Micro-evolution in a polymorphic prosobranch snail ( Clithon oualaniensis (Lesson))

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Abstract
The polymorphic brackish-water snail C. oualaniensis is widely spread in the Indo-Pacific Region (India, Ceylon, Malay Peninsula, Hong Kong and Queensland, Australia). Almost 100,000 individuals were examined. There are great geographical differences in gene frequencies. Purple spirals are polymorphic with an incidence of about 16% in Queensland, 5.5% in the Malay Peninsula and 2% on the east coasts of India and Ceylon, but an abrupt drop to near-absence occurs in the Gulf of Mannar and persists as far as Goa on the Indian west coast. The incidence thus falls from a polymorphic to a monomorphic level. Most variants are polymorphic in parts of the range and monomorphic in others. More gradual clines within the polymorphic phase were observed round the coastline of Malaya. Gene frequencies tend to be uniform over long stretches of coastline which presumably indicates that, for the gene(s) in question, the conditions affecting survival value are similarly uniform. Changes in gene frequency presumably indicate changes in the environment. There is a strong prima facie case that this is so for the major discontinuity between western Clithon from India and Ceylon, and that from the eastern regions. In this instance, surface salinity appears to be one of the factors responsible, and the same parameter may be involved in the changes of incidence in purple spirals. In other instances, there are other environmental differentials whose nature remains to be discovered. The relation between selection and adaptation is discussed, it being maintained that a change in gene frequency brought about by a change in the environment is not, in itself, evidence that the change is adaptive in nature. The incidence of most morphs in Clithon is stable over great distances. This linear stability of gene frequencies is evidently the one-dimensional equivalent of the area effects in land snails like Cepaea. Only a single entity, black (and perhaps giant tongues) shows irregular fluctuations of gene frequency over short distances and thus seems to be influenced by local littoral conditions.