It was Williams who first pointed out that a huge false assumption lay, and indeed still, lies, at the core of most popular treatments of evolution. The old concept of the ladder of progress THE POWER OF PARASITES
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still lingers on in the form of a teleology: Evolution is good for species, and so they strive to make it go faster. Yet it is stasis, not change, that is the hallmark of evolution. Sex and gene repair and the sophisticated screening mechanisms of higher animals to ensure that only defect-free eggs and sperm contribute to the next generation—all these are ways of preventing change. The coelacanth, not the human, is the triumph of genetic systems because it has remained faithfully true to type for millions of generations despite endless assaults on the chemicals that carry its heredity: The old
"Vicar of Bray " model of sex, in which sex is an aid to faster evolution, implies that organisms would prefer to keep their mutation rate fairly high—since mutation is the source of all variety—and then do a good job of sieving out the bad ones: But, as Williams put it, there is no evidence yet found that any creature ever does anything other than try to keep its mutation rate as low as possible.
It strives for a mutation rate of zero: Evolution depends on the fact that it fails. 19
Tangled banks work mathematically only if there is a sufficient advantage in being odd: The gamble is that what paid off in one generation will not pay off in the next and that the longer the generation, the more this is so—which implies that conditions keep changing.
THE RED QUEEN
Enter, running, the Red Queen. This peculiar monarch became part of biological theory twenty years ago and has been growing ever more important in the years since then: Follow me if you will into a dark labyrinth of stacked shelves in an office at the University of Chicago, past ziggurats of balanced books and three-foot Babels of paper. Squeeze between two filing cabinets and emerge into a Sty-gian space the size of a broom cupboard, where sits an oldish man in a checked shirt and with a gray beard that is longer than God 's but not so long as Charles Darwin 's. This is the Red Queen's first prophet, Leigh Van Valen, a single-minded student of evolution.
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The Red' Queen
One day in 1973, before his beard was so gray, Van Valen was searching his capacious mind for a phrase to express a new discovery he had made while studying marine fossils. The discovery was that the probability a family of animals would become extinct does not depend on how long that family has already existed. In other words, species do not get better at surviving (nor do they grow feeble with age, as individuals do). Their chances of extinction are random.
The significance of this discovery had not escaped Van Valen, for it represented a vital truth about evolution that Darwin had not wholly appreciated. The struggle for existence never gets easier. However well a species may adapt to its environment, it can never relax, because its competitors and its enemies are also adapting to their niches., Survival is a zero-sum game. Success only makes one species a more tempting target for a rival species. Van Valen's mind went back to his childhood and lit upon the living chess pieces that Alice encountered beyond the looking glass. The Red Queen is a formidable woman who runs like the wind but never seems to get anywhere:
" Well, in our country," said Alice, still panting a little, " you'd generally get to somewhere else—if you ran very fast for a long time as we've been doing: "
" A slow sort of country! " said the Queen. " Now, here,
you see, it takes all the running you can do to keep in the same place: If you want to get to somewhere else, you must run at least twice as fast as that! "20
"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
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The Red Queen
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.