Cepaea nemoralis is remarkable for its geographic variation for both shell polymorphism and allozymes. An electrophoretic study of 20 loci revealed genetic identities as low as 0-53 between English and Italian populations. Despite this genetic divergence, crosses among American, English, and Italian stocks demonstrate that populations of C. nemoralis are reproductively compatible on a continental scale. This reproductive compatibility in the face of large-scale genetic divergence indicates the danger of using a genetic index to assess reproductive relationships. The decoupling of genetic divergence from speciation emphasizes the limitations of viewing the process of speciation solely in genetic terms. A comparison of Cepaea with another genus of land snails, Partula, suggests that competition may be an important determinant of the likelihood of speciation. C. nemoralis from Lexington, Virginia, were included in the matings and electrophoretic comparisons, in order to test conflicting proposals on the origin of that population. Both sets of data favour a British, rather than an Italian, origin, but are also compatible with earlier evidence that the Lexington population is of hybrid origin.
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