Genotype×environment interactions and evolution of plasticity
A reaction norm describes how a single genotype expresses different phenotypes across an environmental gradient.
G×E interactions arise when genotypes differ in slope (plasticity) — crossing norms indicate disordinal interaction.
Highly plastic genotypes (steep slopes) are favored when the environment varies; canalized genotypes (flat norms)
win in stable environments. Fitness curves show mean fitness at each environment; selection pressure drives
evolution of plasticity (slope of reaction norms).