less beneficial mutant will in fact survive (see Fig. 1). Yet, the probability that either one of the mutants survives does not depend on the timing of their introduction.

A number of authors noted the reduction of fixation probabilities caused by segregating deleterious mutations [@peck94; @manning84; @charlesworth94; @orr2000; @johnson2002], and we observe the same qualitative behavior here. We find that a background of deleterious mutations suppresses the fixation probability, and this suppression is more pronounced for the less beneficial mutant. In the model presented here, there is a simple relationship between beneficial and deleterious mutations: Each mutation occurring on a clone created by a beneficial mutation is reduced to the wild type fitness. This simplified model corresponds to a single-peak landscape, exhaustively studied in the context of quasispecies theory [@tarazona92; @SwetinaSchuster82; @galluccio97; @camposfonta98]. Consequently, we expect a more pronounced decrease of the probability of fixation for small $s$ due to the existence of an error-threshold, i.e., a point at which information is lost from the sequence due to a critically high mutation rate, see [@eigen]. In the stochastic regime of evolution, which occurs for mutation rates above the error threshold, individuals replicate randomly, and each member of the population is 