Fig. 2.

Two methods of sampling beneficial mutations. The shaded curve represents a hypothetical fitness distribution, and the vertical gray bars represent a sample of beneficial mutations. The locations of the vertical bars on the horizontal axis represent their fitnesses, and their heights represent their hypothetical relative probabilities of being observed in a single experiment under the sampling strategy. Note that only the fitnesses, and not their probabilities of being observed, are considered in the test. (a) Under selection, the wild type fitness lies in the tail but sampling is biased toward large-effect mutations. Repeated sampling will yield multiple observations of the same mutations, though those of very small effect may not be observed in a reasonable number of replicates. Shifting alleviates the bias due to missing very small-effect mutations. (b) In gain-of-function experiments, the wild type fitness will not lie in the tail and may in fact be zero, and some beneficial mutations may not have effects large enough to be observed. Mutations with effects above some threshold will have equal probabilities of being sampled, and as these are the fittest mutations, they will lie in the tail. The fitness of the smallest-effect mutation from those sampled provides a threshold in the tail of the distribution