Polymorphism in Cepaea: A Problem with Too Many Solutions?

Abstract

When faced with a problem or a series of related problems, scientists tend to look for a single unifying solution. When alternative solutions present themselves, it is usually accepted that only one is likely to be true. Further research often justifies this assumption. The successes of this type of reasoning have tended to obscure the fact that not all biological observations must necessarily be explicable in a simple and unitary way and that not all hypotheses to be tested need be mutually exclusive. We hope to illustrate this by considering some of the evolutionary processes affecting one well known genetic polymorphism, that of shell pattern in the land snail Cepaea. rhe development of techniques for detecting molecular polymorphisms has led to considerable disagreement between those who believe that such genetic variation is actively maintained by natural selection and those who suggest that random processes are the only significant factor in its control. The most important lesson to be gained from an intensive study of the Cepaea polymorphism is that many types of evolutionary force act upon it and that their relative importance varies between different polymorphic loci, or even when the same locus is studied in different populations. It is largely meaningless to ask whether selection or drift explains the observed variation in gene frequency, or indeed to attempt to identify the single selective mechanism acting on the polymorphism. The nature of the evolutionary process means that the genetic structure of each Cepaea population usually requires a complex and perhaps a unique explanation. This is a point which has not been sufficiently emphasized by students of other polymorphic systems.