Development of Balanced Polymorphism in Laboratory Populations of Drosophila melanogaster

Introduction of a single set of wild type autosomes into experimental populations of D. melanogaster homozygous for sepia, spineless and rough resulted in the quick assumption of a condition in which rough and sepia are balanced in the population at gene frequencies which have not declined progressively thereafter for a period of two and a half years (70 generations). The gene spineless, on the other hand, declined several times to new low levels but has not been eliminated. Its final frequency, about 12 per cent, is sufficiently low that slow elimination cannot be ruled out. These results are explained by the hypothesis that the heterozygotes, at least for rough and sepia, show superior fitness under the conditions of the experiment. As the condition was established almost immediately, it is likely that simple heterosis at certain loci (or possibly adjacent chromosome sections) functions in the immediate establishment of equilibria without the necessity of prior coadaptation.