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Selection in mixed colonies of Cepaea nemoralis and Cepaea hortensis

1967; Springer Nature; Volume: 22; Issue: 1 Linguagem: Inglês

10.1038/hdy.1967.10

1365-2540

M A Carter,

Mollusks and Parasites Studies

CAIN and Sheppard have shown (i) that there is a correlation between the proportion of morphs in a population of C. nemoralis and the background of the habitat on which the snails occur.They have argued that this correlation is most likely to be due to selective visual predation, the most cryptic morph in any habitat being favoured.Clarke (1960) has found a comparable correlation in C. hortensis.As in C. nemoralis the most cryptic morph is at a high frequency but the crypsis is brought about in a different way, i.e. in woods (with dark uniform backgrounds) C. nemoralis has a high proportion of brown and pink unbanded morphs, whereas C. hortensis has a high proportion of banded yellows with fused bands, both species thus having a high proportion of effectively dark morphs.Cain and Sheppard suggested that since the polymorphism was stable and old (Diver, 1940), there was some physiological advantage of the heterozygote maintaining it.However, since there were always rarer non-cryptic morphs present as well as cryptic ones, they considered the possibility of a frequency-dependent visual selective mechanism maintaining the polymorphism.In such a situation phenotypes would be at an advantage when rare.The kind of mechanism that might be acting is suggested by the works of Reighard (1908), De Ruiter and Tinbergen (1960) who showed that predators formed searching images for their prey.In the case of Cepaea the predator would form a searching image for the most common morph, which in the Oxford district would be the most cryptic.Thus the rarer non-cryptic morphs would be at an advantage.Cain and Sheppard could find no evidence that this was the case.Clarke (1962a) in his analysis of Popham's data has shown, however, that frequency dependent visual selection can be effective in