Colour and Banding Morphs in Subfossil Samples of the Snail Cepaea

Abstract

Since Ford’s careful definition (1940) of polymorphism, the interest to population geneticists and evolutionists of this remarkable and widespread phenomenon needs no emphasizing. Its interest is enhanced when studies of its stability or otherwise can be made over long periods of time. The late Cyril Diver realized the importance of subfossil snail shells for this purpose as long ago as 1929. Currey & Cain (1968), using Cepaea nemoralis (L.) and C. hortensis (Muller), two species with an extensive shell-character polymorphism, were able to show a considerable alteration in southern England in the proportions of the bandless form in C. nemoralis from the hypsithermal (c. 4500 BC) to the present day, consistent with climatic selection; C. hortensis, however, showed no consistent signs of change. They showed also in C. nemoralis that the type of geographical distribution of morph frequencies which they called area effects (Cain & Currey 1963a) was probably more widespread in pre-Iron Age times than now, and some area effects may have lessened in intensity. They were able to give considerable evidence concerning these changes for a district including the western Marlborough Downs and the lowlying country adjacent, which is famous for its antiquities and in which several important excavations have taken place.