Selection of viability at loci controlling protein polymorphisms in Drosophila melanogaster is very weak at most.

In an isolated Drosophila population the frequency of chromsomes carrying the second chromosome inversion In(2L)t is about 60%. Three isozyme loci are nearly monomorphic in the inversion-carrying chromosomes but are highly polymorphic in other chromosomes. From known recombination frequencies and the slight polymorphism, it is estimated that the inversion-carrying chromosomes are descended from one or a few identical chromosomes introduced about 1000 generations previously. This is long enougn for lethal and mildly deleterious mutants to reach equilibrium frequencies but not long enough for very weakly selected mutants. Because the difference in viability between homozygous and heterozygous chromosomes is the same, it is inferred that there is at most very weak selection of isozyme loci. This method is sensitive enough to detect selective differences of 0.0005 per locus or less.