Gene frequencies, genetic background and environment in Pyrenean populations of Cepaea nemoralis (L.)

Microgeographical genetic differentiation of populations of the polymorphic land snail Cepaea nemoralis (L.) has been the subject of intensive investigation. The importance of environmental selection acting at the loci which control the shell polymorphism in determining the distribution of genes in this species remains a matter of considerable controversy. Some populations of C. nemoralis in the central Pyrenees show striking local associations of morph frequency (and particularly of the frequency of unhanded shells) with topography which have been ascribed to the action of natural selection.
In the Ter valley (eastern Spanish Pyrenees) populations described here, however, the pattern of association of the frequencies of the shell morphs with topography is quite different from that found in the central Pyrenees. In particular, the distribution of unhanded shells is almost the inverse of that found in some previously studied populations. The general picture of variation in relation to topography in Pyrenean C. nemoralis populations is now seen to be one of great local consistency within valley systems, but of considerable differences between valley systems. This variation in population structure may be due either to cryptic and undetected environmental differences between valley systems or to evolutionary divergence between their C. nemoralis populations. The difficulty of deciding between these two conflicting hypotheses emphasizes the limitations of the technique of simply examining the distribution of individual genes in natural populations as a method of analysing their evolutionary history.