Idea Sex is beneficial because it can eliminate bad mutations

One of the problems faced by asexual organisms is that, after they have a bad mutation, all their descendents will have the same bad mutation. Then, when a second bad mutation occurs in one of these descendents, all its descendents will have two bad mutations, and so on. This suggestion was first made by Hermann Joseph Muller (1890-1967), and the process is referred to as Muller's Ratchet because the increasing number of bad mutations ratchets down the organism's fitness.

This problem would be especially pronounced in small populations, in which random events might more readily result in the fixation of bad mutations (refer to Chapter 6 for info about how random events impact small populations). Sex provides a solution to this problem. Two sexual organisms, each of which had a different bad mutation, could mate and produce offspring that had neither mutation. Problem solved!

Using a virus that attacks bacteria, called phi-6, Lin Chao and coworkers tested the theory that sex can eliminate harmful mutations.

Unlike most organisms that have genomes made of DNA, the phi-6 virus has a genome made of RNA. RNA replication is much more likely to result in errors than DNA replication, and as a result, organisms with RNA genomes have much higher mutation rates. This situation makes phi-6 an excellent subject for a study involving mutations, because many mutations occur over a reasonably short experiment.

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