Fig 1.

(A) Illustration of recombination as a mechanism for purging deleterious mutations. A crossing-over event between two chromosomes carrying different deleterious mutations (a and b) produces a chromosome free of deleterious mutations (A and B). (B) Illustration of the concept of Muller’s ratchet. Without recombination, the number of deleterious mutations per chromosome increases over time in the population, and the chromosomes free of deleterious mutations will be lost by chance. The dark blue shading shows a theoretical original state, with a given mean and variance for the number of deleterious alleles per chromosome in a population. The light blue shading shows the resultant state after many generations without recombination, illustrating the Muller’s ratchet effect of an increase in the mean number of deleterious mutations.