Ectotherms are both the dominant group of organisms on the planet and the most vulnerable to climate warming. Most ectotherms exhibit phenotypic plasticity, but whether plasticity can evolve fast enough to keep up with the rapid pace of warming is, as yet, unknown. The first step in addressing this question is to determine whether there is sufficient genetic variation in the life history traits underlying fitness. Theory predicts that strong stabilizing selection should erode genetic variability in organisms’ typical thermal environment, but that variants producing no phenotypic changes should accumulate. Such cryptic genetic variation (CGV) may prove crucial in adapting to new thermal environments imposed by warming. Our goal is to test the roles of standing and cryptic genetic variation in the evolution of thermal plasticity. We present results from a multigenerational laboratory experiment on the cowpea seed beetle Callosobruchus maculatus, a cosmopolitan pest of stored products. We measured the thermal reaction norms of birth, maturation and mortality rates at five temperatures between 14-40 °C. We quantified the heritability of these life history traits to determine whether heritability, and hence the genetic basis of life history trade-offs, changes when the beetles are exposed to low and high temperature extremes.
Results/Conclusions
We find that C. maculatus’ thermal reaction norms conform to the qualitative shapes exhibited by other ectotherms. For instance, per capita mortality increases monotonically with temperature, the maturation rate is unimodal and left-skewed, and the birth rate is symmetric, unimodal. Our experiments show that warmer temperatures lead to a large birth peak followed almost immediately by death, which deviates from the typical pattern of a peak followed by a slower decline in birth rate with age. We are currently estimating the heritabilities of birth, maturation and mortality rates to determine whether trade-off strength decreases with increasing temperature due to the presence of CGV, or whether it increases due to the lack thereof. We are also developing a trait-based model with variable developmental delays and mechanistic descriptions of life history trait responses to temperature, which we will parameterize with experimental data to predict how trade-offs and genetic variation shape reaction norm evolution and therefore, population dynamics in the face of warming. Our work has the potential to provide important insights into the role of life history traits in driving adaptation to novel thermal environments, which will be important to understand future adaptation and persistence of species under climate change.