Plasticity of size and growth in fluctuating thermal environments: comparing reaction norms and performance curves

Ectothermic animals exhibit two distinct kinds of plasticity in response to temperature: Thermal performance curves (TPCs), in which an individual's performance (e.g., growth rate) varies in response to current temperature; and developmental reaction norms (DRNs), in which the trait value (e.g., adult body size or development time) of a genotype varies in response to developmental temperatures experienced over some time period during development. Here we explore patterns of genetic variation and selection on TPCs and DRNs for insects in fluctuating thermal environments. First, we describe two statistical methods for partitioning total genetic variation into variation for overall size or performance and variation in plasticity, and apply these methods to available datasets on DRNs and TPCs for insect growth and size. Our results indicate that for the datasets we considered, genetic variation in plasticity represents a larger proportion of the total genetic variation in TPCs compared to DRNs, for the available datasets. Simulations suggest that estimates of the genetic variation in plasticity are strongly affected by the number and range of temperatures considered, and by the degree of nonlinearity in the TPC or DRN. Second, we review a recent analysis of field selection studies which indicates that directional selection favoring increased overall size is common in many systems-that bigger is frequently fitter. Third, we use a recent theoretical model to examine how selection on thermal performance curves relates to environmental temperatures during selection. The model predicts that if selection acts primarily on adult size or development time, then selection on thermal performance curves for larval growth or development rates is directly related to the frequency distribution of temperatures experienced during larval development. Using data on caterpillar temperatures in the field, we show that the strength of directional selection on growth rate is predicted to be greater at the modal (most frequent) temperatures, not at the mean temperature or at temperatures at which growth rate is maximized. Our results illustrate some of the differences in genetic architecture and patterns of selection between thermal performance curves and developmental reaction norms.