Evolution of Over-dominance during the Initial Spread of New Alleles

Abstract

Bodmer and Parsons^1–3 have discussed a model for the initial increase in the frequency of newly arising, favourable mutations in a large undivided population and under different mating systems. They argued that over-dominance (higher fitness of heterozygotes over the homozygotes) has evolved in out breeding species during this process as follows: (1) a new allele, say A₁, at locus A₁–A₂ occurs principally in the heterozygotes when it is initially at a low frequency in the population; (2) selection favouring heterozygotes allows a rapid rate of increase in frequency of A₁; (3) hence, the modifiers that primarily enhance the fitness of A₁A₂ particularly when linked, would themselves increase rapidly in frequency and accumulate to form the linked gene complexes; (4) such gene combinations would exhibit so-called segmental (or associative⁴) over-dominance maintaining a balanced polymorphism.