The historical population genetic processes associated
with the divergence of members of the
Drosophila virilis species group were examined
using DNA sequence variation from two loci. New
data on DNA sequence variation from the oskar locus,
taken from within and among all five
closely related taxa in the virilis phylad of the
D. virilis species group, were examined and
compared with similar data previously collected from the
period locus. Overall, the oskar and
period data sets reveal similar patterns of variation.
Both loci support the conclusion that the two
subspecies of D. americana have had a large
historical population size and are exchanging genes in
nature. From these data there is little reason to consider
them as distinct taxa. In the case of D.
novamexicana, from which six lines were sequenced at
each locus, there is an intriguing difference
in the pattern seen at the two loci. Both loci reveal two
distinct groups that are considerably
divergent from each other, with very little evidence of
gene flow between them. However, the
grouping of lines into distinct subgroups based on
oskar is different from the grouping based on
period. The simplest explanation seems to be that
D. novamexicana includes two distinct species,
and that the sample of six lines happens to include cases
of recent gene exchange. Alternatively,
both oskar and period could be
linked to sites of strong balancing selection and limited
recombination.