Change and stasis in urban populations of the land snail <i>Cepaea nemoralis</i> over five to six generations

Repeat surveys of the shell colour and banding polymorphism in populations of the land snail Cepaea nemoralis in the cities of Sheffield (UK) and Wrocław (Poland) and in its surrounding settlements were made after a 13-18 years interval. In all three cases, a substantial proportion of sites had been destroyed by development (18-30%) and a similar proportion no longer held the species, despite being apparently suitable; up to 50% of populations had disappeared. Overall, the pattern was one of stasis in morph-frequencies, and of little change in the range of variation as estimated by a modified version of F st . There were, however, substantial changes in some populations, not consistent in direction, but larger than expected from sampling error. While there was tenuous evidence for selection against yellow shells, the large changes are attributed to bottlenecks, founder effects, and possible local extinction and recolonisation. Urban populations are often transitory and the results are contrasted with those from an area of long-term habitat stability and population connectivity. Newly established and sometimes transitory populations offer opportunities to study underestimated aspects of evolutionary processes.