THE EVOLUTION OF POLYMORPHISM IN COLONIAL INVERTEBRATES AND SOCIAL INSECTS

Morphological polymorphism is widespread in colonial marine invertebrates and social insects. The polymorphs originate as variants on the basic units that comprise the colonies and are often induced by local environmental 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 origination rates and strong natural selection favoring division of labor in colonies where the units are isogenic. Four features of colonial marine invertebrates make them particularly susceptible to high origination rates of morphological novelties: (1) the iterated developmental process, (2) the late differentiation of the germ line, (3) the lability of signal transduction pathways, and (4) the potential for partially functioning ''hopeful monsters'' to be nurtured by the colony. Origination rates of morphological novelties may also be high in colonial marine invertebrates owing to a propensity for environmentally induced heterochronic shifts: All colonial invertebrates produce the units of the colony through an iterated developmental process that allows environmental stimuli to differentiate the functions and morphologies of the units from each other. A small difference in growth rate among the units can be translated into large shifts in morphology. Genetic assimilation of environmentally induced men variants may be facilitated by the late time of differentiation of germ cells and the redifferentiation of germ cells in each newly budded zooid. Environmental induction is common in the triggering of polymorphic transformations. The evolutionary lability of these signal transduction pathways is indicated by the diversity of different combinations of extrinsic and intrinsic cues activating morphological switches. Despite general interest in the evolution of phenotypic plasticity, the particular selective factors maintaining these inducible polymorphisms in nature remain poorly known.