This interference of advantageous mutations has long been recognized as a potential disadvantage of asexual populations [@fisher30; @Muller64; @hill66]. Recently, several groups have worked on an exact quantification of the interference effect, both in theoretical studies [@barton95; @gerrish98; @orr2000; @gerrish2001; @McVeanCharlesworth2000] and in experimental studies with bacteria [@visser99; @rozen2002; @shaver2002] and viruses [@miralles99; @cuevas2002]. A good quantitative understanding of the interference effect is necessary in order to assess the influence that interference has on the patterns of molecular evolution and variation in large populations.

There are two separate dynamics that both contribute to the overall interference effect: First, if two advantageous mutants are both present in sufficiently high concentrations, such that loss to drift can be neglected, then they compete deterministically, and the mutant with the higher selective advantage will replace the other one. Second, if at least one advantageous mutant is still very rare, then we have to consider the influence of other mutants’ presence on the chance that this mutant is lost to drift. To date, there is no single theory that takes into account both dynamics to their full extent. @gerrish98 calculated the speed of adaptation as a function of population size and beneficial mutation rate, under 