THE EVOLUTIONARY HISTORY OF DROSOPHILA-BUZZATII .30. MITOCHONDRIAL-DNA POLYMORPHISM IN ORIGINAL AND COLONIZING POPULATIONS

Both original and colonizer populations of Drosophila buzzatii have be en analyzed for mtDNA restriction polymorphisms. Most of the mtDNA nuc leotide variation in original populations of NW Argentina can be expla ined by intrapopulation diversity and only a small fraction can be acc ounted for by between-population diversity. Similar results are obtain ed using either the estimated number of nucleotide substitutions per s ite or considering each restriction site as a locus. Colonizer populat ions of the Iberian Peninsula are monomorphic and show only the most c ommon haplotype from the original populations. Under the infinite isla nd model and assuming that populations are in equilibrium fixation ind ices indicate enough gene flow to explain why the populations are not structured. Yet, the possibility exists that populations have not reac hed an equilibrium after a founder event at the end of the last Pleist ocene glaciation. Tajima's test suggests that directional selection an d/or a recent bottleneck could explain the present mtDNA differentiati on. Considering the significant population structure found for the chr omosomal and some allozyme polymorphisms, the among-population uniform ity for mtDNA variability argues in favor of the chromosomal and some allozyme polymorphisms being adaptive.