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.26 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 THE POWER OF PARASITES
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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.
BATTLES OF WIT
It is also the inconstant world of sex. 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 these 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 to block easily. Every innovation in attack is soon countered by another in defense.
But of course the usual analogy is an arms race, America builds an atom bomb, so Russia does, too. America builds missiles; so must Russia: Tank after tank, helicopter after helicopter, bomber after bomber, submarine after submarine, the two countries run against each other, yet stay in the same place: Weapons that would have been invincible twenty years before are now vulnerable and obsolete. The bigger the lead of one superpower, the harder the other tries to catch up: Neither dares step off the treadmill while it can afford to stay in the race. Only when the economy of Russia collapses does the arms race cease (or pause):"
These arms race analogies should not be taken too seriously, but they do lead to some interesting insights. Richard Dawkins and John Krebs raised one argument derived from arms races to the level of a "principle": the " life-dinner principle. " A rabbit running from a fox is running for its life, so it has the greater evolutionary incentive to be fast: The fox is merely after its dinner: True enough, but what about a gazelle running from a cheetah? Whereas foxes eat things other than rabbits, cheetahs eat only gazelles. A slow gazelle might never be unlucky enough to meet a cheetah, but a slow chee-
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The Red Queen
tah that never catches anything dies. So the downside is greater for the cheetah. As Dawkins and Krebs put it, the specialist will usually win the race."
Parasites are supreme specialists, but arms race analogies are less reliable for them. The flea that lives in the cheetah 's ear has what economists call an " identity of interest " with the cheetah: If the cheetah dies, the flea dies. Gary Larson once drew a cartoon of a flea walking through the hairs on a dog 's back carrying a placard that read: THE END OF THE DOG IS NEAR: The death of the dog is bad news for the flea, even if the flea hastened it. The question of whether parasites benefit from harming their hosts has vexed para -
sitologists for many years. When a parasite first encounters a new host (myxomatosis in European rabbits, AIDS in human beings, plague in fourteenth-century Europeans) it usually starts off as extremely virulent and gradually becomes less so. But some diseases remain fatal, while others quickly become almost harmless: The explanation is simple: The more contagious the disease, and the fewer resistant hosts there are around, the easier it will be to find a new host. So contagious diseases in unresistant populations need'
not worry about killing their hosts, because they have already moved on. But when most potential hosts are already infected or resistant, and the parasite has difficulty moving from host to host, it must take care not to kill its own livelihood: In the same way an industrial boss who pleads with his workers, " Please don't strike or the company will go bust, " is likely to be more persuasive if unem-ployment is high than if the workers already have other job offers.
Yet, even where virulence declines, the host is still being hurt by the parasite and is still under pressure to improve its defenses, while the parasite is continually trying to get around those defenses and sequester more resources to itself at the host 's expense.'
ARTIFICIAL VIRUSES
Startling proof of the fact that parasites and hosts are locked in evolutionary arms races has come from a surprising source: the innards of computers: In the late 1980s evolutionary biologists THE POWER OF PARASITES
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began to notice a new discipline growing among their more computer-adept colleagues called artificial life: Artificial life is a hubristic name for computer programs that are designed to evolve through the same process of replication, competition, and selection as real life: They are, in a sense, the ultimate proof that life is just a matter of information and that complexity can result from directionless competition, design from randomness.
If life is information and life is riddled with parasites, then information, too, should be vulnerable to parasites. When the history of computers comes to be written, it is possible that the first program to earn the appellation "artificially alive " will be a deceptively simple little two-hundred-line program written in 1983 by Fred Cohen, a graduate student at the California Institute of Technology. The program was a "virus" that would insinuate copies of itself into other programs in the same way a real virus insinuates copies of itself into other hosts. Computer viruses have since become a worldwide problem. It begins to look as if parasites are inevitable in any system of life.'
But Cohen's virus and its pesky successors were created by people. It was not until Thomas Ray, a biologist at the University of Delaware, conceived an interest in artificial life that computer parasites first appeared spontaneously. Ray designed a system called Tierra that consisted of competing programs that were constantly being filled by mutation with small errors. Successful programs would thrive at one another 's expense: The effect was astonishing. Within Tierra, programs began to evolve into shorter versions of themselves. Programs that were seventy-nine instructions long began to replace the original eighty-instruction programs: But then suddenly there appeared versions of the program just forty-five instructions long: They borrowed half of the code they needed from longer programs. These were true parasites. Soon a few of the longer programs evolved what Ray called immunity to parasites: One program became impregnable to the attentions of one parasite by concealing part of: itself. But the parasites were not beaten. A mutant parasite appeared in the soup that could find the concealed lines."