GEOGRAPHIC-DISTRIBUTION AND SHELL COLOR AND BANDING POLYMORPHISM IN MARGINAL POPULATIONS OF CEPAEA-NEMORALIS (GASTROPODA, HELICIDAE)

Occurrence and colour and shell banding polymorphism were investigated in populations of Cepaea nemoralis (L.) at the edge of the species' g eographic distribution in the Czech Republic. Cepaea nemoralis populat ions were found only at intravillan localities. The great majority of town and village localities were occupied by C. nemoralis in three iso lated areas: a northern one integrated with the main geographic area o f the species, and two isolated southern insular areas. A few populati ons were scattered at localities between these areas and further south , where C. nemoralis was replaced by Cepaea hortensis (Muller). The C. nemoralis distribution area and abundance may have increased since 19 50. I speculate that human activity encouraged the spread of C. nemora lis populations, and this species competitively exterminated C. horten sis populations from the intravillan habitats. The frequency of shell colour and banding morphs in local populations was similar to those in oceanic Western Europe (56.2 + 18.6% of pink shell colour), and may b e affected by climatic selection and random drift. There exist small a reas marked by a high frequency of 00000, 00300, 00345 and 12345 shell banding morphs. Their edges were mostly not concordant with areas of geographic distribution.