St^^H»^ First, Chao established that he could actually observe the phenomenon of Muller's ratchet in the laboratory. He produced viruses that had decreased fitness due to the accumulation of harmful mutations. He did this by making sure that his laboratory population sizes would become quite low periodically. When population sizes are low, evolution via genetic drift can lead to fixation of deleterious mutations, which was what Chao observed. (Refer to Chapter 6 for information about genetic drift.)
Then Chao randomly plucked a few viruses out of one flask to populate the next flask, creating the next generation. (Whereas in a large population, the process of natural selection would weed out less-fit genotypes, Chao increased the chances that a harmful mutation would make it to the next generation by randomly choosing a few individuals. In small populations, a virus that's a little less fit still would do fine because it wouldn't have the more-fit strains breathing down its neck.) By repeating this process numerous times, Chao ended up with a population of less-fit viruses on which he could test his idea that sexual reproduction can increase fitness.
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