Population genetics of a sex‐linked locus in Drosophila melanogaster

In population cages set up from laboratory stocks of white (w) and white blood (w bl ), w frequency increased rapidly, rather than decreasing as expected. However, gene frequency changes in two environmental treatments (light and dark) were different. In the light, w frequency increased over four generations to about 0.65, remained polymorphic for about 12 generations, and then declined to zero. In the dark, w frequency increased to higher values over a longer time, but subsequent changes varied among populations. Two of these populations remain polymorphic, but at different frequencies, some 200 generations after initiation. Additional experiments involving 61 cage populations were done to test the hypothesis that the initial polymorphism resulted from associative overdominance due to linkage disequilibrium, and to analyse the nature of this disequilibrium. Results are interpreted in terms of an initial complete and complex linkage disequilibrium, with some loci affecting fitness in one environment, but not in the other. The experiments demonstrate that a complex linkage disequilibrium resulting from the mixing of previously isolated populations can lead to a variety of different polymorphisms, some of which may persist for very long periods of time, and that our understanding of the dynamics of multi‐locus systems is at a very low level.