Models of Wright's theory suggest that mutations of small effect should behave nearly additively, while mutations of large effect will tend to be recessive. This result falsifies Fisher's theory of dominance and provides strong support for the alternative theory. Fisher's theory was laid to rest. Beyond the debate This debate dichotomized views of the evolution of dominance and led to a surprising lack of investigation with the fertile ground between the views of Fisher and Wright. Cases of polymorphism permanently maintained in a species by the stability of the frequency ratio of a pair of allelomorphs supply opportunities peculiarly favourable to the selective evolution of dominance, for in these cases the heterozygotes are not extremely rare.
Selection favors the heterozygote.
Selection favors aa in one environment and AA in another environment. Denis Bourguet and I recently examined the last two cases where heterozygotes can be maintained for prolonged periods of time. Heterozygote Advantage Imagine that the fitness of different genotypes within a population are measured and divided by the average fitness of the whole population. Selection on modifiers that alter dominance levels is stronger when heterozygotes are more frequent, but this may occur in species with intermediate levels of migration. Selection is always much stronger then at the mutation-selection balance considered by Fisher and could shape the evolution of dominance for any gene that is polymorphic for extended periods of time.