Acetylcholine and ciliary movement in the gill plates of Mytilus edulis

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Abstract
There are three essentially different models to explain how dominance might gradually evolve by natural selection. In Fisher's model, the wild-type evolves dominance in response to the occurrence of deleterious mutations: genetic variations in the expression of the mutant heterozygote will be selected to raise its fitness and produce dominance or over-dominance. The rate of this selection is slow and depends on what the fitness of the heterozygote will be at its optimum. In Parsons & Bodmer's model, the fitness of an advantageous mutant heterozygote is raised up to or above that of the mutant homozygote while the mutation is spreading through a population. Whether dominance or over-dominance is reached depends on the heterozygote's genetic variance and optimum fitness. In both these models, the variations in the mutant heterozygote must be caused by modifiers with more or less neutral selective effects on other characters. Fisher's theoretical arguments on the origin of genetic variation and Lewontin & Hubby's experiments show that such modifiers may exist. In Sheppard's model, dominance evolves in a polymorphism maintained by disruptive or frequency-dependent selection. If two different alleles produce two different mimetic forms, the heterozygote and one of the homozygotes will be selected to give the optimum phenotype of one of the mimics and the other homozygote will be selected to give the optimum phenotype of the other one. In this model, the mimic which is the last to appear by mutation evolves dominance. Other small selective forces acting on the modifiers do not stop the evolution of dominance. Sheppard's model should therefore be true in nature whether the modifiers are subject to other selective forces or not. A mathematical model demonstrated that a rare mimic would not evolve dominance if the modifier was maintained in the population by other selective forces producing a heterozygous advantage of more than 1%. Thus it follows from Clarke & Sheppard's discovery of certain rare mimics of Papilio dardanus that this is the greatest possible heterozygous advantage that can be acting on the modifiers.

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