A critical evaluation of the case for competitive selection in cepaea

Abstract

One of several populations showing an association between a low frequency of unbanded Cepaea nemoralis and the distribution of the congeneric species C. hortensis was surveyed in detail in an attempt to determine the selective agent causing the association. The sympatric area was characterized by greater vegeta- tional cover and higher plant species diversity than the area occupied only by C. nemoralis, suggesting that snails in the latter area receive greater insolation. It is thus possible that a microclimatic variable gives rise, independently, to the restricted distribution of C. hortensis and the coincident area of predominantly banded C. nemoralis, rather than the first of these two effects causing the second. The possible extension of this conclusion to other populations is considered and it is seen that climatic selection could produce all the observed associations involving the banding morph-frequency in C. nemoralis. Although associations between banding morph-frequency in C. hortensis and the distribution of C. nemoralis are not readily explicable in terms of climatic selection, the case for a general association of this kind is weak. It is concluded that it is unnecessary to invoke competitive selection to explain the observed patterns of variation in morph-frequency, but that its existence cannot be ruled out. A suggestion is made regarding the choice of model system for future studies of a similar kind. © 1982 The Genetical Society of Great Britain.