MUTATION ACCUMULATION IN FINITE POPULATIONS

Two different processes contribute to mutation accumulation in finite populations: fixation of mutant alleles and Muller's ratchet. In very small random-mating populations, and with tight linkage, fixation of m utant alleles occurs at a high rate. With very restricted recombinatio n, the number of low-frequency mutant alleles per genome in random-mat ing populations also increases over time independently of fixation (Mu ller's ratchet). Increased population size affects the ratchet less th an the fixation process, and the decline in population fitness is domi nated by the ratchet in populations of size greater than about 100, es pecially with high mutation rates. Neither fixation nor the ratchet ha s serious effects unless recombination is severely restricted. The eff ects of differences in the selection parameters (strength of selection , dominance coefficient) can be interpreted in terms of opposing effec ts of selection on individual loci and associations between loci. Stro nger selection slows the accumulation of mutations, although a faster decline in mean fitness sometimes results. Increasing the dominance co efficients of the mutant alleles tends to act similarly. High inbreedi ng slows the ratchet, because with homozygosity there is a higher init ial frequency of the least loaded class, as the increased homozygous e xpression of mutant alleles in inbred populations has effects similar to stronger selection. Fixation of mildly deleterious mutations is acc elerated in highly inbred populations, but fitness decline due to muta tion accumulation was rapid only in very small populations and was alw ays much slower than for asexual populations. The effects of breeding system and rate of recombination on the rate of molecular evolution by the fixation of slightly deleterious alleles are discussed.