the assumption that clonal interference can be neglected during the initial phase of drift. They assumed that the probability that a mutant is not lost to drift corresponds to one minus the standard probability of fixation (as calculated for example by @fisher22, @haldane27, @kimura62) of that mutant. @orr2000 modified the calculations of Gerrish and Lenski to include beneficial mutations that arise in genomes bearing one or more deleterious mutations, but also did not address the effect of interference of other beneficial mutations during the initial phase of drift. In general, the influence of deleterious mutations on the probability of fixation has been studied extensively [@manning84; @charlesworth94; @peck94; @johnson2002], but there are very few studies that consider the effect of interfering beneficial mutations. The problem of interfering advantageous mutations is that there exists no simple theoretical framework with which their influence on drift can be described accurately. @barton95 derived an approximation that allowed him to calculate the probability of fixation of a mutant in a population that is undergoing a selective sweep. However, his approximation is valid only for very small selective advantages. @McVeanCharlesworth2000 studied a similar situation with numerical simulations, and applied their results to codon bias and levels of 