"Parasites invent new keys; hosts change the locks. There is an obvious group-selectionist argument here for sex: At any one time a sexual species will have lots of different locks; members of an asexual one will all have the same locks. So the parasite with the right key will quickly exterminate the asexual species but not the sexual one. Hence the well-known fact: By turning our fields over to monocultures of increasingly inbred strains of wheat and maize, we are inviting the very epidemics of disease that can only be fought by the pesticides we are forced to use in ever larger quantities."