On pseudo-polymorphism

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Abstract
Variation in the shell patterns of marine snails of the genus Umbonium (as also in Clithon oualaniensis) involves two separate categories. A minority of individuals has discontinuous and hence sharply classifiable patterns or colours. Such distinct variants are presumably the result of segregation of a small number of genes with major effects; i.e. they represent conventional or `true' polymorphism. By contrast, in all populations studied, the majority of snails have certain basic patterns that merge step by step into each other. Intergrades exist between individuals, but may also arise gradually or abruptly during growth in one and the same shell. Evidently there is no simple correspondence between genotype and phenotype: a single genotype may be moulded into several phenotypes by influences of the environment and presumably by interaction with a polygenic genetic background, a situation here designated as pseudo-polymorphism.

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