generally get to somewhere else—if you ran very fast for a long time as we've been doing: '
you see, it takes all the running
'A new evolutionary law, ' wrote Van Valen, who sent a man-uscript to each of the most prestigious scientific journals, only to see it rejected. Yet his claim was justified. The Red Queen has become a great personage in the biological court. And nowhere has she won a greater reputation than in theories of sex. 11
Red Queen theories hold that the world is competitive to the death. It does keep changing. But did we not just hear that species are static for many generations and do not change? Yes. The THE POWER OF PARASITES
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point about the Red Queen is that she runs but stays in the same place. The world keeps coming back to where it started; there is change but not progress.
Sex, according to the Red Queen theory, has nothing to do with adapting to the inanimate world—becoming bigger or better camouflaged or more tolerant of cold or better at flying—but is all about combating the enemy that fights back.
Biologists have persistently overestimated the importance of physical causes of premature death rather than biological ones. In virtually any account of evolution, drought, frost, wind, or starvation looms large as the enemy of life. The great struggle, we are told, is to adapt to these conditions. Marvels of physical adaptation—the camel's hump, the polar bear 's fur, the rotifer 's boil-resistant tun—
are held to be among evolution' s greatest achievements. The first ecological theories of sex were all directed at explaining this adapt-ability to the physical environment. But with the tangled bank, a different theme has begun to be heard, and in the Red Queen 's march it is the dominant tune. The things that kill animals or prevent them from reproducing are only rarely physical factors. Far more often other creatures are involved—parasites, predators, and competitors.
A water flea that is starving in a crowded pond is the victim not of food shortage but of competition: Predators and parasites probably cause most of the world 's deaths, directly or indirectly. When a tree falls in the forest, it has usually been weakened by a fungus. When a herring meets its end, it is usually in the mouth of a bigger fish or a in a net. What killed your ancestors two centuries or more ago?
Smallpox, tuberculosis, influenza, pneumonia, plague, scarlet fever, diarrhea. Starvation or accidents may have weakened people, but infection killed them. A few of the wealthier ones died of old age or 22
cancer or heart attacks, but not many.
The 'great war ' of 1914—18 killed 25 million people in four years. The influenza epidemic that followed killed 25 million in four months.23 It was merely the latest in a series of devastating plagues to hit the human species after the dawn of civilization.
Europe was laid waste by measles after A:D. 165, by smallpox after A.D. 251, by bubonic plague after 1348, by syphilis after 1492, and
by tuberculosis after 1800.' And those are just the epidemics.
Endemic diseases carried away additional vast numbers of people.
Just as every plant is perpetually under attack from insects, so every animal is a seething mass of hungry bacteria waiting for an opening. There may be more bacterial than human cells in the object you proudly call ' your ' body. There may be more bacteria in and on you as you read this than there are human beings in the whole world.
Again and again in recent years evolutionary biologists have found themselves returning to the theme of parasites. As 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 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.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
