Experimental observation of overdominance ( heterozygous advantage ) in Drosophila

Natural selection is usually assumed to increase the frequency of an advantageous allele or to decrease the frequency of a deleterious allele over time. There are mechanisms that can maintain genetic variation over time, such as cycling environments that favor one allele for a time and then the other and frequency dependent selection, where rare alleles are selected over the more common allele (see Hedrick, 2005, for a discussion of these selection topics). Fisher (1922) first noted that a stable polymorphism for a pair of alleles of a gene would also occur if the heterozygotes (A1A2) were more fit than either of the homozygotes (A1A1 and A2A2). Fisher (1922) stated: