Rare Events in Population Genetics: Stochastic Tunneling in a Two-Locus Model with Recombination

We study the evolution of a population in a two-locus genotype space, in which the negative effects of two single mutations are overcompensated in a high-fitness double mutant. We discuss how the interplay of finite population size N and sexual recombination at rate r affects the escape times t(esc) to the double mutant. For small populations demographic noise generates massive fluctuations in tesc. The mean escape time varies nonmonotonically with r, and grows exponentially as lnt(esc) similar to N(r - r*)(3/2) beyond a critical value r*.