Genetic variation ln the spread of Drosophila subobscura from a nonequilibrium population

Drosophila subobscura was first identified in North America in the early 19 80s, and a newer D. subobscura population in Utah appears to have been esta blished more than 10 years later. In this study, we use nuclear microsatell ite allele frequencies, mitochondrial restriction fragment length polymorph ism (RFLP) allele frequencies, and computer simulations to investigate poss ible scenarios of how this species has spread across North America. Our met hod develops a 95% confidence interval for the maximum and minimum number o f founders that could have colonized the new population given various scena rios for spread. Unlike many other methods, it may be applied to nonequilib rium source populations given certain conditions. We find that observed all ele frequency differences between newer and older D. subobscura populations are consistent with very few inseminated females being transported east fr om the coast in a single step or with larger numbers of colonizers invading after several intermediate steps. They are not consistent with a large, pa nmictic population of D. subobscura colonizing Utah in a single step.