Environmental stress and evolution: a theoretical study

A haploid population subject to recurrent deleterious mutations and two environments that provide two different profiles of selection coefficients over loci are modeled. The population is supposed to inhabit one "home" environment where it evolves the corresponding genetic constitution. One generation of the population is then exposed to the second, "foreign" environment. The decline in the mean population fitness is considered as a measure of stress in the population caused by the foreign environment. I define the relative strength of stress as 1 minus the ratio of the mean fitnesses in the foreign and home environments, and give the corresponding analytical expression. The stress strength is composed of three different contributors: the environmental component of stress, which is determined by purely external, non-genetic factors of the foreign environment (its carrying capacity), and two genetic components. The latter consists of the environment x genetic component, caused by the direct influence of the foreign environment on selection coefficients, and of the evolutionary component that is due to the adaptation of the population to the home environment. Among others, it is shown that even if the home and foreign environments were equivalent, so that both the environmental and environmental x genetic components of stress were absent, stress would still occur in the foreign environment due to the evolutionary component. The model also predicts that stressful foreign environments cause an increase in the genotypic variance of fitness. Some other features of population variability in stressful environments are discussed. A general conclusion that can be drawn from this model is that a certain environment may be claimed as stressful only if considered with respect to both a given population and the environment in which the population has evolved.