| 英文摘要: | Global climate is changing rapidly, and the degree to which natural populations respond genetically to these changes is key to predicting ecological responses1, 2, 3. So far, no study has documented evolutionary changes in the thermal tolerance of natural populations as a response to recent temperature increase. Here, we demonstrate genetic change in the capacity of the water flea Daphnia to tolerate higher temperatures using both a selection experiment and the reconstruction of evolution over a period of forty years derived from a layered dormant egg bank. We observed a genetic increase in thermal tolerance in response to a two-year ambient +4 °C selection treatment and in the genotypes of natural populations from the 1960s and 2000s hatched from lake sediments. This demonstrates that natural populations have evolved increased tolerance to higher temperatures, probably associated with the increased frequency of heat waves over the past decades, and possess the capacity to evolve increased tolerance to future warming. Average global temperatures have substantially increased during the past 50 years and are expected to increase 0.4 to 4.8 °C in the next hundred years4. Populations, communities and ecosystems have responded to these changes in various ways, including shifts in species ranges, and in flowering and migration times1, 5. Although natural populations may adapt genetically to climate change, their ability to evolve fast enough to keep pace with observed rates of climate change is debated3, 5, 6, 7. Understanding whether evolution can mediate species responses to climate change is fundamental because the ability to genetically track changes in climate can reduce extinction rates. In addition, local adaptation to changing temperatures can also modulate the relative importance of local and regional processes, as it changes competitive interactions between residents and immigrants, potentially reducing establishment success of immigrants2, 8. Climate change and associated increases in frequency of heat waves9 induce physiological stress owing to increased metabolic rates and oxygen demand10, 11. Thermal performance curves and measures of heat tolerance have been used to predict organisms’ responses to climate change10, 12, 13. A widely used index of heat tolerance is the critical thermal maximum (CTMax), which is the upper temperature at which animals lose motor function. CTMax is heritable in the fruit fly Drosophila, is a measure of performance under field conditions14, and is informative for climate change responses in several species15, 16. Yet, no study of natural populations has so far documented a genetic increase in CTMax in response to recent climate change. Here, we first use an experimental evolution approach to test whether a natural population of the water flea Daphnia magna Straus (1820), a key zooplankton organism in shallow freshwater ecosystems17, harbours sufficient genetic variation in CTMax to adapt genetically to increased water temperature. D. magna typically inhabits shallow lakes and ponds, which are vulnerable to changes in temperature. Next, we use resurrection ecology to quantify evolutionary changes in CTMax. Resurrection ecology uses the layered dormant propagule bank in sediments as a genetic archive that allows the reconstruction of microevolution in a given population through time18, 19, 20. Lake sediments are typically layered and contain dormant stages of zooplankton that represent snapshots of the population through time. By hatching genotypes produced during these different time periods we can study the genetic changes that occurred in the population. D. magna were resurrected from a half-century-old egg bank (1955–1965) of a natural population and their CTMax values were compared with those of more recent (1995–2005) D. magna from the same pond. As such, we were able to determine realized shifts in CTMax in response to recent climate change. This approach has successfully been applied to reconstruct evolution in phenology and physiology in response to climate change in the plant Brassica rapa21, 22. In the experimental evolution trial, we exposed a genetically diverse population of D. magna (150 clones), to either ambient temperature or to an ambient +4 °C temperature treatment in outdoor tanks (mesocosms) that mimicked small ponds. After two years, sediments were collected from two heated- and two ambient-temperature mesocosms and twenty genotypes were hatched from dormant eggs in these sediments—that is, probing post-selection populations (see Supplementary Information I.A for more details). The resulting clones were scored for CTMax. Results confirmed that natural Daphnia populations have the evolutionary potential to respond genetically to a strong but environmentally relevant temperature increase. Clones from heated mesocosms had, on average, a 3.6 °C higher CTMax than clones from the ambient-temperature mesocosms (temperature treatment effect P < 0.001; Table 1 and Fig. 1a). The difference in CTMax between the populations from the two temperature treatments matched the +4 °C temperature difference between the two selection regimes.
- Corrected online 03 September 2015
- In the version of this Letter originally published, in Fig. 1a, the boxplot for the Ambient +4 °C treatment was incorrect. This error has been corrected in the online versions.
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