Shaking the ‘deleterious mutations’ dogma?

Estimating the genome-wide mutation rate and the distribution of mutation effects on fitness is important both for the evolution of sex and the fate of small populations. To date, most studies have suggested that most mutations with phenotypic effects tend to be deleterious to fitness. However, Shaw et al., studying the effect of mutations on fitness in Arabidopsis thaliana, now suggest that mutations can often have beneficial effects. These findings have been questioned by Keightley and Lynch.