The genetics of phenotypic plasticity I. Heritability

Methods for estimating the genetic component of phenotypic plasticity are presented. In the general case of clonal replicates or full‐sibs raised in several environments, the heritability of plasticity can be measured as the ratio of the genotype‐environment interaction variance to the total phenotypic variance. In the special case of only two environments plasticity also can be measured as the difference among environments in genotype or family means. In that case, the heritability of plasticity can be measured as either a ratio of variance components or as the slope of a parent‐offspring regression. The general measure suffers because no least‐square standard errors have been developed, although they can be calculated by maximum‐likelihood or bootstrapping techniques. For the other two methods least‐square standard errors can be calculated but require very large experiments for statistical significance to be achieved. The heritability measures are compared using data on plasticity of thorax size in response to temperature in Drosophila melanogaster . The heritability estimates are all in close agreement. Models of the evolution of phenotypic plasticity have treated it as a trait in its own right and as a cross‐environment genetic correlation. Although the first approach is the one used here, neither one is preferred.