Fitness Effects of New Mutations

The distribution of the fitness effects of new mutations plays a role in many aspects of evolutionary biology. Patterns of synonymous and non-synonymous mutation are discussed in terms of constraint and Fisher’s geometrical model. Experiments demonstrate that synonymous mutations can be subject to selection. Experiments show that the distribution of fitness effects is J-shaped near wild type, a result that supports Ohta’s nearly neutral theory. Reasons why so many mutations have near wild-type fitness are presented. Muller’s Ratchet predicts that mutation pressure alone can cause fitness to erode in small populations. Experiments show that this is indeed the case and that genetic exchange can mitigate the declines. Estimating the distribution of beneficial mutations is difficult because so few are beneficial. A simple model yields that L-shaped, linear and near-uniform fitness distributions are possible and that the distribution can change as evolution proceeds. Experiments illustrate the three kinds of distributions of beneficial effects: 1) new mutations, 2) those that become established in populations and 3) those that escape clonal interference and fix. Experimental difficulties with experimental protocols are discussed. Experiments confirm theoretical predictions that recombination can accelerate adaption by mitigating the effects of clonal interference.