell, 1975; Throckmorton, 1975 among others). However, a phylogeny including all or at least the great majority of the species of the group was not available until Lakovaara et al. (1972, 1976) constructed two phylogenetic hypotheses, first for the obscura subgroup and subsequently for the whole group. Based on genetic distances between species, these trees were constructed using enzymatic and other protein variants as taxonomic characters and assuming that the overall rate of evolutionary divergence is approximately homogeneous over all phyletic lines. Their results are, in general, in agreement with previous studies: the European species of the obscura subgroup were shown to be more closely related to each other than to their relatives in the New World, and similarly the species of the affinis subgroup of the New World were shown to be more closely related to each other than to those of the obscura subgroup, both of the Old as well as the New World. However, using the data of Lakovaara et al. (1972) but using methods which do not assume constant evolutionary rates for the species, Farris (1974) constructed a tree in which one of the Palearctic species, D. bifasciata, appears to be more closely related to the New World species of the subgroup than to those of the Old World. More recently, Marinkovic et al. (1978) established phylogenetic relationships among some of the American and European species of the obscura subgroup and found a European species, D. subobscura, to be apparently more closely related to American species of the subgroup than to other European species. It is of interest, therefore, to determine if these proposed close relationships between some American and European species of the obscura subgroup are consistent since it would greatly contribute to our understanding of the evolutionary history of the subgroup. The present work is an attempt to establish the relationships between a Nearctic species, D. pseudoobscura, and four Palearctic species, two of which have a wide geographical distribution, D. subobscura and D. ambigua. The other two are recently discovered species which are endemic to some of the Maccaronesian Islands, D. guanche Monclus (Monclus,