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occur within genomes lacking recombination. The combination of mutation, genetic drift, and natural selection results in the progressive loss of the class of individuals in a population with the fewest mutations, in a phenomenon called Muller's Ratchet (Muller 1964; Maynard Smith 1978; Charlesworth & Charlesworth 1997). The name is an analogy to a mechanical device like a ratchet wrench that permits rotation in only one direction. Muller's Ratchet results in the accumulation of more and more mutations in a population, which leads to ever-declining average fitness in populations if most mutations are deleterious. Thus, Muller's Ratchet demonstrates a selective advantage of recombination under some conditions.

To see how Muller's Ratchet works in detail, consider a finite population of haploid individuals that reproduce clonally. Assume that all mutations at all loci are equally deleterious and acted against by natural selection to the same degree. The selective disadvantage is s at each locus with a mutation and the total selection coefficient against an individual with n mutated loci is (1 - s)n. Further, assume that mutation is irreversible and can only make deleterious alleles from wild-type alleles but not wild-type alleles from deleterious ones. Initially, all individuals in the population start off with no mutations. Mutations that occur decrease the proportion of individuals with no mutations and increase the frequencies of individuals with 1,2,3 ... n mutations. Over time, the frequency of the zero mutation category declines while the frequencies of individuals with one or more mutations increases. This process can be seen in the top two panels of Fig. 5.7.

Figure 5.7 Simulation results show the action of Muller's Ratchet in increasing the number of deleterious mutations in the absence of recombination. Initially, all haploid individuals in the population have zero mutations. Mutations occur randomly over time and continually reduce the frequency of individuals with fewer mutations. Genetic drift causes sampling error and the stochastic loss of mutation classes with few individuals. Individuals with more mutations are less likely to reproduce, due to natural selection against deleterious alleles. Once the category with fewest mutations (e.g. the zero mutations class) is lost due to genetic drift and mutation, there is no process that can repopulate it. Therefore, the distribution of the number of mutations continually moves to the right but can never move back to the left. The simulation parameters were Ne = 200 and = 0.06, each mutation reduced the chance of reproduction by 1%, and each individual had 100 loci.

Figure 5.7 Simulation results show the action of Muller's Ratchet in increasing the number of deleterious mutations in the absence of recombination. Initially, all haploid individuals in the population have zero mutations. Mutations occur randomly over time and continually reduce the frequency of individuals with fewer mutations. Genetic drift causes sampling error and the stochastic loss of mutation classes with few individuals. Individuals with more mutations are less likely to reproduce, due to natural selection against deleterious alleles. Once the category with fewest mutations (e.g. the zero mutations class) is lost due to genetic drift and mutation, there is no process that can repopulate it. Therefore, the distribution of the number of mutations continually moves to the right but can never move back to the left. The simulation parameters were Ne = 200 and = 0.06, each mutation reduced the chance of reproduction by 1%, and each individual had 100 loci.

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