Population Extinction by Mutational Load and Demographic Stochasticity

Genetic aspects are important in the evaluation of the risk of extinction for small populations. Using estimates of rates and effects of slightly deleterious mutations, we calculate the mean time to extinction under the joint action of mutation load and density-dependent stochastic population regulation. Accumulation of mutations diminishes the individual survival probability, which leads to a reduction in population size. This, in turn, progressively facilitates the fixation of future deleterious mutations by random genetic drift. This synergistic interaction has been called the mutational melt-down. In asexual populations, the probability of extinction increases as the mutational effect increases and as actual population size decreases. As reference points for sexual populations, we present the expected extinction times without mutational load but with stochastic fecundity and sex-ratio under a logistic population regulation. Selection and recombination does not prevent mutational melt-down in small sexual populations; slightly deleterious mutations reduce the mean time to extinction by several orders of magnitude. Stochastic fecundity is a minor direct source of extinction in sexual populations, but it leads to temporary reductions in effective population size, which increases the risk of extinction due to stochastic variations of the sex-ratio.