Development of Heterosis Through Natural Selection in Experimental Populations of Drosophila pseudoobscura

Hybrids of the California and the Mexican races of Drosophila pseudoobscura were kept in population cages for approximately 15 generations. A preliminary study of viability, and the early course of the population in the cages, showed no heterosis for the heterozygotes which carried a California and a Mexican third chromosome. Heterosis has, however, developed during the experiment, as indicated by the attainment of equilibrium and by a study of the viability of flies derived from the cage. Both tests gave statistically significant results. Methods were developed for estimating adaptive values and viability in such experiments, and for investigating the possible range of values of these constants consistent with the sampling error involved.