Table 1.

Definitions of terms used in studies of plasticity and selection on plasticity.

term	definition (as used here)
phenotype	the observed realization of a single or multiple traits (characteristics) of an individual organism, resulting from both genetic and environmental influences
genotype	the realization of genetic differences among individuals of a species, which could be variation in a particular gene, nucleotide, or genome region (this could constitute an individual, clone, family, or line)
fitness	the contribution of an individual or genotype to the gene pool of subsequent generations. In practical terms, one of the best measures of an individual's fitness may be the number of surviving offspring produced through an organism's entire life (‘lifetime reproductive success’, LRS), but fitness is often assessed by components of LRS such as survival or fecundity rates
global fitness	the fitness of a genotype averaged or summed across the range of environments experienced
(phenotypic) plasticity	the ability of a single genotype to express different phenotypes under different environmental conditions. Often measured as the slope of a regression of the phenotypic trait across environments. Property of a reaction norm when the reaction norm slope is non-zero
reaction norm	describes the shape or form of the phenotypic response to environment: the value of the trait as a function of the environmental variable
acclimation	plasticity that encompasses short term physiological changes in response to a changed environment
canalization	weak or no phenotypic response to environmental variation (i.e. lack of plasticity in a trait)
genotype × environment interactions (G×E)	interactions between genotypes and the environment determine the phenotype such that reaction norm slopes vary among genotypes
adaptive plasticity	the scenario when a plastic response to the environment results in higher fitness than if a genotype maintains a constant phenotype across environments
non-adaptive plasticity	the scenario when a genotype shows a plastic response to the environment but this does not affect fitness
maladaptive plasticity	the scenario when a genotype shows a plastic response to the environment and this results in a decrease in fitness
selection	the primary mechanism leading to adaptive evolution. Measured as the covariance between a phenotypic trait (or plasticity) and fitness
directional selection	selection that changes the mean trait value, typically measured as the covariance between trait and fitness (selection differential, S), or the linear slope of the regression of fitness on the trait (selection gradient, β)
stabilizing selection	selection that favours intermediate values of a trait, typically measured as the quadratic selection gradient (γ)
selection on plasticity	an association between phenotypic plasticity (slope of the reaction norm) and fitness, i.e. selection on the response rather than the value of the phenotypic trait itself