Populations that decline towards extinction can be evolutionary rescued by a natural selection acceleration of the growth rate
As natural selection in the absence of interactive competition approaches Fisher’s (1930) fundamental theorem, populations that decline in numbers should eventually converge on a natural selection acceleration of growth rates. If this acceleration is sufficiently large compared with the rate of decline, the population is evolutionary rescued by natural selection before it goes extinct.
The process of evolutionary rescue was introduced by Pease et al. (1989), and the term was used for the first time by Gomulkiewicz and Holt in 1995. Evolutionary rescue is typically studied in relation to populations that decline due to environmental changes (Ramsayer et al., 2013; Bell, 2017). But the process is likely to be much more common. Selection-regulated populations with cyclic dynamics e.g., are often saved from extinction by natural selection during the low phase of a cycle.
Selection accelerating populations that decline in numbers are often evolutionarily rescued due to the continued natural selection acceleration of their growth rate. The passenger pigeon in North America, however, is an example of a species that wasn’t evolutionary rescued as it continued on a drastic decline from about 1870 until extinction in the wild around 1901 (the last individual in captivity died in 1914).
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