Figure 3.

Relative versus absolute stress. The frequencies of the different alleles are plotted as a function of time from the introduction of the allele A. Every 10 000 mating seasons, 10 seasons of ‘environmental stress’ occurred, decreasing the fertility of the whole population by 10%. (a) Susceptibility to relative stress. An allele for stress-related mortality based on relative stress, which causes the ‘A’ individuals to suffer increased mortality only when their fitness is lower than the deme's average. This allele increases to fixation, and the increase is accompanied by a decrease in the frequency of the allele b, and, therefore, an increase in the average population fitness. (b) Susceptibility to absolute stress. The rare events of environmental stress are sufficient to eliminate the allele for stress-related mortality based on absolute stress. The expected longevity is 10 mating seasons for stress-susceptible individuals when under stress, and 50 mating seasons for all other individuals, with n=5, θ=0.5, α=0.1, r=0.05, μ=0.01. The results do not depend on initial conditions.