Visual and other selection in cepaea: A further example

Abstract

Populations of Cepaea nemoralis and C. hortensis have been sampled in a small area in north-west Worcestershire. No clear ecological separation between the two species is apparent. Morphs of C. nemoralis that one would expect to vary in frequency between woods and open habitats on the basis of visual selection do so in the manner expected. The variation shown by C. hortensis in the region is quite different and much reduced, compared with that of C. nemoralis. The expected differences between woods and open habitats still occur, however. Tests for any relations between morph-frequency and two other environmental variables (altitude and proximity to water) have been made. Only altitude shows such relations: unbanded in C. hortensis tends to rise in frequency with altitude and listeria in C. nemoralis may do so. The Index of Polymorphism of C. nemoralis is always high if C. hortensis is present, but may be low if the latter is absent. Several mechanisms which could explain this are suggested. Linkage disequilibria are apparent in C. nemoralis. Banding is deficient in browns and at an excess in yellows, compared with pinks, in all habitats. It is concluded that the diversity of selective forces acting affectively on Cepaea populations, not random events, is responsible for the wide variation between them in morph-frequencies. © 1974 The Genetical Society of Great Britain.