SummaryThe acute thermal tolerance of ectotherms has been measured in a variety of ways; these include assays where organisms are shifted abruptly to stressful temperatures and assays where organisms experience temperatures that are ramped more slowly to stressful levels. Ramping assays are thought to be more relevant to natural conditions where sudden abrupt shifts are unlikely to occur often, but it has been argued that thermal limits established under ramping conditions are underestimates of true thermal limits because stresses due to starvation and/or desiccation can arise under ramping. These confounding effects might also impact the variance and heritability of thermal tolerance. We argue here that ramping assays are useful in capturing aspects of ecological relevance even though there is potential for confounding effects of other stresses that can also influence thermal limits in nature. Moreover, we show that the levels of desiccation and starvation experienced by ectotherms in ramping assays will often be minor unless the assays involve small animals and last for many hours. Empirical data illustrate that the combined effects of food and humidity on thermal limits under ramping and sudden shifts to stressful conditions are unpredictable; in Drosophila melanogaster the presence of food decreased rather than increased thermal limits, whereas in Ceratitis capitata they had little impact. The literature provides examples where thermal limits are increased under ramping presumably because of the potential for physiological changes leading to acclimation. It is unclear whether heritabilities and population differentiation will necessarily be lower under ramping because of confounding effects. Although it is important to clearly define experimental methods, particularly when undertaking comparative assessments, and to understand potential confounding effects, thermotolerance assays based on ramping remain an important tool for understanding and predicting species responses to environmental change. An important area for further development is to identify the impact of rates of temperature change under field and laboratory conditions.

Summary1. 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 (h 2 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.

Dispersal and phenotypic plasticity are two main ways for species to deal with rapid changes of their environments. Understanding how genotypes (G), environments (E), and their interaction (genotype and environment; G × E) each affects dispersal propensityis therefore instrumental for predicting the ecological and evolutionary responses of species under global change. Here we used an actively dispersing ciliate to quantify the contributions of G, E, and G × E on dispersal propensity, exposing 44 different genotypes to three different environmental contexts (densities in isogenotype populations). Moreover, we assessed the condition dependence of dispersal, that is, whether dispersal is related to morphological, physiological, or behavioral traits. We found that genotypes showed marked differences in dispersal propensity and that dispersal is plastically adjusted to density, with the overall trend for genotypes to exhibit negative density-dependent dispersal. A small, but significant G × E interaction indicates genetic variability in plasticity and therefore some potential for dispersal plasticity to evolve. We also show evidence consistent with condition-dependent dispersal suggesting that genotypes also vary in how individual condition is linked to dispersal under different environmental contexts thereby generating complex dispersal behavior due to only three variables (genes, environment, and individual condition). K E Y W O R D S :Condition-dependent dispersal, context-dependent dispersal, density dependence, genotype × environment, phenotypic plasticity.Dispersal is a key trait for the ecological and evolutionary dynamics of a given species. It is broadly defined as the exchange of individuals between the natal and one or more reproductive sites (Matthysen 2012). Dispersal is crucially important for the spatial functioning of (meta)populations and (meta)communities, and can have profound impacts on both the ecological and evolutionary dynamics of populations and species (Hanski and Gaggiotti 2004;Leibold et al. 2004;Holyoak et al. 2005). Dispersal can compensate for local extinctions by recolonization of vacant habitats (Hanski and Gaggiotti 2004) or simply augment or "rescue" small populations (Brown and Kodric-Brown 1977). The exchange of individuals between populations influences gene flow and hence has implications for local adaptation, drift, genetic diversity, and population divergence of species (Ronce 2007). Understanding the factors that shape dispersal strategies and their genetic underpinnings is of great importance as dispersal is a crucially important means for species to mitigate global change both through direct movement and the subsequent gene flow that may facilitate local adaptation to new conditions (Berg et al. 2010;Chevin et al. 2010;Chaine and Clobert 2012).

Summary1. Acclimation and hardening represent examples of phenotypic plasticity, the extent to which phenotypes produced by the same genotype vary under different environments. Widespread species are expected to differ in thermal plasticity from narrowly distributed tropical species, but this has rarely been tested particularly when species are reared under the same conditions. 2. We investigated acclimation and hardening responses of 11 widespread or tropically restricted Drosophila species from Australia using estimates of heat resistance where temperatures were increased suddenly (static measure) or slowly (ramping measure), and after controlling for phylogenetic relatedness. We predicted that restricted species would show little acclimation regardless of the method used, whilst widespread species would respond well after a hardening treatment (35°C for 1 h) particularly under ramping. 3. These predictions were partially supported. There was a tendency for the tropically restricted species to be less plastic than the widespread species, although variation among species within the two groups was generally greater than between the groups. For acclimation and stress resistance measured under ramping acclimation, there was an association between the southernmost latitude at which species were found (reflecting variability in climatic conditions they encountered) and knockdown resistance after controlling for phylogeny. There was also evidence of significant divergence from the ancestral state in the ramping trait, likely reflecting a history of direct or indirect selection for ramping knockdown resistance in Drosophila. 4. There was a significant negative association between basal resistance and hardening capacity for static acclimation in the widespread species, suggesting a limit to the extent that plastic responses vary independently of basal resistance. 5. The reduced plastic response in tropically restricted species and negative association between hardening and basal resistance suggest a limit to the effectiveness of plastic responses in changing upper thermal limits for countering increases in thermal stress under future climate change.