A two–stage model for Cepaea polymorphism

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Abstract
The history of the study of snails in the genusCepaeais briefly outlined.Cepaea nemoralisandC. hortensisare polymorphic for genetically controlled shell colour and banding, which has been the main interest of the work covered. Random drift, selective predation and climatic selection, both at a macro– and micro–scale, all affect gene frequency. The usual approach to understanding maintenance of the polymorphism, has been to look for centripetal effects on frequency. Possible processes include balance of mutation pressure and drift, heterozygote advantage, relational balance heterosis, frequency–dependent predation, multi–niche selective balance, or some combination of these. Mutational balance is overlaid by more substantial forces. There is some evidence for heterosis. Predation by birds may protect the polymorphism, and act apostatically to favour distinct morphs. Although not substantiated forCepaea, many studies show that predators behave in the appropriate manner, while shell colour polymorphisms in molluscs occur most commonly in species exposed to visually searching predators. It is not known whether different thermal properties of the shells help to generate equilibria. Migration between colonies is probably greater than originally thought. The present geographical range has been occupied for less than 5000 generations. Climatic and human modification alter snail habitats relatively rapidly, which in turn changes selection pressures. A simple simulation shows that migration coupled with selection which fluctuates but is not centripetal, may retain polymorphism for sufficiently long to account for the patterns we see today. There may therefore be a two–stage basis to the polymorphism, comprising long–term but weak balancing forces coupled with fluctuating selection which does not necessarily balance but results in very slow elimination. Persistence of genetic variants in this way may provide the conditions for evolution of a balanced genome.