"Parasites provide exactly the incentive to change genes every generation that sex seems to demand. The success of the genes that defended you so well in the last generation may be the best of reasons to abandon the same gene combinations in the next. By the time the next generation comes around, the parasites will have surely evolved an answer to the defense that worked best in the last generation. It is a bit like sport. In chess or in football, the tactic that proves most effective is soon the one that people learn easily to block. Every innovation in attack is soon countered by another in defense."