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Temperature and water availability are independently hypothesised to be important abiotic drivers of the evolution of metabolic rates and gas exchange patterns, respectively. Specifically, the metabolic cold adaptation hypothesis (MCA) predicts that cold environments select for faster metabolic rates to counter the thermodynamics of biochemical reactions while the hygric hypothesis predicts that dry environments select for discontinuous gas exchange to reduce water loss. Although these two hypotheses consider different physiological traits and how they vary along different abiotic gradients, metabolic rate drives gas exchange patterns in insects meaning these two traits are inherently linked. Despite this link, the MCA and hygric hypotheses are rarely considered together and the extent to which metabolic rates and respiratory patterns vary and co-vary with temperature and aridity along climatic gradients remains unclear. We tested the MCA and hygric hypotheses within a species of endemic Fijian bee, Homalictus fijiensis, across an altitudinal gradient of 1100 m, and among four Fijian bee species, including H. fijiensis, that inhabit different altitudinal bands. In Fiji, environmental temperature is ~5°C lower in the central highlands than in the coastal lowlands with the highlands receiving ~100 mm of additional precipitation than the lowlands each month. We found an MCA-like pattern within H. fijiensis and among Fijian bee species, where metabolic rate decreased with increasing temperature, but precipitation also explained variation in metabolic rate. We also found support for the hygric hypothesis within H. fijiensis and across species. Lastly, we found that the relationship between metabolic rate and ventilation rate changed depending on precipitation of the driest month within H. fijiensis and among bee species. Bees that inhabited wet regions had low ventilation rates of greater volume but bees that inhabited drier locations had rapid ventilations of smaller volume potentially as a mechanism to exchange gas without desiccating. As the correlation between metabolic rate and ventilation rate changes across the precipitation gradient, it is possible that these two traits can evolve independently of each other in response to abiotic variables despite being positively correlated.

Studies suggest that many species are already living close to their upper physiological thermal limits. Phenotypic plasticity is thought to be an important mechanism for species to counter rapid environmental change, yet the extent to which plastic responses may buffer projected climate change - and what limits the evolution of plasticity - is still unclear. The tolerance-plasticity trade-off hypothesis predicts that the evolution of plasticity may be constrained by a species' thermal tolerance. Empirical evidence is equivocal, but we argue that inconsistent patterns likely reflect problems in experimental design/analysis, limiting our ability to detect and interpret trade-off patterns. Here, we address why we may, or may not see tolerance-plasticity trade-offs and outline a framework addressing current limitations, focusing on understanding the underlying mechanisms.

Sex-based differences in physiological traits may be influenced by both evolutionary and environmental factors. Here we used male and female flies from >80 Drosophila species reared under common conditions to examine variance in a number of physiological traits including size, starvation, desiccation and thermal tolerance. Sex-based differences for desiccation and starvation resistance were comparable in magnitude to those for size, with females tending to be relatively more resistant than males. In contrast thermal resistance showed low divergence between the sexes. Phylogenetic signal was detected for measures of divergence between the sexes, such that species from the Sophophora clade showed larger differences between the sexes than species from the Drosophila clade. We also found that sex-based differences in desiccation resistance, body size and starvation resistance were weakly associated with climate (annual mean temperature/precipitation seasonality) but the direction and association with environment depended on phylogenetic position. The results suggest that divergence between the sexes can be linked to environmental factors, while an association with phylogeny suggests sex-based differences persist over long evolutionary time-frames.

The resilience of ecosystem function under global climate change is governed by individual species vulnerabilities and the functional groups they contribute to (e.g. decomposition, primary production, pollination, primary, secondary and tertiary consumption). Yet it remains unclear whether species that contribute to different functional groups, which underpin ecosystem function, differ in their vulnerability to climate change.It is important to examine if functional group vulnerability differs across space (e.g. tropical vs temperate latitudes) to determine if some regions will be more vulnerable to loss of ecosystem function than others, and to examine whether localized effects of particular community compositions override global patterns of functional group vulnerability.We used existing upper thermal limit data across a range of terrestrial species (N = 1,743) to calculate species warming margins (degrees distance between a species upper thermal limit and the maximum environmental temperature they inhabit), as a metric of climate change vulnerability, to determine whether species that comprise different functional groups exhibit differential vulnerability to climate change, and if vulnerability trends change across geographic space.We found that primary producers had the broadest warming margins across the globe (μ = 21.85 °C) and that tertiary consumers had the narrowest warming margins (μ = 4.37 °C), where vulnerability tended to increase with trophic level.Species that contribute towards primary production were more vulnerable in low-latitude than mid-latitude regions, but warming margins across all other functional groups did not differ across regions when evolutionary history was considered. However, when evolutionary history was excluded from the analyses (as closely related species often play similar functional roles within ecosystems demonstrating true variation in functional group warming margins) we found that pollinators are more vulnerable in mid-latitude regions and that primary producers are more vulnerable in low-latitude environments.This study provides a critical first step in linking individual species vulnerabilities with whole ecosystem responses to climate change.

The resilience of ecosystem function under global climate change is governed by individual species vulnerabilities and the functional groups they contribute to (e.g. decomposition, primary production, pollination, primary, secondary and tertiary consumption). Yet it remains unclear whether species that contribute to different functional groups, which underpin ecosystem function, differ in their vulnerability to climate change. We used existing upper thermal limit data across a range of terrestrial species (N = 1701) to calculate species warming margins (degrees distance between a species upper thermal limit and the maximum environmental temperature they inhabit), as a metric of climate change vulnerability. We examined whether species that comprise different functional groups exhibit differential vulnerability to climate change, and if vulnerability trends change across geographic space while considering evolutionary history. Primary producers had the broadest warming margins across the globe (μ = 18.72 °C) and tertiary consumers had the narrowest warming margins (μ = 9.64 °C), where vulnerability tended to increase with trophic level. Warming margins had a nonlinear relationship (second-degree polynomial) with absolute latitude, where warming margins were narrowest at about 33°, and were broader at lower and higher absolute latitudes. Evolutionary history explained significant variation in species warming margins, as did the methodology used to estimate species upper thermal limits. We investigated if variation in body mass across the trophic levels could explain why higher trophic level organisms had narrower warming margins than lower trophic level organisms, however, we did not find support for this hypothesis. This study provides a critical first step in linking individual species vulnerabilities with whole ecosystem responses to climate change.

Anthropogenic climate change and invasive species are two of the greatest threats to biodiversity, affecting the survival, fitness and distribution of many species around the globe. Invasive species are often expected to have broad thermal tolerance, be highly plastic, or have high adaptive potential when faced with novel environments. Tropical island ectotherms are expected to be vulnerable to climate change as they often have narrow thermal tolerance and limited plasticity. In Fiji, only one species of endemic bee, Homalictus fijiensis, is commonly found in the lowland regions, but two invasive bee species, Braunsapis puangensis and Ceratina dentipes, have recently been introduced into Fiji. These introduced species pollinate invasive plants and might compete with H. fijiensis and other native pollinators for resources. To test whether certain performance traits promote invasiveness of some species, and to determine which species are the most vulnerable to climate change, we compared the thermal tolerance, desiccation resistance, metabolic rate and seasonal performance adjustments of endemic and invasive bees in Fiji. The two invasive species tended to be more resistant to thermal and desiccation stress than H. fijiensis, while H. fijiensis had greater capacity to adjust their CTmax with season, and H. fijiensis females tended to have higher metabolic rates than B. puangensis females. These findings provide mixed support for current hypotheses for the functional basis of the success of invasive species; however, we expect the invasive bees in Fiji to be more resilient to climate change because of their increased thermal tolerance and desiccation resistance. Refereed/Peer-reviewed

While species distribution models (SDMs) are widely used to predict the vulnerability of species to climate change, they do not explicitly indicate the extent to which plastic responses ameliorate climate change impacts. Here we use data on plastic responses of 32 species of

Understanding the capacity for different species to reduce their susceptibility to climate change via phenotypic plasticity is essential for accurately predicting species extinction risk. The climatic variability hypothesis suggests that spatial and temporal variation in climatic variables should select for more plastic phenotypes. However, empirical support for this hypothesis is limited. Here, we examine the capacity for ten Drosophila species to increase their critical thermal maxima (CTMAX ) through developmental acclimation and/or adult heat hardening. Using four fluctuating developmental temperature regimes, ranging from 13 to 33 °C, we find that most species can increase their CTMAX via developmental acclimation and adult hardening, but found no relationship between climatic variables and absolute measures of plasticity. However, when plasticity was dissected across developmental temperatures, a positive association between plasticity and one measure of climatic variability (temperature seasonality) was found when development took place between 26 and 28 °C, whereas a negative relationship was found when development took place between 20 and 23 °C. In addition, a decline in CTMAX and egg-to-adult viability, a proxy for fitness, was observed in tropical species at the warmer developmental temperatures (26-28 °C); this suggests that tropical species may be at even greater risk from climate change than currently predicted. The combined effects of developmental acclimation and adult hardening on CTMAX were small, contributing to a

The thermal biology of ectotherms is often used to infer species' responses to changes in temperature. It is often proposed that temperate species are more cold-tolerant, less heat-tolerant, more plastic, have broader thermal performance curves (TPCs) and lower optimal temperatures when compared to tropical species. However, relatively little empirical work has provided support for this using large interspecific studies. In the present study, we measure thermal tolerance limits and thermal performance in 22 species of Drosophila that developed under common conditions. Specifically, we measure thermal tolerance (CT min and CT max ) as well as the fitness components viability, developmental speed and fecundity at seven temperatures to construct TPCs for each of these species. For 10 of the species, we also measure thermal tolerance and thermal performance following developmental acclimation to three additional temperatures. Using these data, we test several fundamental hypotheses about the evolution and plasticity of heat and cold resistance and thermal performance. We find that cold tolerance (CT min ) varied between the species according to the environmental temperature in the habitat from which they originated. These data support the idea that the evolution of cold tolerance has allowed species to persist in colder environments. However, contrary to expectation, we find that optimal temperature ( T opt ) and the breadth of thermal performance ( T breadth ) are similar in temperate, widespread and tropical species and we also find that the plasticity of TPCs was constrained. We suggest that the temperature range for optimal thermal performance is either fixed or under selection by the more similar temperatures that prevail during growing seasons. As a consequence, we find that T opt and T breadth are of limited value for predicting past, present and future distributions of species. This article is part of the theme issue ‘Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen’.

Based on the sensitivity of species to ongoing climate change, and numerous challenges they face tracking suitable conditions, there is growing interest in species' capacity to adapt to climatic stress. Here, we develop and apply a new generic modelling approach (AdaptR) that incorporates adaptive capacity through physiological limits, phenotypic plasticity, evolutionary adaptation and dispersal into a species distribution modelling framework. Using AdaptR to predict change in the distribution of 17 species of Australian fruit flies (Drosophilidae), we show that accounting for adaptive capacity reduces projected range losses by up to 33% by 2105. We identify where local adaptation is likely to occur and apply sensitivity analyses to identify the critical factors of interest when parameters are uncertain. Our study suggests some species could be less vulnerable than previously thought, and indicates that spatiotemporal adaptive models could help improve management interventions that support increased species' resilience to climate change.

Summary 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. 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. 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 1·01 °C. These results imply that overheating risk will only be minimally reduced by plasticity. 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. 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. A lay summary is available for this article.

Thermal performance curves (TPCs) are intended to approximate the relationship between temperature and fitness, and are commonly integrated into species distributional models for understanding climate change responses. However, TPCs may vary across traits because selection and environmental sensitivity (plasticity) differ across traits or because the timing and duration of the temperature exposure, here termed time scale, may alter trait variation. Yet, the extent to which TPCs vary temporally and across traits is rarely considered in assessments of climate change responses. Using a common garden approach, we estimated TPCs for standard metabolic rate (SMR), and activity in Drosophila melanogaster at three test temperatures (16, 25 and 30°C), using flies from each of six developmental temperatures (16, 18, 20, 25, 28 and 30°C). We examined the effects of time scale of temperature exposure (minutes/ hours versus days/weeks) in altering TPC shape and position, and commonly used descriptors of the TPC: thermal optimum (Topt), thermal limits (Tmin and Tmax) and thermal breadth (Tbr). In addition, we collated previously published estimates of TPCs for fecundity and egg-to-adult viability in D. melanogaster. We found that the descriptors of the TPCs varied across traits (egg-to-adult viability, SMR, activity and fecundity), but variation in TPCs within these traits was small across studies when measured at the same time scales. The time scale at which traits were measured contributed to greater variation in TPCs than the observed variance across traits, although the relative importance of time scale differed depending on the trait (activity versus fecundity). Variation in the TPC across traits and time scales suggests that TPCs using single traits may not be an accurate predictor of fitness and thermal adaptation across environments.

Recently, heterogeneity of the environment has been suggested as an important player in the evolution of life span variation. Established ageing theories propose that life span variation is the result of coevolution with other traits, such as stress resistance. This study aimed to compare these alternative hypotheses by examining the relationship between four environmental variables and different types of stress resistance traits with life span in 13 Drosophila species originating from tropical, subtropical and temperate environments (ecotypes). Average life span was found to differ significantly both between species and sexes, but only male life span correlated with the environment and cold resistance. While controlling for phylogeny, the environmental variable precipitation seasonality and resistance against cold-induced stress explained most variation in male life span. Furthermore, male life span varied between species in a manner represented by environmental variables linked to the different ecotypes, such that tropical species lived longer and were less cold resistant. The current results suggest that general mechanisms underlying stress resistance and life span are unlikely. In addition, our results point to the environment independently shaping variation in life span and cold resistance rather than genetic interactions.

Understanding the capacity for different species to reduce their susceptibility to climate change via phenotypic plasticity is essential for accurately predicting species extinction risk. The climatic variability hypothesis suggests that spatial and temporal variation in climatic variables should select for more plastic phenotypes. However, empirical support for this hypothesis is limited. Here, we examine the capacity for ten Drosophila species to increase their critical thermal maxima (CT

The long-term survival of species and populations depends on their ability to adjust phenotypic values to environmental conditions. In particular, the capability of dealing with environmental stress to buffer detrimental effects on fitness is considered to be of pivotal importance. Resistance traits are readily modulated by a wide range of environmental factors. In the present study, Drosophila melanogaster Meigen is used to investigate plastic responses to temperature and photoperiod in stress resistance traits. The results reveal that stress resistance traits (cold, heat, starvation and desiccation resistance) are affected by the factors temperature and sex predominantly. Cooler temperatures compared with warmer temperatures increase cold tolerance, desiccation and starvation resistance, whereas they reduce heat tolerance. Except for heat resistance, females are more stress-resistant than males. Stress resistance traits are also affected by photoperiod. Shorter photoperiods decrease cold tolerance, whereas longer photoperiods enhance desiccation resistance. Overall, thermal effects are pervasive throughout all measured resistance traits, whereas photoperiodic effects are of limited importance in the directly developing (i.e. nondiapausing) flies used here, suggesting that pronounced photoperiodic effects on stress resistance traits may be largely limited to, and triggered by, diapause-inducing effects.

To forecast the responses of species to future climate change, an understanding of the ability of species to adapt to long-term shifts in temperature is crucial. We present a review on evolutionary adaptation and phenotypic plasticity of temperature-related traits in terrestrial invertebrates. The evidence for adaptive evolution in melanization is good, but we caution that genetic determination needs to be tested in each individual species, and complex genetic correlations may exist. For phenological traits allochronic data sets provide powerful means to track climate-induced changes; however, rarely are responses deconstructed into evolutionary and plastic responses. Laboratory studies suggest climate change responses in these traits will be driven by both. For stress resistance, the evidence for shifts in traits is poor. Studies leaning heavily on Drosophila have demonstrated potential limits to evolutionary responses in desiccation and heat resistance. Quantifying the capacity for these species to respond plastically and extending this work to other taxa will be an important next step. We also note that, although not strictly speaking a species trait, the response of endosymbionts to heat stress requires further study. Finally, while clearly genetic, and possibly adaptive, the anonymous nature of latitudinal shifts in clines of genetic markers in Drosophila prevents further interpretation.

Fluctuating environments are expected to select for individuals that have highest geometric fitness over the experienced environments. This leads to the prediction that genetically determined environmental robustness in fitness, and average fitness across environments should be positively genetically correlated to fitness in fluctuating environments. Because quantitative genetic experiments resolving these predictions are missing, we used a full-sib, half-sib breeding design to estimate genetic variance for egg-to-adult viability in Drosophila melanogaster exposed to two constant or fluctuating temperatures that were above the species' optimum temperature, during development. Viability in two constant environments (25°C or 30°C) was used to estimate breeding values for environmental robustness of viability (i.e., reaction norm slope) and overall viability (reaction norm elevation). These breeding values were regressed against breeding values of viability at two different fluctuating temperatures (with a mean of 25°C or 30°C). Our results based on genetic correlations show that average egg-to-adult viability across different constant thermal environments, and not the environmental robustness, was the most important factor for explaining the fitness in fluctuating thermal environments. Our results suggest that the role of environmental robustness in adapting to fluctuating environments might be smaller than anticipated.

Clinal studies are a powerful tool for understanding the genetic basis of climatic adaptation. However, while clines in quantitative traits and genetic polymorphisms have been observed within and across continents, few studies have attempted to demonstrate direct links between them. The gene methuselah in Drosophila has been shown to have a major effect on stress response and longevity phenotypes based largely on laboratory studies of induced mutations in the mth gene. Clinal patterns in the most common mth haplotype and for lifespan (both increasing with latitude) have been observed in North American populations of D. melanogaster, implicating climatic selection. While these clinal patterns have led some to suggest that mth influences ageing in natural populations, limited evidence on the association between the two has so far been collected. Here, we describe a significant cline in the mth haplotype in eastern Australian D. melanogaster populations that parallel the cline in North America. We also describe a cline in mth gene expression. These findings further support the idea that mth is itself under selection. In contrast, we show that lifespan has a strong nonlinear clinal pattern, increasing southwards from the tropics, but then decreasing again from mid-latitudes. Furthermore, in association studies, we find no evidence for a direct link between mth haplotype and lifespan. Thus, while our data support a role for mth variation being under natural selection, we found no link to naturally occurring variation in lifespan and ageing in Australian populations of D. melanogaster. Our results indicate that the mth locus likely has genetic background and environment-specific effects.

A common practice in thermal biology is to take individuals directly from the field and estimate a range of thermal traits. These estimates are then used in studies aiming to understand broad scale distributional patterns, understanding and predicting the evolution of phenotypic plasticity, and generating predictions for climate change risk. However, the use of field-caught individuals in such studies ignores the fact that many traits are phenotypically plastic and will be influenced by the thermal history of the focal individuals. The current study aims to determine the extent to which estimates of upper thermal limits (CTmax), a frequently used measure for climate change risk, are sensitive to developmental and adult acclimation temperatures and whether these two forms of plasticity are reversible. Examining a temperate and tropical population of Drosophila melanogaster we show that developmental acclimation has a larger and more lasting effect on CTmax than adult acclimation. We also find evidence for an interaction between developmental and adult acclimation, particularly when flies are acclimated for a longer period, and that these effects can be population specific. These results suggest that thermal history can have lasting effects on estimates of CTmax. In addition, we provide evidence that developmental and/or adult acclimation are unlikely to contribute to substantial shifts in CTmax and that acclimation capacity may be constrained at higher temperatures.

Metabolic cold adaptation (MCA) is a controversial hypothesis suggesting that cold adapted species display an elevated metabolic rate (MR) compared to their warm climate relatives. Here we test for the presence of MCA in 65 species of drosophilid flies reared under common garden conditions. MR was measured at both 10 and 20 °C for both sexes and data were analyzed in relation to the natural thermal environment of these species. We found considerable interspecific variation in MR ranging from 1.34 to 8.99 µW mg−1 at 10 °C. As predicted by Bergmann’s rule body mass of fly species correlated negatively with annual mean temperature (AMT), such that larger species were found in colder environments. Because larger flies have a higher total MR we found MR to vary with AMT, however, after inclusion of mass as a co-variate we found no significant effect of AMT. Furthermore, we did not find that thermal sensitivity of MR (Q10) varied with AMT. Based on this broad collection of species we therefore conclude that there is no adaptive pattern of metabolic cold adaptation within drosophilid species ranging from sub-arctic to tropical environments.

Species distributions are often constrained by climatic tolerances that are ultimately determined by evolutionary history and/or adaptive capacity, but these factors have rarely been partitioned. Here, we experimentally determined two key climatic niche traits (desiccation and cold resistance) for 92-95 Drosophila species and assessed their importance for geographic distributions, while controlling for acclimation, phylogeny, and spatial autocorrelation. Employing an array of phylogenetic analyses, we documented moderate-to-strong phylogenetic signal in both desiccation and cold resistance. Desiccation and cold resistance were clearly linked to species distributions because significant associations between traits and climatic variables persisted even after controlling for phylogeny. We used different methods to untangle whether phylogenetic signal reflected phylogenetically related species adapted to similar environments or alternatively phylogenetic inertia. For desiccation resistance, weak phylogenetic inertia was detected; ancestral trait reconstruction, however, revealed a deep divergence that could be traced back to the genus level. Despite drosophilids' high evolutionary potential related to short generation times and high population sizes, cold resistance was found to have a moderate-to-high level of phylogenetic inertia, suggesting that evolutionary responses are likely to be slow. Together these findings suggest species distributions are governed by evolutionarily conservative climate responses, with limited scope for rapid adaptive responses to future climate change.

It has frequently been suggested that trait heritabilities are environmentally sensitive, and there are genetic trade-offs between tolerating different environments such as hot and cold or constant and fluctuating temperatures. Future climate predictions suggest an increase in both temperatures and their fluctuations. How species will respond to these changes is uncertain, particularly as there is a lack of studies which compare genetic performances in constant vs. fluctuating environments. In this study, we used a nested full-sib/half-sib breeding design to examine how the genetic variances and heritabilities of egg-to-adult viability differ at high and low temperatures with and without daily fluctuations in temperatures using Drosophila melanogaster as a model organism. Although egg-to-adult viability was clearly sensitive to developmental temperatures, heritabilities were not particularly sensitive to developmental temperatures. Moreover, we found that egg-to-adult viabilities at different developmental temperatures were positively correlated, suggesting a common genetic background for egg-to-adult viability at different temperatures. Finding both a uniform genetic background coupled with rather low heritabilities insensitive to temperatures, our results suggest evolutionary responses are unlikely to be limited by temperature effects on genetic parameters or negative genetic correlations, but by the direct effects of stressful temperatures on egg-to-adult viability accompanied with low heritabilities.

Laboratory experiments on Drosophila have often demonstrated increased heritability for morphological and life-history traits under environmental stress. We used parent–offspring comparisons to examine the impact of humidity levels on the heritability of a physiological trait, resistance to heat, measured as knockdown time at constant temperature. Drosophila melanogaster were reared under standard nonstressful conditions and heat-shocked as adults at extreme high or low humidity. Mean knockdown time was decreased in the stressful dry environment, but there was a significant sex-by-treatment interaction: at low humidity, females were more heat resistant than males, whereas at high humidity, the situation was reversed. Phenotypic variability of knockdown time was also lower in the dry environment. The magnitude of genetic correlation between the sexes at high humidity indicated genetic variation for sexual dimorphism in heat resistance. Heritability estimates based on one-parent–offspring regressions tended to be higher under desiccation stress, and this could be explained by decreased environmental variance of heat resistance at low humidity. There was no indication that the additive genetic variance and evolvability of heat resistance differed between the environments. The pattern of heritability estimates suggests that populations of D. melanogaster may have a greater potential for evolving higher thermal tolerance under arid conditions.

Genetic prerequisites for the evolution of sexual dimorphism, sex-specific heritabilities and low or negative genetic correlations between homologous traits in males and females are rarely found. However, sexual dimorphism is evolving rapidly following environmental change, suggesting that sexual dimorphism and its genetic background could be environmentally sensitive. Yet few studies have explored the sensitivity of the genetic background of sexual dimorphism on environmental variation. In this study, on Drosophila melanogaster, we used a large nested full-sib–half-sib breeding design where families were split into four different developmental temperatures: two constant temperature treatments of 25 and 30 °C and two cycling temperatures with means of 25 and 30 °C, respectively. After emergence, we tested heat shock tolerance of adult flies. We found that sexual dimorphism was strongly affected by temperature during development. Moreover, we found that female heritability was significantly lower in flies developing at hot temperature and more so under hot and cycling temperatures. Interestingly, most of the genetic variation for heat shock tolerance was orthogonal (i.e. noncorrelated) between sexes, allowing independent evolution of heat shock tolerance in males and females. These findings give support to the hypothesis that the evolution of sexual dimorphism can be influenced by the environments experienced during development.

Summary 1. Acclimation or hardening to one stress in arthropods can lead to a plastic response, which confers increased resistance to other stresses. Such cross-resistance may indicate shared physiological resistance mechanisms and a possibility of joint evolution for resistance traits. 2. In this study, we tested for cross-resistance using Drosophila melanogaster as a model system. Adult females were assayed for resistance to cold, heat, desiccation and starvation following cold acclimation, heat hardening, desiccation hardening and starvation acclimation, resulting in 12 pretreatment-by-test combinations for estimating potential cross-resistance effects. The acclimation/hardening regimes were chosen so that flies enhanced their resistance to the same type of stress as that used for the particular pretreatment. 3. Two cases of cross-resistance were found, with desiccation-hardened and starvation-acclimated flies being more resistant than control ones in heat and desiccation resistance tests, respectively. In four cases, no acclimation/hardening effect was observed, and for six pretreatment-by-test combinations, the effect of acclimation/hardening was negative. 4. We also revealed that heat and desiccation hardening as well as acclimation to starvation had a cost under non-stressful conditions leading to reduced longevity. Cold acclimation did not affect longevity, although its effect was difficult to estimate precisely: during pretreatment at a low temperature, biological ageing of the flies might be delayed. 5. The pattern of acclimation/hardening responses considered in the context of literature data on stress resistance indicates that expression of heat-shock proteins is not likely to contribute to the observed cross-resistance, but other probable general stress resistance mechanisms such as reduction of metabolic rate and accumulation of energy reserves might be involved. 6. The lack of cross-resistance induced by acclimation/hardening treatments suggests that in an environment with multiple stresses, evolution of shared protective systems associated with plastic responses may be constrained.

Several evolutionary hypotheses help explain why only some species adapt readily to new conditions and expand distributions beyond borders, but there is limited evidence testing these hypotheses. In this study, we consider patterns of neutral (microsatellite) and quantitative genetic variation in traits in three species ofDrosophilafrom themontiumspecies group in eastern Australia. We found little support for restricted or asymmetrical gene flow in any species. In rainforest-restrictedDrosophila birchii, there was evidence of selection for increased desiccation and starvation resistance towards the southern border, and a reduction in genetic diversity in desiccation resistance at this border. No such patterns existed forDrosophila bunnanda, which has an even more restricted distribution. In the habitat generalistDrosophila serrata, there was evidence for geographic selection for wing size and development time, although clinal patterns for increased cold and starvation resistance towards the southern border could not be differentiated from neutral expectations. These findings suggest that borders in these species are not limited by low overall genetic variation but instead in two of the species reflect patterns of selection and genetic variability in key traits limiting borders.

Experimental evolution can be a useful tool for testing the impact of environmental factors on adaptive changes in populations, and this approach is being increasingly used to understand the potential for evolutionary responses in populations under changing climates. However, selective factors will often be more complex in natural populations than in laboratory environments and produce different patterns of adaptive differentiation. Here we test the ability of laboratory experimental evolution under different temperature cycles to reproduce well-known patterns of clinal variation in Drosophila melanogaster. Six fluctuating thermal regimes mimicking the natural temperature conditions along the east coast of Australia were initiated. Contrary to expectations, on the basis of field patterns there was no evidence for adaptation to thermal regimes as reflected by changes in cold and heat resistance after 1-3 years of laboratory natural selection. While laboratory evolution led to changes in starvation resistance, development time, and body size, patterns were not consistent with those seen in natural populations. These findings highlight the complexity of factors affecting trait evolution in natural populations and indicate that caution is required when inferring likely evolutionary responses from the outcome of experimental evolution studies.

Most quantitative traits are thought to exhibit high levels of genetic variance and evolutionary potential. However, this conclusion may be biased by a lack of studies on nonmodel organisms and may not generalize to restricted species. A recent study on a single, southern population of the rainforest-restricted Drosophila birchii failed to find significant additive genetic variance for the desiccation resistance trait; however, it is unclear whether this pattern extends to other D. birchii populations or to other rainforest species. Here we use an animal model design to show very low levels of additive genetic variance for desiccation resistance in multiple populations of two highly sensitive rainforest species of Drosophila from tropical northeastern Australia. In contrast, relatively high levels of genetic variance were found for morphological traits in all populations of the species tested. This indicates limited evolutionary potential for evolving increased desiccation resistance in these rainforest restricted species.

There are several evolutionary reasons why species distributions are locally and geographically limited, and these mostly revolve around gene flow and levels of genetic variation in populations. While there has been progress in developing models assessing the impact of gene flow, empirical data on hypotheses about factors limiting distributions remain rare. A few recent plant studies have highlighted that levels of additive genetic variance are not limiting, suggesting a role for gene flow. However, some animal studies, mostly onDrosophilaspecies, have refocused attention on heritable variation as an evolutionary reason for distribution limits. There is a need for comparative studies that examine levels of heritable variation across related species groups with broad and narrow distributions, and also across a range of traits including those likely to be under selection at borders. These studies will help to test specific predictions about the relative importance of genetic variance and gene flow in limiting current borders and also help in predicting the impact of future environmental changes on distribution shifts.

Comparative analyses of ectotherm susceptibility to climate change often focus on thermal extremes, yet responses to aridity may be equally important. Here we focus on plasticity in desiccation resistance, a key trait shaping distributions ofDrosophilaspecies and other small ectotherms. We examined the extent to which 32Drosophilaspecies, varying in their distribution, could increase their desiccation resistance via phenotypic plasticity involving hardening, linking these responses to environment, phylogeny and basal resistance. We found no evidence to support the seasonality hypothesis; species with higher hardening plasticity did not occupy environments with higher and more seasonal precipitation. As basal resistance increased, the capacity of species to respond via phenotypic plasticity decreased, suggesting plastic responses involving hardening may be constrained by basal resistance. Trade-offs between basal desiccation resistance and plasticity were not universal across the phylogeny and tended to occur within specific clades. Phylogeny, environment and trade-offs all helped to explain variation in plasticity for desiccation resistance but in complex ways. These findings suggest some species have the ability to counter dry periods through plastic responses, whereas others do not; and this ability will depend to some extent on a species' placement within a phylogeny, along with its basal level of resistance.

Thermal tolerance is an important factor influencing the distribution of ectotherms, but we still have limited understanding of the ability of species to evolve different thermal limits. Recent studies suggest that species may have limited capaity to evolve higher themal limits in response to slower, more ecologically relevant rates of warming. However these conclusions are based on univarite estimates of adaptive capacity. To test these findings within an explicitly multivariate context, we used a paternal half-sibling breeding design to estimate the multivariate evolutionary potential for upper thermal limits in Drosophila melanogaster. We assessed heat tolerance using static (basal and hardened) and ramping assays. Additive genetic variances were significantly different from zero only for the static measures of heat tolerance. Our G matrix analysis revealed that any response to selection for increased heat tolerance will largely be driven by static basal and hardened heat tolerance, with minimal contribution from ramping heat tolerance. These results suggest that the capacity to evolve upper thermal limits in nature may depend on the type of thermal stress experienced.

The ability to respond evolutionarily to increasing temperatures is important for survival of ectotherms in a changing climate. Recent studies suggest that upper thermal limits may be evolutionary constrained. We address this hypothesis in a laboratory evolution experiment, encompassing ecologically relevant thermal regimes. To examine the potential for species to respond to climate change, we exposed replicate populations of Drosophila melanogaster to increasing temperatures (0.3 °C every generation) for 20 generations, whereas corresponding replicate control populations were held at benign thermal conditions throughout the experiment. We hypothesized that replicate populations exposed to increasing temperatures would show increased resistance to warm and dry environments compared with replicate control populations. Contrasting replicate populations held at the two thermal regimes showed (i) an increase in desiccation resistance and a decline in heat knock-down resistance in replicate populations exposed to increasing temperatures, (ii) similar egg-to-adult viability and fecundity in replicate populations from the two thermal regimes, when assessed at high stressful temperatures and (iii) no difference in nucleotide diversity between thermal regimes. The limited scope for adaptive evolutionary responses shown in this study highlights the challenges faced by ectotherms under climate change.

Species distributions are often constrained by climatic tolerances that are ultimately determined by evolutionary history and/or adaptive capacity, but these factors have rarely been partitioned. Here, we experimentally determined two key climatic niche traits (desiccation and cold resistance) for 92-95 Drosophila species and assessed their importance for geographic distributions, while controlling for acclimation, phylogeny, and spatial autocorrelation. Employing an array of phylogenetic analyses, we documented moderate-to-strong phylogenetic signal in both desiccation and cold resistance. Desiccation and cold resistance were clearly linked to species distributions because significant associations between traits and climatic variables persisted even after controlling for phylogeny. We used different methods to untangle whether phylogenetic signal reflected phylogenetically related species adapted to similar environments or alternatively phylogenetic inertia. For desiccation resistance, weak phylogenetic inertia was detected; ancestral trait reconstruction, however, revealed a deep divergence that could be traced back to the genus level. Despite drosophilids' high evolutionary potential related to short generation times and high population sizes, cold resistance was found to have a moderate-to-high level of phylogenetic inertia, suggesting that evolutionary responses are likely to be slow. Together these findings suggest species distributions are governed by evolutionarily conservative climate responses, with limited scope for rapid adaptive responses to future climate change.

Upper thermal limits vary less than lower limits among related species of terrestrial ectotherms. This pattern may reflect weak or uniform selection on upper limits, or alternatively tight evolutionary constraints. We investigated this issue in 94 Drosophila species from diverse climates and reared in a common environment to control for plastic effects that may confound species comparisons. We found substantial variation in upper thermal limits among species, negatively correlated with annual precipitation at the central point of their distribution and also with the interaction between precipitation and maximum temperature, showing that heat resistance is an important determinant of Drosophila species distributions. Species from hot and relatively dry regions had higher resistance, whereas resistance was uncorrelated with temperature in wetter regions. Using a suite of analyses we showed that phylogenetic signal in heat resistance reflects phylogenetic inertia rather than common selection pressures. Current species distributions are therefore more likely to reflect environmental sorting of lineages rather than local adaptation. Similar to previous studies, thermal safety margins were small at low latitudes, with safety margins smallest for species occupying both humid and dry tropical environments. Thus, species from a range of environments are likely to be at risk owing to climate change. Together these findings suggest that this group of insects is unlikely to buffer global change effects through marked evolutionary changes, highlighting the importance of facilitating range shifts for maintaining biodiversity.

Adaptive Limits Species adapt to a changing environment as a result of selection acting on current genetic variation. However, the degree of variation underlying traits that can respond to selection is unclear. Kellermann et al. (p. 1244 ; see the Perspective by Merilä ) investigated the degree of genetic variation available to fruit flies for cold and desiccation tolerance. Species from the tropics tended to have low variability for these traits, while flies from more temperate climates showed higher levels of variation. However, overall genetic variability did not differ, suggesting that the tropical species lacked the alleles that confer tolerance to these environmental extremes and restrict their potential range.

Periodically, populations will experience stressful conditions. Adaptation to these conditions requires adequate levels of heritable genetic variance. Other factors including genetic interactions, plasticity, nongenetic inheritance and gene flow may also prevent evolutionary responses. Although stressful conditions can lead to a loss of biodiversity following extinction, periods of stress may also lead to rapid evolution and diversification through novel/intense selection pressures and through the release of variability following an increase in mutation and recombination rates and/or the disruption of canalized systems. Over the last century, anthropogenic disturbances have increasingly imposed severe and novel environmental stresses on natural populations. Evolutionary shifts in response to these recent disturbances (including climate change) have already been detected as shifting phenotypes and altered allele frequencies along gradients.