Distribution patterns of morph frequencies in the snail Cepaea hortensis in Iceland

In Iceland Cepaea hortensis Müll. reaches its northern limit of distribution occurring in restricted areas along the south coast. Samples of snails collected along steep cliffs in two such restricted areas were effectively monomorphic for shell colour (yellow), but were polymorphic for banding pattern and revealed strong area effects. In both localities samples from sites facing east contained varying frequencies of unbanded snails whereas in ecologically similar sites facing south unbanded were absent. In an experimental population released at a south-facing site unbanded snails failed to become established whereas banded individuals survived and bred. It is suggested that the area effects are maintained through non-visual (most likely climatic) selection though other possibilities cannot be ruled out.