  The outcomes of evolution are determined by which mutations occur and fix. In
rapidly adapting microbial populations, this process is particularly hard to
predict because lineages with different beneficial mutations often spread
simultaneously and interfere with one another's fixation. Hence to predict the
fate of any individual variant, we must know the rate at which new mutations
create competing lineages of higher fitness. Here, we directly measured the
effect of this interference on the fates of specific adaptive variants in
laboratory Saccharomyces cerevisiae populations and used these measurements to
infer the distribution of fitness effects of new beneficial mutations. To do
so, we seeded marked lineages with different fitness advantages into replicate
populations and tracked their subsequent frequencies for hundreds of
generations. Our results illustrate the transition between strongly
advantageous lineages which decisively sweep to fixation and more moderately
advantageous lineages that are often outcompeted by new mutations arising
during the course of the experiment. We developed an approximate likelihood
framework to compare our data to simulations and found that the effects of
these competing beneficial mutations were best approximated by an exponential
distribution, rather than one with a single effect size. We then used this
inferred distribution of fitness effects to predict the rate of adaptation in a
set of independent control populations. Finally, we discuss how our
experimental design can serve as a screen for rare, large-effect beneficial
mutations.
