Surviving environmental change: when increasing population size can increase extinction risk

Abstract

Populations threatened by an abrupt environmental change—due to rapid climate change, pathogens or invasive competitors—may survive if they possess or generate genetic combinations adapted to the novel, challenging condition. If these genotypes are initially rare or non-existent, the emergence of lineages that allow a declining population to survive is known as ‘evolutionary rescue’. By contrast, the genotypes required for survival could, by chance, be common before the environmental change. Here, considering both of these possibilities, we find that the risk of extinction can be lower in very small or very large populations, but peaks at intermediate population sizes. This pattern occurs when the survival genotype has a small deleterious effect before the environmental change. Since mildly deleterious mutations constitute a large fraction of empirically measured fitness effects, we suggest that this unexpected result—an intermediate size that puts a population at a greater risk of extinction—may not be unusual in the face of environmental change.

Evolutionary rescue on genotypic fitness landscapes

“…To allow for analytical progress, we studied ER via de novo mutation only. Standing genetic variation is another important contributor to evolutionary rescue [13,16] that can produce counterintuitive effects in the rescue process [9]. We also narrowed our focus to the effects of single mutations from the wild-type (but see [26]).…”

“…The fate of populations under stressful environmental conditions is determined by a complex interplay among demographic, genetic and extrinsic factors. The size of the initial population is a crucial determinant of evolutionary rescue [7][8][9], along with the rate of population decline, which is determined by the degree of maladaptation or level of stress in the adversely changed (or changing) environment [10][11][12]. The population is likely to face a geometrical decline in abundance if sufficiently maladapted [13], and may decline to critically small sizes at which it will be highly susceptible to extinction via demographic stochasticity [1].…”

A lack of genetic diversity and minimal adaptive evolutionary divergence in introduced Mysis shrimp after 50 years

“…In fact, several studies have demonstrated rapid adaptive evolution in introduced and invasive species despite a loss of genetic variation (Colautti & Lau, 2015; Dlugosch et al., 2015; Schrieber & Lachmuth, 2017; Tsutsui et al., 2000; Willoughby et al., 2018). The selective landscape of novel environments and a small founding population size can even foster adaptation through either demographic founder effects (Szűcs et al., 2017), where adaptive alleles drifting to higher frequency (Tanaka & Wahl, 2022) accelerates the rate of evolution (Schlaepfer et al., 2009; Tsutsui et al., 2000), and/or allowing founding populations to purge deleterious alleles (Facon et al., 2011). This counterintuitive pattern of founding populations with reduced genetic variation adapting to novel environments is known as the genetic paradox of biological invasions (Estoup et al., 2016).…”

How density dependence, genetic erosion and the extinction vortex impact evolutionary rescue

“…However, they did not investigate effects of density dependence on adaptation from standing genetic variation. This ignores the influence of drift, which at reduced population size can cause the loss of favoured alleles (but see [ 22 ] for possible increase in frequency). Chevin & Lande [ 9 ] found that density dependence impedes rescue by accelerating population decline.…”