THE EVOLUTION OF POLYMORPHISM IN COLONIAL INVERTEBRATES AND SOCIAL INSECTS

Morphological polymorphism is widespread in colonial marine invertebra tes and social insects. The polymorphs originate as variants on the ba sic units that comprise the colonies and are often induced by local en vironmental cues. This review examines (1) the incidence of polymorphi sm within invertebrate phyla and social insects, (2) the cues triggeri ng polymorphs in the colonial marine invertebrates and the social inse cts, (3) the roles of heterochrony and genetic assimilation in the ori gins of polymorphs, and (4) the factors favoring the maintenance of po lymorphism. The incidence of developmental polymorphism is high, but e pisodically distributed in three phyla of colonial marine invertebrate s, the Cnidaria, the Bryozoa, and the Urochordata. Although polymorphi sm is well known in the social insects, the incidence appears lower th an in some clades of colonial marine invertebrates. I suggest that the high incidence of polymorphism in some taxa of colonial marine invert ebrates results from both unusually high origination rates and strong natural selection favoring division of labor in colonies where the uni ts are isogenic. Four features of colonial marine invertebrates make t hem particularly susceptible to high origination rates of morphologica l novelties: (1) the iterated developmental process, (2) the late diff erentiation of the germ line, (3) the lability of signal transduction pathways, and (4) the potential for partially functioning ''hopeful mo nsters'' to be nurtured by the colony. Origination rates of morphologi cal 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 iter ated developmental process that allows environmental stimuli to differ entiate the functions and morphologies of the units from each other. A small difference in growth rate among the units can be translated int o large shifts in morphology. Genetic assimilation of environmentally induced men variants may be facilitated by the late time of differenti ation of germ cells and the redifferentiation of germ cells in each ne wly budded zooid. Environmental induction is common in the triggering of polymorphic transformations. The evolutionary lability of these sig nal transduction pathways is indicated by the diversity of different c ombinations of extrinsic and intrinsic cues activating morphological s witches. Despite general interest in the evolution of phenotypic plast icity, the particular selective factors maintaining these inducible po lymorphisms in nature remain poorly known.