The environment plays a crucial role in the developing organism, first in defining the developmental trajectory from genotype to phenotype, then by modifying that trajectory by natural selection. Nearly all traits exhibit some degree of phenotypic plasticity: the capacity to change, or to develop in response to, the environment. The plasticity of a trait can itself evolve, and some of the most specialized adaptations include evolved responses to environmental variation. Plasticity has long been theorized to potentiate adaptive evolution, by environmental induction of phenotypes that boosts the potential for subsequent genetic evolution or by revealing cryptic alleles in new environments that in turn generate new adaptive phenotypes. A plastic trait may vary continuously, which can be described by norms of reaction, or it may produce discrete types as a polyphenism, a codified adaptive response to specific environmental signals. The concept of plasticity can also be applied to variation in phenotype associated with a single genotype in a single environment. Such microenvironmental plasticity defines in part the robustness of a trait. In the evolution of complex traits, tension between plasticity and its opposite, canalization, may be crucial for rapid evolution, adaptation, and the emergence of novelty.
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