The degree of genetic variability exhibited by the thermal reaction norms of life history traits is an important question in evolutionary ecology. Strong stabilizing selection toward optimal performance in a given thermal environment should erode genetic variability, but genetic variants producing no phenotypic changes under ambient conditions could accumulate. Such cryptic genetic variation (CGV) has no fitness effects in a typical environment, but may be important for adaptation to new extreme environments such as will result from climate warming.
We present results from a laboratory experiment in which we measure the genetic variability in the reaction norms of life history traits (i.e., mortality, development, and fecundity). We reared full sibling families of the harlequin bug, an herbivore of the California coastal sage scrub, at five temperatures within its natural range (18 – 30°C). We tested the following hypotheses. If stabilizing selection reduces genetic variability and CGV is the main source of variability, variance between genetic lines should be greater at extreme temperatures compared with typical temperatures. Traits under internal biochemical constraints (e.g., development) should exhibit lower levels of standing genetic variation at typical temperatures than traits subject to external selection pressures such as resources and natural enemies (e.g., fecundity, mortality).
Using quadratic regression, we fit thermal response curves for each life history trait treating genetic line as a random effect. Reaction norms for fecundity, development and mortality conform to functions derived from underlying biochemical processes. Mortality increases monotonically with increasing temperature, maturation rate is unimodal and left-skewed, and fecundity is symmetric unimodal (Gaussian). Variance among genetic lines was higher overall for mortality and less so for fecundity, but there was little variance among genetic lines for development, which is likely due to strong stabilizing selection and physiological constraints. However, there was higher variance, measured by the coefficient of variation, at extreme temperatures compared with optimal temperatures for development. A similar but weaker pattern was found for fecundity and survival, likely due to increased variation overall.
Our results emphasize that the genetic variability underlying plasticity varies by trait and contribute to our larger goal of quantifying the genetic variability underlying the thermal response of fitness. Constrained traits may be under stronger stabilizing selection than more plastic traits, but CGV may exist that allows for a broader range of phenotypes in new environments. Given climate warming CGV may be particularly important for constrained traits to help organisms respond rapidly to warming environments.