Change and Stability in Cepaea Populations over 25 Years: A Case of Climatic Selection

Sites surveyed for Cepaea populations in Hertfordshire, England, in 1964—66 were revisited in 1990. Some sites had been destroyed, but among those containing populations of Cepaea hortensis on both occasions there were examples both of change and of stability in shell colour and banding polymorphisms. Populations in valley bottoms showed a consistent and significant decline in the frequency of yellow shells. This remained significant when sites in which large and significant changes believed to result from disturbance were excluded. A weaker trend for a decrease in banded shells in the same valley-bottom sites was also detected. The frequency of yellow varies with topography in the area, and circumstantial evidence suggests that the changes are a response to climatic selection, or to its relaxation. The particular circumstances that allowed a relatively small change in frequency to be detected are discussed, and are related to earlier claims of temporal stability in Cepaea populations.