or mutant) is replaced by a mutant of the other type. Throughout this paper, we will understand $\Delta t$ to be the difference in generations between the appearance of mutant 2 and mutant 1, so that negative values of $\Delta t$ indicate that mutant 2 appeared before mutant 1.

The population is finite of size $N$, and replication takes place in discrete generations, according to the Wright-Fisher model: All individuals in generation $t$ are direct descendants of the individuals of the previous generation; the probability that an individual is the offspring of a particular parent is proportional to the parent’s fitness. We introduce deleterious mutations into the offspring organisms with probability $u$. If a mutant of type 1 or 2 is hit by a mutation, then its fitness is set to 1 (that is, it reverts to the lower fitness wild type). We do not consider deleterious mutations in the wild type genotype.

We consider a genotype to be fixed if it has become the most-recent common ancestor of the whole population, regardless of whether some individuals in the population have a different genotype. This definition of fixation has been used recently to study fixation of beneficial mutations in a 