MUTATION AND EXTINCTION - THE ROLE OF VARIABLE MUTATIONAL EFFECTS, SYNERGISTIC EPISTASIS, BENEFICIAL MUTATIONS, AND DEGREE OF OUTCROSSING

Recent theoretical studies have illustrated the potential role of spon taneous deleterious mutation as a cause of extinction in small populat ions. However, these studies have not addressed several genetic issues , which can in principle have a substantial influence on the risk of e xtinction. These include the presence of synergistic epistasis, which can reduce the rate of mutation accumulation by progressively magnifyi ng the selective effects of mutations, and the occurrence of beneficia l mutations, which can offset the effects of previous deleterious muta tions. In stochastic simulations of small populations (effective sizes on the order of 100 or less), we show that both synergistic epistasis and the rate of beneficial mutation must be unrealistically high to s ubstantially reduce the risk of extinction due to random fixation of d eleterious mutations. However, in analytical calculations based on dif fusion theory, we show that in large, outcrossing populations (effecti ve sizes greater than a few hundred), very low levels of beneficial mu tation are sufficient to prevent mutational decay. Further simulation results indicate that in populations small enough to be highly vulnera ble to mutational decay, variance in deleterious mutational effects re duces the risk of extinction, assuming that the mean deleterious mutat ional effect is on the order of a few percent or less. We also examine the magnitude of outcrossing that is necessary to liberate a predomin antly selfing population from the threat of long-term mutational deter ioration. The critical amount of outcrossing appears to be greater tha n is common in near-obligately selfing plant species, supporting the c ontention that such species are generally doomed to extinction via ran dom drift of new mutations. Our results support the hypothesis that a long-term effective population size in the neighborhood of a few hundr ed individuals defines an approximate threshold, below which outcrossi ng populations are vulnerable to extinction via fixation of deleteriou s mutations, and above which immunity is acquired.