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Effective Population Numbers in the Snail Cepaea nemoralis

J. J. D. Greenwood
Evolution
Vol. 28, No. 4 (Dec., 1974), pp. 513-526
DOI: 10.2307/2407278
Stable URL: http://www.jstor.org/stable/2407278
Page Count: 14
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Effective Population Numbers in the Snail Cepaea nemoralis
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Abstract

Populations of Cepaea nemoralis usually occur at densities of 0.1-1.5/m2, and often up to 10/m2. The variance of dispersal appears to be related inversely to density, so the range of neighborhood sizes tends to be less than that of densities. Neighborhood numbers corresponding to the three figures of density quoted are about 380, 5,800 and 13,000. The main factor causing the effective population number to depart from the actual neighborhood number is probably non-random variance in progeny number. The estimate of this is complicated by the snails being perennial but an estimate of the lifetime mean and variance of progeny number has been obtained. Its value suggests the N$_e \bumpeq$ 0.5 N. It is concluded that these estimates are reasonably reliable. They indicate that most populations of this species cannot be considered small. Even low rates of migration will result in the effective numbers of semi-isolated colonies being higher than one would expect from their actual numbers. Relatively rare longdistance dispersal may be important in maintaining heterogeneity in Cepaea populations. A comparison is made with results obtained for other species and it is concluded that more studies of effective population numbers are needed.

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