Development of Balanced Polymorphism in Laboratory Populations of Drosophila melanogaster

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Abstract
Equilibrated populations of se ss ro stock were inoculated with a single set of haploid wild-type autosomes extracted from a single wild-caught male fly. Very quickly (3 generations), se and ro assumed gene frequencies which did not undergo further progressive decline thereafter for a period of 2-1/2 years (70 generations). The gene ss has declined several times to new low levels but has not been eliminated. These results are explained by the hypothesis that heterozygotes, at least for ro and se, show superior fitness under the conditions of the experiment. As the balanced condition was established almost immediately, it is likely that simple heterosis for certain loci at least for adjacent chromosome sections functions in the immediate establishment of heterotic equilibria without the necessity of prior coadaptation.