A 2-STAGE MODEL FOR CEPAEA POLYMORPHISM

The history of the study of snails in the genus Cepaea is briefly outl ined. Cepaea nemoralis and C. hortensis are polymorphic for geneticall y controlled shell colour and banding, which has been the main interes t of the work covered. Random drift, selective predation and climatic selection, both at a macro- and microscale, all affect gene frequency. The usual approach to understanding maintenance of the polymorphism, has been to look for centripetal effects on frequency. Possible proces ses include balance of mutation pressure and drift, heterozygote advan tage, relational balance heterosis, frequency-dependent predation, mul ti-niche selective balance, or some combination of these. Mutational b alance is overlaid by more substantial forces. There is some evidence for heterosis. Predation by birds may protect the polymorphism, and ac t apostatically to favour distinct morphs. Although not substantiated for Cepaea, many studies show that predators behave in the appropriate manner, while shell colour polymorphisms in molluscs occur most commo nly in species exposed to visually searching predators. It is not know n whet-her different thermal properties of the shells help to generate equilibria. Migration between colonies is probably greater than origi nally thought. The present geographical range has been occupied for le ss 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 whic h fluctuates but is not centripetal, may retain polymorphism for suffi ciently long to account for the patterns we see today. There may there fore be a two-stage basis to the polymorphism, comprising long-term bu t weak balancing forces coupled with fluctuating selection which does not necessarily balance but results in very slow elimination. Persiste nce of genetic variants in this way may provide the conditions for evo lution of a balanced genome.