2 Mechanism of evolutionary response to changing environments: genetic variation, heritability, and selection
The genetic architecture of a trait under selection will in part determine the potential for adaptive evolution, as well as the impact of plasticity on adaptive tracking. The rate at which a trait responds to selection is determined by the number of genes that affect the trait, and the magnitude of their impact. Traits determined by many genes of small effect can respond rapidly if most of the loci are variable at intermediate frequencies[42–44]. To understand plasticity’s role in adaptive evolution, we first need to consider how different environmental changes impact the mechanisms of evolutionary tracking in the absence of plasticity. For adaptive evolution to occur, natural selection must act on variation in a heritable trait that affects fitness. Most traits that mediate population dynamics are determined by many genes each of which has small effects: they are quantitative traits, not Mendelian, traits. One way to assess whether or not a quantitative trait may evolve is with the breeder’s equation , which states that the change in a trait equals the selection differential multiplied by its narrow-sense heritability. Heritability is a function of both genetic variation[45,46] and the environment in which that variation is expressed[47]. The contributions of environmental change/variation to phenotypic and genetic variation are often relegated to an error term that absorbs unmeasured uncertainties in quantitative genetic models([48], but see [49]). By making explicit the ways in which rate of mean change, variability, and temporal autocorrelation in the environment each influence heritability, genetic variation, and selection, we aim to understand the ability of genetic change to track an evolutionary optimum in changing environments, and the role of plastic responses in decreasing phenotypic lag.