Mutation supplies new genetic variation. Variation is the raw material upon which natural selection operates. Most mutations are harmful, causing crucial dysfunctions and bringing the mutant forms to an evolutionary dead end. But occasionally a mutation happens to be useful and adaptive. And the more mutations occurring, the greater chance that good ones will turn up. (More mutations also mean more chance of harmful ones, lethal to the virus; this puts a cap on the maximum sustainable mutation rate.) RNA viruses therefore evolve quicker than perhaps any other class of organism on Earth. It’s
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