Reexamination of diversifying selection of polymorphic allozyme genes by using population cages in Drosophila melanogaster

Several investigators have reported increased heterozygosity for allozyme markers in laboratory experiments simulating a variable environment and have concluded that there is diversifying selection for allozyme variants. We found no change in heterozygosity in 28 large population cages where temperature, food, and light were each set at three levels, two constant and one variable, in a factorial design. Specifically, there was no change in heterozygosity when 0, 1, 2, or 3 factors were variable. The discrepancy between our negative results and earlier positive ones is that our experiments were started from a large caged population of flies which had been maintained for 6 years - long enough to be very near to linkage equilibrium. We suggest that the earlier results may have been the result of linkage between neutral, or nearly neutral, allozyme markers and selected loci or in some cases, inversions.