Colour polymorphism in northern peripheral populations of Cepaea hortensis.

The colour polymorphism of Cepaea hortensis (Muller) (Gastropoda, Helicidae) was investigated in peripheral populations in five geographical regions in Finland. Yellow-shelled individuals were found in all the regions, pinks in two and browns in none. In all five regions the five-banded morph 12345 was in the majority, and in all but one the unbanded morph 00000 was second in frequency. The morphs 10305 and 10345 were found to be confined to the same two regions where pinks were present. The regions in question include old urban centres. Here populations may have received anthropochorous additions to their gene pools. Several banded morphs, including the midbanded 00300, which is common in some parts of Europe further south, are absent or extremely rare in Finland. This situation appears to be the same in peripheral populations in the other Northern Countries, suggesting that selection is due to climatic factors. In northern peripheral populations, selection often operates mainly on the ratio yellow 00000/yellow 12345. The groups of populations in the several geographical regions of Finland show distinct area effects with regard to this ratio.