Studies oncepaea,IV. Climate and selection of banding morphs incepaeafrom the climatic optimum to the present day

Abstract

A number of samples of subfossilCepaea nemoralisandhortensisfrom sites in southern Britain of archaeological interest, ranging in date from about 4500 b .c . to Romano-British and Anglo-Saxon, have been scored for frequency of the major banding morphs, and compared with present-day samples taken on each site or as near to it as these species could be found. In C.nemoralisthere is a significant decrease of unbandeds from pre-iron Age samples to the corresponding ones for the present day, but no indication of systematic change from Iron Age samples to the present day. Spread banded also shows changes from pre-iron Age samples to present-day ones, but very little change from the early Iron Age to the present day. The smaller samples of C.hortensisavailable give no sign of a trend although there is much change from pre-iron Age times to the present; Iron Age samples and the corresponding present-day ones do not show the relative constancy of composition seen in C.nemoralis—as usual these two very closely related species are behaving differently. At the present day there is evidence (experimental and distributional) that the frequencies of banding morphs of C.nemoralisare affected by climate, unbandeds and mid-bandeds being favoured by better summers than those normal in Britain at present. The available evidence, from pollen analysis and other sources, of changes in the climate of southern Britain in the last 6500 years suggests that the observed differences in morph frequencies can be related to known climatic changes, in agreement with present-day evidence. One area effect (south-west M arlborough Downs) has contracted and become less intense since pre-iron Age times, as perhaps have others; in some cases a site has remained in an area effect, but the effect itself has changed. Two pairs of samples from lowland sites appear to have changed from frequencies indicating area effects in pre-iron Age times to others consistent with visual selection at the present day. Area effects seem to have been rather constant from the Iron Age to the present day.