A comparison of model fit for the gamma model and the two null models for the B mutation (A) and the F mutation (B).

Fitness effects were measured in units of doublings per hour. For the null fitness model, we assumed that the fitnesses of the mutants were normally distributed and estimated the mean and variance. For the null fitness effect model, we assumed that the effects of mutations were normally distributed with mean zero and estimated the variance. The numbers of parameters in the models are given by Note that the null fitness model was used as the basis of our calculations and therefore has no value associated with it.