Species that are habitat specialists make up much of biodiversity, but the evolutionary factors that limit their distributions have rarely been considered. We show that in Drosophila, narrow and wide ranges of desiccation and cold resistance are closely associated with the distributions of specialist and generalist species, respectively. Furthermore, our data show that narrowly distributed tropical species consistently have low means and low genetic variation for these traits as compared with those of widely distributed species after phylogenetic correction. These results are unrelated to levels of neutral variation. Thus, specialist species may simply lack genetic variation in key traits, limiting their ability to adapt to conditions beyond their current range. We predict that such species are likely to be constrained in their evolutionary responses to future climate changes.

1. Increases in average temperature and the frequency of extreme temperature events are likely to pose a major risk to species already close to their upper physiological thermal limits. The extent to which thermal phenotypic plasticity can buffer these changes and whether plasticity is constrained by basal tolerance levels remains unknown. 2. We examined the effect of developmental temperature under both constant and fluctuating thermal regimes (developmental acclimation), as well as short-term heat hardening, on upper thermal limits (CTmax) in a tropical and temperate population of Drosophila melanogaster. 3. We found evidence for thermal plasticity in response to both developmental acclimation and hardening treatments; CTmax increased at warmer developmental temperatures and with a prior heat hardening treatment. However, hardening and acclimation responses were small, improving CTmax by a maximum of 1Á01°C. These results imply that overheating risk will only be minimally reduced by plasticity. 4. We observed significant associations between developmental temperature and both basal CTmax and hardening capacity (a measure of the extent of the plastic response). Basal CTmax increased, while hardening capacity decreased, with increasing developmental acclimation temperature. This indicates that increases in basal heat resistance at warmer temperatures may come at the cost of a reduced capacity to harden. 5. While plasticity in CTmax is evident in both populations of D. melanogaster we studied, plastic increases in upper thermal limits, particularly at warmer temperatures, may not be sufficient to keep pace with temperature increases predicted under climate change.

Understanding and predicting how adaptation will contribute to species' resilience to climate change will be paramount to successfully managing biodiversity for conservation, agriculture, and human health‐related purposes. Making predictions that capture how species will respond to climate change requires an understanding of how key traits and environmental drivers interact to shape fitness in a changing world. Current trait‐based models suggest that low‐ to mid‐latitude populations will be most at risk, although these models focus on upper thermal limits, which may not be the most important trait driving species' distributions and fitness under climate change. In this review, we discuss how different traits (stress, fitness and phenology) might contribute and interact to shape insect responses to climate change. We examine the potential for adaptive genetic and plastic responses in these key traits and show that, although there is evidence of range shifts and trait changes, explicit consideration of what underpins these changes, be that genetic or plastic responses, is largely missing. Despite little empirical evidence for adaptive shifts, incorporating adaptation into models of climate change resilience is essential for predicting how species will respond under climate change. We are making some headway, although more data are needed, especially from taxonomic groups outside of Drosophila, and across diverse geographical regions. Climate change responses are likely to be complex, and such complexity will be difficult to capture in laboratory experiments. Moving towards well designed field experiments would allow us to not only capture this complexity, but also study more diverse species.

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