What is Evolutionary Rescue？

Basic concepts in evolutionary biology provide the framework for addressing this issue.One such concept is that of ‘evolutionary rescue’ (ER).

It describes the capacity of populations to survive rapid environmental change by evolving adaptations that allow survival in the new environment and restore positive population growth (Bell & Collins 2008; Gonzalez et al. 2013). If evolutionary rescue occurs, then the population dynamics are typically defined by a U-shaped curve, where a strong initial population decline is followed by an increase, as surviving (and adapted) individuals rapidly reproduce (Gomulkiewicz & Holt 1995; Bell & Gonzalez 2009; Martin et al. 2013).

The likelihood of ER depends on the effect size of the environmental stress and the rate at which environmental conditions deteriorate (Gonzalez et al. 2013). If an environmental change is sudden, then a large, rapid population decline is probable, increasing the chance of extinction. In this case, ER likely relies on existing genetic variation within the population or better adapted variants quickly entering the population through migration or mutation. In this scenario, it is argued that adaptation relies on a small number of mutations with large effects that fix rapidly (Bell 2013).

Conversely, if environmental change is more gradual, then adaptation may proceed by the slower fixation of many mutations of small effect as the population tracks the changing environmental conditions (Lynch & Lande 1993; Bell 2013). Gradual environmental degradation increases the likelihood of successful evolutionary rescue (Bell & Gonzalez 2011). Experimental studies have demonstrated that slower rates of environmental change can result in more adapted and thus more resilient, final populations or fewer extinctions (Huey et al. 1991; Perron et al. 2008; Collins & de Meaux 2009; Bell & Gonzalez 2011).

Rapidly deteriorating environments can limit both the availability of mutations due to reduced population size and can make some adapted genotypes evolutionarily inaccessible (Lindsey et al. 2013). Thus, the rate of environmental change affects both the likelihood of evolutionary rescue occurring, and the evolutionary trajectory and genetic architecture which results.

That text above was copied from the paper:

Killeen, J., Gougat-Barbera, C., Krenek, S., and Kaltz, O. (2017). Evolutionary rescue and local adaptation under different rates of temperature increase: a combined analysis of changes in phenotype expression and genotype frequency in Paramecium microcosms. Mol Ecol 26, 1734–1746.

 

Evolutionary rescue (ER) occurs when populations, which have declined due to rapid environmental change, recover through genetic adaptation. The success of this process and the evolutionary trajectory of the population strongly depend on the rate of environmental change. Here we investigated how different rates of temperature increase (from 23 to 32 °C) affect population persistence and evolutionary change in experimental microcosms of the protozoan Paramecium caudatum. Consistent with theory on ER, we found that those populations experiencing the slowest rate of temperature increase were the least likely to become extinct and tended to be the best adapted to the new temperature environment. All high-temperature populations were more tolerant to severe heat stress (35, 37 °C), indicating a common mechanism of heat protection. High-temperature populations also had superior growth rates at optimum temperatures, leading to the absence of a pattern of local adaptation to control (23 °C) and high-temperature (32 °C) environments. However, high-temperature populations had reduced growth at low temperatures (5–9 °C), causing a shift in the temperature niche. In part, the observed evolutionary change can be explained by selection from standing variation. Using mitochondrial markers, we found complete divergence between control and high-temperature populations in the frequencies of six initial founder genotypes. Our results confirm basic predictions of ER and illustrate how adaptation to an extreme local environment can produce positive as well as negative correlated responses to selection over the entire range of the ecological niche.