Why be different from your parents? . . . . [P]arasites evolve rapidly: they have short lifespans and heaving populations. It doesn’t take them long to adapt to their host at the most intimate molecular scale—protein to protein, gene to gene. Failure to do so costs them their life; success gives them the freedom to grow and replicate. If the host population is genetically identical, then the successful parasite has the run of the entire population and may well obliterate it. If the hosts very among themselves, however, there is a chance, indeed a probability, that some individuals will have a rare genotype that happens to resist the parasite. They will thrive until the parasite is obliged to focus its attention on this new genotype or face extinction itself. And so it goes on, generation after generation, cycling genotype after genotype, forever running and getting nowhere, as the Red Queen herself. So sex exists to keep parasites at bay.
Nick Lane, Life Ascending: The Ten Great Inventions of Evolution 135 (2009).
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