New paper, just out. Some key points

> Populations subject to severe stress may be rescued by natural selection, but its operation is restricted by ecological and genetic constraints.

> Genostasis expresses the lack of variation that prevents many populations from adapting to stress. While the role of relative fitness in adaptation is well understood, evolutionary rescue emphasizes the need to recognize explicitly the importance of absolute fitness. 

> 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. This principle implies that population size is an important determinant of rescue. 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. 

> 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.

Evolutionary rescue and the limits of adaptation
Graham Bell, Biology Department, McGill University
Phil. Trans. R. Soc. B January 19, 2013

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