Reaction norm
A reaction norm shows how a genotype produces different phenotypes across an environmental range.
What a reaction norm is
A reaction norm, also called a norm of reaction, is a way to describe how a genotype expresses a trait across environments. The trait might be height, development time, body size, enzyme activity, flowering date, behavior, or fitness. Instead of asking what phenotype a genotype has in isolation, a reaction norm asks how that phenotype changes when the environment changes.
How to read the graph
A typical reaction-norm graph places an environmental variable on the horizontal axis and a phenotype on the vertical axis. Each line or curve represents one genotype, family, strain, clone, or individual. A flat line means the phenotype stays similar across the measured environments. A sloped or curved line means the phenotype responds to the environmental gradient.
Plasticity in the slope
The slope of a reaction norm is often used as a simple measure of phenotypic plasticity. If two genotypes have the same average trait value but one changes strongly with temperature while the other barely changes, their plasticity differs. Curved reaction norms add another layer: an organism may respond strongly only after a threshold, or it may level off near a physiological limit.
Genotype-by-environment interaction
Reaction norms are especially useful when lines are not parallel. Nonparallel lines mean genotypes do not respond to the environment in the same way. Lines may cross, showing that one genotype has the higher trait value in one environment while another has the higher value elsewhere. This pattern is called genotype-by-environment interaction, and it matters in ecology, breeding, medicine, and evolution.
Continuous and threshold responses
Many reaction norms are drawn as smooth changes across a gradient, but not all responses are gradual. Some traits switch between alternative forms when an environmental cue crosses a threshold. Developmental biology often contrasts reaction norms with polyphenism: reaction norms emphasize a range of trait values, while polyphenism emphasizes distinct environmentally triggered forms.
How researchers estimate them
Researchers estimate reaction norms by raising related genotypes, clones, strains, families, or populations in controlled environments, or by measuring the same individuals across changing conditions. Common-garden experiments, reciprocal transplants, breeding trials, and longitudinal field studies can all reveal whether trait differences come from genotype, environment, or their interaction.
Evolutionary meaning
A reaction norm can itself evolve when genetic variation affects environmental responsiveness. Selection may favor a broad, flexible response in variable environments, a flatter response in stable environments, or different reaction-norm shapes in different habitats. Because natural selection acts on phenotypes, the shape of the genotype-to-phenotype relationship can influence evolutionary outcomes.
Why it matters
Reaction norms make trait variation easier to interpret. They help explain why the same crop variety performs differently across farms, why animals mature at different sizes or ages in different habitats, and why laboratory phenotypes may not match field phenotypes. In a changing climate, reaction norms also help scientists ask which organisms can respond quickly and which may need genetic adaptation to keep pace.