Parasititic Influence on Evolution

Again and again in recent years evolutionary biologists have found themselves returning to the theme of parasites. As Is Richard Dawkins put it in a recent paper: "Eavesdrop [over] morning coffee at any major centre of evolutionary theory today, and you will find 'parasite' to be one of the commonest words in ti the language. Parasites are touted as the prime movers in the evolution of sex. promising a final solution to that problem of problems.'"

Parasites have a deadlier effect than predators for two reasons. One is that there are more of them. Human beings have no predators except great white sharks and one another, but they have lots of parasites. Even rabbits, which are eaten by stoats, weasels, foxes, buzzards, dogs, and people, are host to far more fleas, lice. ticks, mosquitoes, tapeworms, and uncounted varieties of protozoa. bacteria, fungi, and viruses. The myxomatosis virus has killed far more rabbits than have foxes. The second reason, which is the cause of the first, is that parasites are usually smaller than their hosts. while predators are usually larger. This means that the parasites live shorter lives and pass through more generations in a given time than their hosts. The bacteria in your gut pass through six times as many generations during your lifetime as people have passed through since they were apes.^^ As a consequence, they can multiply faster than their hosts and control or reduce the host population. The predator merely follows the abundance of its prey.

Parasites and their hosts are locked in a close evolutionary embrace. The more successful the parasite's attack (the more hosts it infects or the more resources it gets from each), the more the host's chances of survival will depend on whether it can invent a defense. The better the host defends, the more natural selection will promote the parasites that can overcome the defense. So the advantage will always be swinging from one to the other: The more dire the emergency for one, the better it will fight. This is truly the world of the Red Queen, where you never win, you only gain a temporary respite.

Parasites are small and numerous, evolving faster than we do and putting incredible evolutionary pressure on the species they plague.