GEOGRAPHIC VARIATION IN NORTH AMERICAN COLONIES OF CEPAEA NEMORALIS.

Abstract

Because of their highly polymorphic shell patterns, snails in the genus Cepaea, principally C. nemoralis, have been the subject of numerous studies in ecological genetics in Europe for decades. The proportions of the various color and banding morphs often differ noticeably from one colony to another, and various workers have attempted to explain this variation. Selection by predators against the most conspicuous morph is evidently an important factor in some areas (Cain and Sheppard, 1950, 1954; Clarke, 1960, 1962), while genetic drift and founder effects (Lamotte, 1959; Goodhart, 1963), cryptic environmental factors (Cain and Currey, 1963), the action of modifier genes (Clarke, 1966), and climatic selection (Arnold, 1968, 1969, 1971; Jones, 1973) have also been implicated. Many conflicts of interpretation over the relative importance of these forces have yet to be adequately resolved. Introduced colonies of C. nemoralis are established in various localities in North America (Pilsbry, 1939; Reed, 1964), but they have received relatively little attention. The colony at Lexington, Virginia, is probably the best known. Its existence was noted simultaneously by Cockerell (1889) and Pilsbry (1889); and in a later paper, Cockerell (1897) listed the various color and banding morphs which had been collected there. Howe (1898) was the first to publish a detailed study of the variation in this colony, and McConnell (1935) reported on the changes in morph frequencies which had occurred since Howe's time. Other papers which list shell morph frequencies in North American Cepaea colonies are those of Brooks and Brooks (1934), Judd (1953, 1955), Landman (1956), and Reed (1964). However, no study on the variation of C. nemoralis throughout its range on this continent had ever been done. The primary objective of this study was to describe the pattern of geographic variation in shell polymorphisms and allozymes in a substantial number of North American C. nemoralis colonies, and to determine, if possible, the most likely reasons for the observed variation.