MULLERS RATCHET UNDER EPISTATIC SELECTION

In a finite asexual population mean fitness may decrease by a process known as Muller's ratchet, which proceeds if all individuals with the minimum number of deleterious alleles are randomly lost. If these alle les have independent effects on fitness, previous analysis suggested t hat the rate of this decrease either remains constant or, if accumulat ion of mutations leads to the decline of the population size, grows. H ere I show that this conclusion is quite sensitive to the assumption o f independence. If deleterious alleles have synergistic fitness effect s, then, as the ratchet advances, the frequency of the best available genotype will necessarily increase, making its loss less and less prob able. As a result, sufficiently strong synergistic epistasis can effec tively halt the action of Muller's ratchet. Instead of being driven ex tinct, a finite asexual population could then survive practically inde finitely, although with lower mean fitness than without random drift.