Correlated phenotypic responses to habitat difference in Cepaea nemoralis (L.)

Using data from the Evolution Megalab Project paired samples of Cepaea nemoralis (L.) coming respectively from woodland and open habitats have been examined for joint response to habitat difference at different polymorphic loci. Throughout the range of the species there is a tendency for open habitat samples to have different frequencies at shell colour and pattern loci from those in neighbouring woods. In Britain, the chance that the frequency of yellow is higher in open than in wooded habitats is about 67 per cent. There is a 41 per cent chance that they will have both higher frequency of yellow and a lower frequency of unbanded at the linked banding locus. Responses of unbanded and the unlinked mid-banded locus are to a large extent independent, however. The chance that open habitats have higher yellow and a lower value for the sum of unbanded and mid-banded (effectively unbanded) is 42 per cent, while the chance that the open habitat sample is more yellow, less unbanded and less mid-banded is no more than 19 per cent. The colour, but not the banding difference was also found in the data for continental Europe. The effect of habitat acts within a polymorphic system. For Britain closely spaced sample pairs have an average frequency difference (Euclidean distance) between habitats at the three loci of about 0.26. As a result of other factors affecting the polymorphism this difference increases to 0.43 for pairs 1 km apart and 0.59 at 10 km apart. These results extend the original findings of CAIN & SHEPPARD (1954) and others but show clearly that the habitat is only part of the explanation for polymorphism in Cepaea.