Abstract
Introduction of a single set of haploid autosomes into an experimental P melanogaster population equilibrated by natural selection, results in an approximately threefold rise in the collective size and production of the population. The environment was rigidly controlled throughout. The biomass of the experimental populations transgressed also the level maintained by that control population which was used as a donor of the introduced wild/type chromosomes. The data suggest that simple heterosis without coadaptation (= luxuriance) may contribute directly to fitness. Natural selection appears not only to prevent the breakdown of the immediate heterosis but also to maintain it through favoring the development of balanced polymorphism.