"Sexual reproduction exists solely as a means to defeat parasites. By mixing male and female genes, sex produces offspring not exactly like either the male or female - making each generation different from the last, and presenting a moving target to intruders intent on compromising this system. ... Even with this variation, parasites continue to pose a threat... and parasitism evolves and moves through any system - not just living things. The less variation there is in a system, the more readily parasites will evolve to infest it."