Climate change has the potential to affect the ecology and evolution of every species on Earth. Although the ecological consequences of climate change are increasingly well documented, the effects of climate on the key evolutionary process driving adaptationnatural selection-are largely unknown. We report that aspects of precipitation and potential evapotranspiration, along with the North Atlantic Oscillation, predicted variation in selection across plant and animal populations throughout many terrestrial biomes, whereas temperature explained little variation. By showing that selection was influenced by climate variation, our results indicate that climate change may cause widespread alterations in selection regimes, potentially shifting evolutionary trajectories at a global scale.C limate affects organisms in ways that ultimately shape patterns of biodiversity (1). Consequently, the rapid changes in Earth's recent climate impose challenges for many organisms, often reducing population fitness (2-4). Although some species may migrate and undergo range shifts to avoid climate-induced declines and potential extinction (5), an alternative outcome is adaptive evolution in response to selection imposed by climate (6). However, we lack a general understanding of whether local and global climatic factors such as temperature, precipitation, and water availability influence selection (2, 7). Understanding these effects is critical for predicting the consequences of increasing droughts, heat waves, and extreme precipitation events that are expected in many regions (8, 9).To quantify how climate variation influences selection, we assembled a large database of standardized directional selection gradients and differentials from spatially [mean = 4.6 ± 5.4 (SD) populations, range = 2 to 59 populations] and temporally [mean = 5.2 ± 6.8 (SD) years, range = 2 to 45 years] replicated selection studies (N = 168) in plant and animal populations (Table 1 and database S1). We focused on directional selection that can generate increases or decreases in trait values because it is well characterized and is likely to drive rapid evolution (10) in response to variation in climatic factors. However, selection acting on trait combinations and trait variance may also be affected by climate (7). Selection gradients estimate the strength and direction of selection acting directly on a trait, whereas differentials estimate "total selection" on a trait via both direct and indirect selection because of trait correlations (11). These standardized selection coefficients describe selection in terms of the relationship between relative fitness and quantitative traits measured in standard deviations, thus facilitating cross-study comparisons (11,12).Geographically, the database contains many estimates of selection from temperate, mid-latitude regions centered at 40°N (Fig. 1A). The populations in this database span many terrestrial biomes on Earth, with the exception of tundra and tropical rainforests where selection has rarely been quantified (Fig. 1B...

Most terrestrial ectotherms experience diurnal and seasonal variation in temperature. Because thermal performance curves are non-linear, mean performance can differ in fluctuating and constant thermal environments. However, time-dependent effects -effects of the order and duration of exposure to temperature -can also influence mean performance. We quantified the non-linear and time-dependent effects of diurnally fluctuating temperatures for larval growth rates in the tobacco hornworm, Manduca sexta L., with four main results. First, the shape of the thermal performance curve for growth rate depended on the duration of exposure: for example, optimal temperature and thermal breadth were greater for growth rates measured over short (24 h during the last instar) compared with long (the entire period of larval growth) time periods. Second, larvae reared in diurnally fluctuating temperatures had significantly higher optimal temperatures and maximal growth rates than larvae reared in constant temperatures. Third, for larvae maintained at three mean temperatures (20, 25 and 30°C) and three diurnal temperature ranges (±0, ±5 and ±10°C), diurnal fluctuations had opposite effects on mean growth rates at low versus high mean temperature. Fourth, both short-and long-term thermal performance curves yielded poor predictions of the non-linear effects of fluctuating temperature on mean growth rates (compared with our experimental results) at higher mean temperatures. Our results suggest caution in using constant temperature studies to model the consequences of variable thermal environments.

Although many selection estimates have been published, the environmental factors that cause selection to vary in space and time have rarely been identified. One way to identify these factors is by experimentally manipulating the environment and measuring selection in each treatment. We compiled and analyzed selection estimates from experimental studies. First, we tested whether the effect of manipulating the environment on selection gradients depends on taxon, trait type, or fitness component. We found that the effect of manipulating the environment was larger when selection was measured on life-history traits or via survival. Second, we tested two predictions about the environmental factors that cause variation in selection. We found support for the prediction that variation in selection is more likely to be caused by environmental factors that have a large effect on mean fitness but not for the prediction that variation is more likely to be caused by biotic factors. Third, we compared selection gradients from experimental and observational studies. We found that selection varied more among treatments in experimental studies than among spatial and temporal replicates in observational studies, suggesting that experimental studies can detect relationships between environmental factors and selection that would not be apparent in observational studies.

In many ectotherms, exposure to high temperatures can improve subsequent tolerance to higher temperatures. However, the differential effects of single, repeated or continuous exposure to high temperatures are less clear. We measured the effects of single heat shocks and of diurnally fluctuating or constant rearing temperatures on the critical thermal maximum (CT max ) for final instar larvae of Manduca sexta. Brief (2 h) heat shocks at temperatures of 35°C and above significantly increased CT max relative to control temperatures (25°C). Increasing mean temperatures (from 25 to 30°C) or greater diurnal fluctuations (from constant to ±10°C) during larval development also significantly increased CT max . Combining these data showed that repeated or continuous temperature exposure during development improved heat tolerance beyond the effects of a single exposure to the same maximum temperature. These results suggest that both acute and chronic temperature exposure can result in adaptive plasticity of upper thermal limits.

Climate change is increasing the frequency of heat waves and other extreme weather events experienced by organisms. How does the number and developmental timing of heat waves affect survival, growth and development of insects? Do heat waves early in development alter performance later in development? We addressed these questions using experimental heat waves with larvae of the Tobacco Hornworm, Manduca sexta. The experiments used diurnally fluctuating temperature treatments differing in the number (0-3) and developmental timing (early, middle and/or late in larval development) of heat waves, in which a single heat wave involved three consecutive days with a daily maximum temperature of 42 °C. Survival to pupation declines with increasing number of heat waves. Multiple (but not single) heat waves significantly reduced development time and pupal mass; the best models for the data indicated that both the number and developmental timing of heat waves affected performance. In addition, heat waves earlier in development significantly reduced growth and development rates later in larval development. Our results illustrate how the frequency and developmental timing of sublethal heat waves can have important consequences for life history traits in insects.

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The comment by Myers-Smith and Myers focuses on three main points: (i) the lack of a mechanistic explanation for climate-selection relationships, (ii) the appropriateness of the climate data used in our analysis, and (iii) our focus on estimating climate-selection relationships across (rather than within) taxonomic groups. We address these critiques in our response.