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Fine, Distribution of Morph Frequencies in the Snail, Cepaea nemoralis Near Groningen

1969; Wiley; Volume: 38; Issue: 2 Linguagem: Inglês

10.2307/2773

1365-2656

H. Wolda,

Isotope Analysis in Ecology

Some years ago at Groningen we started to study the distribution, abundance and genetic variation in the polymorphic land-snail Cepaea nemoralis (L.). Such a study necessarily has two components. Firstly the description of distribution, abundance and genetic variation as it occurs in nature, and secondly an experimental approach to the explanation of the phenomena observed. The present paper deals with some of the results of our programme of sampling natural populations in the neighbourhood of Groningen, in the north of the Netherlands. Polymorphism in C. nemoralis has been studied by several authors throughout the range of the species. Sacchi (1963) has done so in Italy, Lamotte (1951, 1959, 1966) throughout France, Guerrucci-Henrion (1966) in Brittany, Schilder S Cain & Currey 1963a, b; Goodhart 1962; Clarke & Murray 1962; Currey, Arnold & Carter 1964; Carter 1967). Important differences in mean morph frequency between large areas in France and in Germany have been found. The causes of these large-scale differences are not known, though there are indications that climatic factors may be of some significance (Lamotte 1959; Wolda 1967). But as there is great variation within each large area, gross climatic factors cannot be the only ones involved. Differences have also been found between much smaller areas that are much closer together. The transition between two such areas may be gradual, but there can be very steep clines. The causes of these between-area differences are again unknown. Morph frequencies within such a small area can also vary a great deal. Sometimes part of this within-area variation could be correlated with differences in habitat, and such a correlation is at least partly explained by visual selection by thrushes, which are important predators of these snails. Whether other factors that vary between the different habitats, such as microclimate, also play a part here is not known. In other areas no correlation between morph frequencies and habitat has been found although thrushes do prey upon Cepaea and have sometimes even been shown to select in these areas. Apparently the effect of visual selection, if present, is being overridden by other factors that work alike in all habitats. Sometimes considerable within-area variation in morph frequencies has been observed without any apparent corresponding changes in habitat (Goodhart 1962; Clarke & Murray 1962). The causes of this variation are still obscure. This short summary of the literature shows that we still know very little about the causes of genetic variation in populations of Cepaea. Experimental work is probably the most promising approach to a solution of the problems at hand, but surveying the