Probable Instance of Genetic Polymorphism in the Graptolites

Abstract

A SPECIES, in the biological sense, is a community of individuals with a similar genetic structure, drawing on a common gene pool and having the ability to interbreed. Individual variation within such a community can be accounted for, in large part, by the segregation and recombination of genes according to Mendelian rules. This variation may be continuous and capable of being expressed in terms of a unimodal distribution curve, or it may be discontinuous, with sharply contrasted differences which either do not overlap or else give rise to a bimodal (or multimodal) curve. Variation of the latter kind is polymorphism, and if the two or more distinct forms of a species co-exist in the same habitat, that is, if they are genetically determined rather than a product of geographical variation, this phenomenon can be called genetic polymorphism.