LAUSR

In ecosystems around the globe, humans are driving rapid alterations to natural habitats across multiple biotic and abiotic variables (1). Understanding biological responses to this highly multivariate change is one of the great challenges of modern biology. Responses to multivariate change are difficult to predict because combined stressors often have nonadditive effects, with impacts that cannot be projected from single-stressor studies (2, 3). These responses are made even more complex by the fact that multivariate selection pressures can alter the capacity for adaptation, with limits to evolution incurred by trade-offs among traits under selection (4, 5). Thus, responses to multivariate change are doubly difficult to predict, both because of their potentially synergistic physiological effects and because of the potential for evolutionary trade-offs among stress tolerance traits to stymie adaptation. In PNAS, Brennan et al. (6) use a powerful experimental evolution approach to investigate responses to multivariate environmental change in marine zooplankton. In the past 10 y, experimental evolution has emerged as a valuable tool for investigating evolutionary responses to climate change (7). In these studies, investigators directly expose populations of organisms to controlled sources of selection and test their capacity to evolve. These types of experiments are especially useful for investigating responses to multiple stressors, because sources of selection can be varied alone and in combination to test for synergistic effects. Experimental evolution can also be combined with genomic sequencing to identify the targets of selection, a technique termed “evolve and resequence” (8). Evolve and resequence experiments offer the possibility of identifying the genetic basis of complex adaptations, a central goal of modern evolutionary genetics. In PNAS, Brennan et al. (6) provide a striking example of the insights that can be gained by combining experimental evolution with genomic sequencing. They report the results of a 25-generation evolve and resequence experiment, testing the response of the copepod Acartia tonsa to acidification, warming, and combined ocean warming and acidification (OWA) conditions (Fig. 1). Marine organisms are especially vulnerable to the potential synergistic effects of multiple anthropogenic stressors. In particular, they face the combined threats of warming and acidification, a decline in ocean pH caused by the absorption of anthropogenic CO2 from the atmosphere (9). The combination of warming and acidification has multiple potential interactive effects, including an increased cost of acid–base regulation combined with an energy debt driven by increased overall metabolism under ocean warming. It is especially important to understand the impacts of these two stressors on key players in marine food webs, where even small changes in growth and survival will have potentially large cascading ecological effects. In this regard, A. tonsa is a prime target for study, as Fig. 1. Brennan et al. (6) report the results of 25 generations of experimental evolution, where populations of the marine copepod A. tonsa were allowed to adapt to warming, acidification, and the combination of the two stressors. Sequencing of experimental lines and controls shows that adaptation to each condition occurred through “soft sweeps,” or an increase in the frequency of genetic variants that were already present in the wild source population.