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In bisexual organisms, pattern polymorphism may affect both sexes equally (unimodal polymorphism), or be expressed differently in the two sexes (bimodal polymorphism). Bimodal species may or may not have one or more forms that are identical in both sexes (shared morphs), in addition to those pattern forms which only occur in a particular sex (sex-limited morphs). Sex-limited morphs may occur in the male sex only, the female sex only, or be found in both sexes. Bimodal species in which there are no shared morphs show complete sex-limitation of pattern (dualism), in contrast to those in which there are shared morphs, which may be said to exhibit partial sex-limitation. Sexual dimorphism is here divided into two categories, partial sexual dimorphism (where both similar and dissimilar prospective pairs coexist), and complete sexual dimorphism (where only dissimilar prospective pairs coexist). From these considerations a comprehensive classification of nine categories of phaneromorphic polymorphism is developed, including monomorphism as the limiting case. This classification is presented graphically for convenient reference and visualisation. Selected examples of each class as they occur in butterflies are given, for both mimetic and non-mimetic species. The necessity for such a classification as a tool in the study of the evolution of polymorphism, sexual dimorphism and mimicry in butterflies is briefly discussed. This classification should also prove relevant to polymorphism studies in all non-parthenogenetic, bisexually reproducing species of organisms.