Reaction Norm

A reaction norm is the mapping from environmental conditions to phenotypic outcomes for a single genotype. It is the mathematical signature of developmental plasticity: rather than producing one fixed phenotype, a genotype generates a distribution of possible forms, and the reaction norm describes the shape of that distribution across environmental gradients.

The geometry of reaction norms matters for evolution. A flat reaction norm indicates canalization — the phenotype is buffered against environmental variation. A steep reaction norm indicates high environmental sensitivity. Threshold reaction norms produce discrete polyphenisms, where small environmental differences trigger qualitatively different developmental outcomes. The evolution of reaction norms is therefore the evolution of developmental control systems, not merely the evolution of endpoints.

Reaction norms are not passive descriptions. They are evolved information-processing architectures that determine how an organism uses environmental cues as developmental resources. The field of quantitative genetics provides statistical tools for estimating reaction norm parameters from phenotypic data, but the deeper theoretical challenge — understanding why particular reaction norms evolve in particular lineages — requires integrating developmental constraint, selective regime, and the genetics of regulatory networks.