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Is Muller's Ratchet A Useful Tool In Explaining The Evolution Of Sexual Reproduction?

1155 words - 5 pages

Muller's ratchet is a hypothesis that helps explain the advantage of sexual reproduction (Chao, 1997). Muller's ratchet is a very useful tool in explaining the evolution of sexual reproduction, however it has its limits. In order to understand this, an examination of Muller's ratchet, completed experiments, and its limitations must be accomplished. Mutation and drift create problems in an asexual society and the drift theory of sex declares that a sexual society will solve that problem (Freeman, 2001).
Muller's ratchet is one of the most renowned drift models (Freeman, 2001). It claims that "asexual populations are doomed to accumulate deleterious mutations" (Freeman, 2001). In 1964 Hermann Joseph Muller thought of a small asexual population where the individuals of it occasionally have "deleterious mutations" (Freeman, 2001). The asexual population could contain individuals that did not carry any mutations, one mutation, and two mutations, and so on and so forth (Freeman, 2001). He believed that over time the asexual population would pass the deleterious mutation on to its progeny (Freeman, 2001). The group of individuals that have no mutations would be small, if there were events that occur to prevent this group from reproducing. They will eventually all die out. The group that have no mutations would have had the "highest-fitness" (Freeman, 2001). Upon their extinction, the group with one mutation would then have the highest-fitness. As time passes, "highest-fitness group after highest-fitness group is lost from the population, the average fitness of the population declines over time" (Freeman, 2001). Eventually, the asexual population would consist of lots of individuals that carry the deleterious mutation (Freeman, 2001). This would then result in an eventual extinction of the species because of this deleterious mutation gene (Freeman, 2001). However, if sexual reproduction was introduced before the group that had no mutations became extinct then the population would no longer be at risk (Freeman, 2001). The odds of an asexual group producing an offspring with a deleterious mutation are greater than that of a sexual group (Freeman, 2001). "In Muller's view, the genes responsible fore sex are maintained in populations because they help to create zero-mutation genotypes" (Freeman, 2001). When the "zero-mutation genotypes increase in frequency, the genes for sex increase in frequency with them" (Freeman, 2001). According to Muller, the "linkage disequilibrium is created by drift" (Freeman, 2001). Therefore, according to Muller, "sex reduces linkage disequilibrium by re-creating the missing genotypes" (Freeman, 2001). Now that we have examined what Muller's ratchet entails we can take a look at some completed experiments.
There have been many experiments completed to test the validity of Muller's ratchet theory. One experiment completed by Lin Chao involved the molecular clock in RNA viruses (Chao, 1990). It examined the...

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