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Variation in a Colony of the Snail Cepaea nemoralis (L.)

C. B. Goodhart
Journal of Animal Ecology
Vol. 31, No. 2 (Jun., 1962), pp. 207-237
DOI: 10.2307/2137
Stable URL: http://www.jstor.org/stable/2137
Page Count: 33
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Variation in a Colony of the Snail Cepaea nemoralis (L.)
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Abstract

1. A study was made of local variation in a large and effectively linear colony of the polymorphic land snail C. nemoralis, living along 2 miles (3.2 km) of an artificial river bank near Cambridge. 2. This straight bank provides a uniform habitat along its whole length, but a road divides the bank into upper and lower sections, which are ecologically distinct. 3. The abundant snail population was sampled at intervals of 220 yd (201 m) along both sides of the road. There are wide fluctuations in the proportions of different shell colours and banding patterns in the population along the bank, although the habitat appears to be uniform. Pairs of samples from either side of the road are generally similar, despite the consistent habitat difference. 4. The chief predators in this colony are thought to have been rats, rather than thrushes. There is no evidence, from the snail shells found eaten by rats, of any predator selection for shell colour or banding patterns. 5. An analysis of the samples according to estimated age shows that certain phenotypes are disproportionately represented in the older age-groups, and this is probably an indication of differential survival. Since, however, for any particular phenotype this effect is the same throughout the colony, it is unlikely to have been the cause of the significant local variation observed. 6. Marking-recapture experiments show that the rate of dispersion of the snails, believed to result from random movement, is slow; the estimated standard deviation for displacement after 1 year is 6 yd (5.5 m). Adult mortality appears to be about 50% per year. 7. The population is now probably too large for random genetic drift to occur, although it is not panmictic because of the slow rate of dispersion. Numbers may have been reduced by flooding 5 years before the present investigation, and the local variation, apparently not correlated with habitat differences, may have resulted from random genetic assortment in the small numbers surviving the floods. This would provide an example of the Sewall Wright effect operating in a natural animal population.

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