Cepaea nemoralis (L.) in southeastern Poland: association of morph frequencies with habitat

Cepaea nemoralis shell colour and banding morphs were studied in southeastern Poland, at the eastern edge of its distribution in Europe. Overall, 73% of shells were yellow, 94% were banded and 73% of banded shells were mid-banded. The frequency of yellow shells was significantly higher in open (median 85%) than in shaded habitats (median 60%). Distribution of banding frequencies was consistent with climatic rather than visual selection. Colonies in open habitats were significantly less polymorphic than colonies in shaded habitats, indicating that in more extreme climatic conditions, selection can act against certain genotypes. Analysis of a Wroclaw diagram suggests different possible ways of adaptation to the same habitat type, depending on the initial gene pools. These results conform to a pattern over the whole range, where an increase in the frequency of the yellow phenotype is observed from the north to the south of Europe both within open and shaded habitats, but with significant differences between those habitat types. This can have important consequences for the interpretation of distributional data; results must be considered habitat by habitat.