The evolutionary significance of phenotypic plasticity: phenotypic sources of variation among organisms can be described by developmental switches and reaction norms

Variation, the fuel that feeds evolutionary change, originates at the levels of both the genotype and the phenotype. Genetically identical organisms reared under different conditions may display quite distinct characteristics. Until recently, the types and sources of such phenotypic variation have been given little consideration in evolutionary theory. But a knowledge of the mechanisms and developmental patterns underlying phenotypic variation is crucial to the understanding of important evolutionary phenomena. Therefore some biologists are predicting an increasing focus on this variation leading to a Renaissance of the Phenotype.