Phenotypic Plasticity and the Origins of Diversity'

Phenotypic plasticity is the ability of a single genotype to produce more than one alternative form of morphology, physiological state, and/or behavior in response to environmental conditions. "Plasticity" and "development" are related terms that are becoming increasingly common in evolutionary biology and ecology. Both phenomena have passed through a period of neglect. In the 1960s Wigglesworth (228, p. 107) described some geneticists as being "apologetic" about environmentally cued polymorphisms, which they considered examples of unfortunate defects in the delicate genetic apparatus: "As R. A. Fisher once said to me, it is not surprising that such elaborate machinery should sometimes go wrong." And Bradshaw (19, p. 148) noted that botanists were carefully avoiding any mention of plasticity; environmental effects in experiments were considered "only an embarrassment." Until recently, genetic considerations have predominated in discussions of evolution and selection. Compared to the enormous progress made in genetics, there has been relatively little systematic effort to analyze environmental effects on the phenotype, and their evolutionary consequences. The plastic phenotype, stigmatized by poorly understood environmental influences and the ghost of Lamarck, has sometimes been lost from view as the focus of selection (e.g. 46; but see 48, 49). Much recent progress has been made toward integrating developmental and evolutionary biology, especially in vertebrate morphology (2, 12, 16, 2 16), developmental genetics (16, 163, 164), and molecular biology (103; also see 10, 11 1). "Developmental constraints" is a term symptomatic of this progress, though an unfortunate one because it seems to imply that the main effect of