Thermal ramping rate influences evolutionary potential and species differences for upper thermal limits in Drosophila

P>1. Thermal tolerance is a key factor limiting insect distributions, but there is limited information on the ability of species to evolve different thermal limits. Recent studies indicate that the experimental protocol influences upper limits, with slower but ecologically relevant rates of warming lowering estimates of tolerance. These effects could also influence genetic and environmental variances that define evolutionary potential. 2. To determine the influence of experimental protocol on estimates of narrow sense heritability (h2(n)) and other measures of evolutionary potential in Drosophila melanogaster, we conducted family studies on knockdown time when flies were immediately exposed to a high temperature (static) or when temperature was increased to an upper limit (ramping). 3. Estimates of variance components in two populations were obtained using the animal model approach that incorporates information from all relationships among relatives. Coefficients of variation were higher when flies were exposed to a static stress, as were estimates of additive genetic variance and measures of evolvability where genetic variances were standardized by trait means. In contrast, levels of environmental variance were higher under ramping conditions. These effects mean that the narrow sense heritability of thermal resistance was low under slow ramping and did not differ significantly from zero. 4. Differences in thermal limits under both methods were detected among Drosophila species. There was a significant positive relationship between the fast and slow ramping estimates of thermal resistance across species after correction for phylogeny, suggesting similar underlying mechanisms or a history of correlated evolution. However, this result was caused by the strong influence of two taxa. 5. These results suggest that natural populations exhibit lower adaptive potential for upper thermal limits under ramping than estimated from traditional (static) estimates of heat resistance. Even the highly adaptable Drosophila melanogaster appears to have little evolutionary potential to extend its upper thermal range under ramping conditions although species have diverged for this measure.