Reaction Norms

A 'reaction norm' describes the sensitivity of an organism, or of a set of organisms of the same genotype (e.g., the members of a clone), to some specific environmental variable. It quantifies phenotypic change (or lack of change) in a selected aspect ofthe phenotype as a function of variation in the environmental factor of interest (Figure 1). The reaction norm approach treats environmental sensitivity as a continuously variable response. Discrete, threshold-governed responses like conditional alternative phenotypes in morphology (e.g., polyphen-isms) and behavioral traits are not usually described in terms of reaction norms.

Again, it is quite common, in discussions ofindividual differences in norms ofreaction, to describe individuals as genotypes, implying that the differences in their norms of reaction are due to differences in genotype, or genetic variation. It cannot be overemphasized, however, that degree of plasticity (responsiveness) and the norm of reaction are properties of the 'phenotype', and the

Temperature (°C)

Figure 1 Reaction norms. Modified from West-Eberhard MJ (2003) Developmental Plasticity and Evolution. New York: Oxford University Press, after Gupta AP and Lewontin R (1982) A study of reaction norms in natural populations of Drosophila pseudoobscura. Evolution 36: 934-948.

Temperature (°C)

Figure 1 Reaction norms. Modified from West-Eberhard MJ (2003) Developmental Plasticity and Evolution. New York: Oxford University Press, after Gupta AP and Lewontin R (1982) A study of reaction norms in natural populations of Drosophila pseudoobscura. Evolution 36: 934-948.

phenotype, including its responsiveness to a given environmental variable, can be influenced by environmental conditions as well as by the genotype during development. Thus, while the terminology suggests that an analysis of reaction norms can separate nature from nurture (genetic from environmental influence), the separation is in fact not so clear. This is particularly important in behavioral ecology, where the social environment (e.g., the presence of other, differentially aggressive or dominant individuals, as well as opportunities for imitation and learning) and maternal effects (e.g., hormonal factors transmitted to offspring) -along with genotype - can affect the behavioral responses of individuals.

Behavioral ecologists seldom use a reaction norm approach, even in quantitative genetic analyses. This is perhaps because behavioral plasticity so often involves the expression of discrete actions, or sequences of them. So quantitative genetic studies of behavior more often focus on heritability and evolutionary responses to selection on discrete traits rather than their continuously variable properties.

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