Developmental Plasticity: Developmental ConversionversusPhenotypic Modulation

Abstract

The developmental mechanisms by which the environment may alter the phenotype during development are reviewed. Developmental plasticity may be of two forms: developmental conversion or phenotypic modulation. In developmental conversion, organisms use specific environmental cues to activate alternative genetic programs controlling development. These alternative programs may either lead to alternative morphs, or may lead to the decision to activate a developmental arrest. In phenotypic modulation, nonspecific phenotypic variation results from environmental influences on rates or degrees of expression of the developmental program, but the genetic programs controlling development are not altered. Modulation, which is not necessarily adaptive, is probably the common form of environmentally induced phenotypic variation in higher organisms, and adaptiveness of phenotypic plasticity therefore cannot be assumed unless specific genetic mechanisms can be demonstrated. The genetic mechanisms by which developmental plasticity may evolve are reviewed, and the relationship between developmental plasticity and evolutionary plasticity are examined.