The Evolution of Polymorphism in Colonial Invertebrates and Social Insects

Abstract

Morphological polymorphism is widespread in colonial marine invertebrates and socia insects. The polymorphs originate as variants on the basic units that comprise the colonies and are often induced by local environmentl cues. This review examines (1) the incidence of polymorphism within invertebrate phyla and social insects, (2) the cues triggering polymorphs in the colonial marine invertebrates and the social insects, (3) the roles of heterochrony and genetic assimilation in the origins of polymorphs, and (4) the factors favoring the maintenance of polymorphism. The incidence of developmental polymorphism is high, but episodically distributed in three phyla of colonial marine invertebrates, the Cnidaria, the Bryozoa, and the Urochordata. Although polymorphism is well known in the social insects, the incidence appears lower than in some clades of colonial marine invertebrates. I suggest that the high incidence of polymorphism in some taxa of colonial marine invertebrates results from both unusually high o...