ABSTRACT
Natural selection can bring about the replacement of one genotype by another, as in the case of industrial melanism and insecticide resistance. It can also maintain a balanced polymorphism in the population. Many of the earliest reports on polymorphism came from morphological studies or chromosomal studies of Drosophila. In the 1960s – 1980s, many more populations were described as polymorphic in electrophoretic studies of many species. In the 1960s, population geneticists were greatly interested in the report that in Drosophila, mating success was a function of genotype frequencies. Rare-male advantage has rarely been reported in organisms other than Drosophila – perhaps because testing for it requires laboratory populations that can be easily-handled, and few organisms are available for this kind of research. A process of frequency-dependent selection can occur also in ecological situations.
