Phenotypic plasticity is the ability of an organism to change in response to stimuli or inputs from the environment. Synonyms are phenotypic responsiveness, flexibility, and condition sensitivity. The response may or may not be adaptive, and it may involve a change in morphology, physiological state, or behavior, or some combination of these, at any level of organization, the phenotype being all of the characteristics of an organism other than its genes.
There is a great potential for confusion regarding definitions of phenotypic plasticity, including this one. Even though the phenotype is defined here to exclude the genome, in fact phenotypic plasticity always involves a change in gene expression or gene-product use (morphological, physiological, and behavioral traits always being products, in part, of gene expression). Some definitions of phenotypic plasticity refer to the environmental sensitivity of a genotype, a potentially confusing terminology because it uses the word 'genotype' to mean 'organism bearing a particular gene or set of genes' and may be mistakenly understood to imply that the organism's genome, rather than its phenotype (whose nature has been influenced by both environment and genome), responds to the environment. Such definitions attempt to convey the correct idea that phenotypic plasticity involves a change in some aspect of the phenotype without a change in the individual's genes, or the genetic underpinnings ofa particular trait. Thus, one could examine the phenotypes of genetically identical individuals and find that they differ phenotypically in different environments, indicating phenotypic plasticity for particular traits. Or, conversely, one could begin with individuals from phenotypically different populations found in different environments, and subject them to the same environment in a 'common garden' experiment designed to control environmental variables, and see to what degree the phenotypic differences are maintained, indicating the degree to which genetic differences between the populations, rather than plasticity, account for the phenotypic differences between them.
Phenotypic plasticity can be a source of 'noise', or confounding variation, in genetic experiments. Such experiments are therefore often designed to control environmental variation and reduce the effects of plasticity. But research in behavioral ecology, rather than eliminating plasticity, often focuses on it. Behavioral phe-notypes are eminently plastic, often in adaptively appropriate ways. Plasticity of behavioral responses -the occurrence of complex, condition-sensitive behavioral repertoires - can increase the diversity of phenotypes within populations. But behavioral plasticity can also reduce phenotypic variation, as when behavioral responses are stability increasing or homeostatic in their effects. For example, individuals may adopt postures or move to locations that help reduce extremes of variation in body temperature. Homeostatic behavior can be quite elaborate: some social insects engage in behaviors (water transport and application to nest surfaces, followed by fanning wing movements that promote evaporative cooling) and effectively lower the temperature of nest and brood.
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