Frequency-dependent selection: Minority male mating advantage in Drosophila

In frequency-dependent selection the fitness of a given genotype depends on its frequency. Frequency dependence may be positive or negative. In the po sitive case any rare variant is at a disadvantage, whereas rare forms are f avoured in the negative case, One of the best-studied examples of frequency -dependent selection is the rare male mating advantage or minority male mat ing advantage (negative frequency-dependent selection). This could be of co nsiderable evolutionary significance as it can maintain high levels of gene tic polymorphism without any genetic load at equilibrium. Rare male mating advantage has been extensively studied in the genus Drosophila where its ex istence has been proved in twelve species. It has also been demonstrated in other insects and vertebrates, It has been shown to occur in morphological mutants, inversion karyotypes, isozyme variants, geographic strains, strai ns reared at different temperatures and those having behavioural difference s. Rare male mating advantage has been shown to occur in inversion karyotyp es of natural populations of certain species of Drosophila, In certain case s, the rare male mating advantage was tested for, but its existence could n ot be proved. The expression of rare male mating advantage may be affected by age of the females, temperature or experimental protocol, exposure to ot her flies, previous mating experience and sex ratio, The generality of this phenomenon where rare males are favoured in mating has been accepted, but there is still much discussion and controversy concerning its causes, A num ber of explanations have been proposed to account for the rare male effect.