Developmental Plasticity: Developmental Conversion versus Phenotypic Modulation

The developmental mechanisms by which the environment may alterthe phenotype during development are reviewed. Developmentalplasticity may be of two forms: developmental conversion orphenotypic modulation. In developmental conversion, organismsuse specific environmental cues to activate alternative geneticprograms controlling development. These alternative programsmay either lead to alternative morphs, or may lead to the decisionto activate a developmental arrest. In phenotypic modulation,nonspecific phenotypic variation results from environmentalinfluences on rates or degrees of expression of the developmentalprogram, but the genetic programs controlling development arenot altered. Modulation, which is not necessarily adaptive,is probably the common form of environmentally induced phenotypicvariation in higher organisms, and adaptiveness of phenotypicplasticity therefore cannot be assumed unless specific geneticmechanisms can be demonstrated. The genetic mechanisms by whichdevelopmental plasticity may evolve are reviewed, and the relationshipbetween developmental plasticity and evolutionary plasticityare examined.