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.