Reflections on molluscan shell polymorphisms

The best-studied example of polymorphism of shell colour and shell pattern in molluscs is Cepaea nemoralis (L.). Colour and pattern are determined by linked loci in disequilibrium. A number of factors including the relation of disequilibrium to gene frequency indicate that the most likely explanation for the pattern of polymorphism in this species is a balance between selection in different habitats and migration. The selection is visual, exerted by predators, and non-visual arising from difference in thermal properties of shells of different colours. Non-visual selection probably explains much of the variation in frequency between populations. Other polymorphic molluscs from a wide range of groups for which the genetics is known have similar multilocus systems determining shell colour and pattern in disequilibrium. Where explanations for polymorphism have been proposed they usually involve some level of disruptive selection and gene exchange between locations. The species concerned live in mosaic environments in which pale backgrounds favour pale morphs and dark backgrounds dark morphs. The adaptation is thermal or cryptic or, most probably, both. In the very long term visible polymorphisms are due to visual predation, resulting in different physiological costs for different morphs. Genetic colour and pattern systems evolved to accommodate these features by developing supergene combinations in linkage disequilibrium. It is not clear, however, why determination of shell colour and shell pattern should necessarily be genetically distinct.