Again, the idea is by no means new: the evolution of antibiotic resistance by bacteria is an excellent example of evolutionary rescue, although the term was rarely used in this context. In particular, the potential of evolutionary rescue to mitigate the effects of anthropogenic stress has been widely discussed. It has become more prominent in the past few years, however, and has attracted a growing body of theory and experiment, reviewed in the other articles in this volume. The interplay of ecological and evolutionary processes is not a new observation, of course: it has been appreciated for as long as the two fields have existed. This is the phenomenon of evolutionary rescue. The second is that populations may adapt to conditions that would have been lethal to their ancestors, allowing the population to persist when in the absence of genetic variation it would have become extinct. A population may well become steadily better adapted to a deteriorating environment while irreversibly declining in abundance. The first is that natural selection may act to modify characters and yet have no permanent result because the population becomes extinct. Evolutionary change may be modulated by a trend in abundance, whereas ecological change may respond to a trend in mean character state.Įco-evolutionary dynamics have two principal consequences. In many cases, therefore, the dynamics of a population exposed to a stress, or a stimulus, will have both ecological and evolutionary components: an overall shift in abundance and a change in composition. This has been amply confirmed in the past decade by reports of rapid evolution in a broad range of organisms. That is, the genetic variance of fitness uncovered by selection greatly exceeds that estimated from screens. The success of field studies of natural selection subsequent to the 1950s, however, made it clear that selection was often much stronger than had been expected, and could cause rapid modification in response to environmental change. The removal of evolution from ecology, and ecology from evolution, were useful simplifying devices that made it possible to lay out distinct foundations for both fields. In particular, the dynamics of allele frequency under selection were conventionally analysed by assuming that the population size is fixed (or infinite), implying that agents of selection have no appreciable effect on abundance. At the same time, evolutionary theory was often almost devoid of ecological context. Any species can then be regarded as having a fixed set of attributes during the period of an ecological study. It implies that ecological processes can be studied without reference to natural selection, at least to a good approximation, because genetic variation is inadequate to fuel appreciable change in the short term of a few dozen generations. More fundamentally, extreme gradualism uncouples evolution from ecology. It follows that field and laboratory studies of selection are likely to be fruitless, and very few were attempted for the first hundred years of evolutionary biology. The gradualist view of evolution is that natural selection is almost always very weak, causing very gradual change over long periods of time. Neither the standing genetic variation of small populations nor the mutation supply of large populations, however, may be sufficient to provide evolutionary rescue for most populations. In gradually deteriorating environments, survival at lethal stress may be procured by prior adaptation to sublethal stress through genetic correlation. The overall number of individuals exposed to selection will be greater when the population declines gradually under a constant stress, or is progressively challenged by gradually increasing stress. This principle implies that population size is an important determinant of rescue. Permanent adaptation requires a range of genetic variation in absolute fitness that is broad enough to provide a few extreme types capable of sustained growth under a stress that would cause extinction if they were not present. While the role of relative fitness in adaptation is well understood, evolutionary rescue emphasizes the need to recognize explicitly the importance of absolute fitness. ![]() Genostasis expresses the lack of variation that prevents many populations from adapting to stress. The cost of natural selection expresses the limited capacity of a population to sustain the load of mortality or sterility required for effective selection. ![]() ![]() Populations subject to severe stress may be rescued by natural selection, but its operation is restricted by ecological and genetic constraints.
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