Understanding Evolutionary Rescue and Parallelism in Response to Environmental Stress.

Abstract

Evolutionary rescue, the process by which populations facing environmental stress avoid extinction through genetic adaptation, is a critical area of study in evolutionary biology. The order in which mutations arise and get established will be relevant to the population's rescue. This study investigates the degree of parallel evolution at the genotypic level between independent populations fac- ing environmental stress and subject to different demographic regimes. Under density regulation, two regimes exist: in the first, the population can restore positive growth rates by adjusting its population size or through adaptive mu- tations, whereas in the second regime, the population is doomed to extinction unless a rescue mutation occurs. Analytical approximations for the likelihood of evolutionary rescue are obtained and contrasted with simulation results. We show that the initial level of maladaptation and the demographic regime signifi- cantly affect the level of parallelism. There is an evident transition between these two regimes. Whereas in the first regime, parallelism decreases with the level of maladaptation, it displays the opposite behavior in the rescue/extinction regime. These findings have important implications for understanding population persis- tence and the degree of parallelism in evolutionary responses as they integrate demographic effects and evolutionary processes.