Animal behavior has evolved to battle parasites. In fact, we have parasites to thank for the existence of sex. Sex is a costly and time-consuming method of reproduction. Experiments have shown that, in the absence of parasites, species evolve toward parthenogenesis—or cloning—as the reproductive method of choice. In parthenogenesis each individual is able to self-replicate. But this produces almost no genetic variation. In the presence of parasites, cloning, while more energy-efficient, is not a viable reproductive strategy. It presents a stationary genetic target to parasites, who, once introduced into such a system, will quickly dominate it....

Chapter 31: Red Queen Hypothesis, Character: Sobol - Daemon (2006)