Matt Ridley

Some species of pests then evolve ways of breaking down the tox-ins, and so on. An arms race has begun.

Antibiotics are chemicals produced naturally by fungi to kill their rivals: bacteria. But when man began to use antibiotics, he found that, with disappointing speed, the bacteria were evolving the ability to resist the antibiotics. There were two startling things about antibiotic resistance in pathogenic bacteria. One, the genes for resistance seemed to jump from one species to another, from harmless gut bacteria to pathogens, by a form of gene transfer not THE POWER OF PARASITES

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unlike sex. And two, many of the bugs seemed to have the resistance genes already on their chromosomes; it was just a matter of reinventing the trick of switching them on. The arms race between bacteria and fungi has left many bacteria with the ability to fight antibiotics, an ability they no longer ' thought they would need '

when inside a human gut.

Because they are so short-lived compared with their hosts, parasites can be quicker to evolve and adapt. In about ten years, the genes of the AIDS virus change as much as human genes change in 10 million years. For bacteria, thirty minutes can be a lifetime.

Human beings, whose generations are an eternal thirty years long, are evolutionary tortoises.

PICKING DNA'S LOCKS

Evolutionary tortoises nonetheless do more genetic mixing than evolutionary hares. Austin Burt's discovery of a correlation between generation length and amount of recombination is evidence of the Red Queen at work. The longer your generation time, the more genetic mixing you need to combat your parasites.' Bell and Burt also discovered that the mere presence of a rogue parasitic chromosome called a 'B-chromosome' is enough to induce extra recombination (more genetic mixing) in a species.' Sex seems to be an essential part of combating parasites. But how?

Leaving aside for the moment such things as fleas and mosquitoes, let us concentrate on viruses, bacteria, and fungi, the causes of most diseases. They specialize in breaking into cells—either to eat them, as fungi and bacteria do, or, like viruses, to subvert their genetic machinery for the purpose of making new viruses: Either way, they must get into cells. To do that they employ protein molecules that fit into other molecules on cell surfaces; in the jargon, they 'bind. ' The arms races between parasites and their hosts are all about these binding proteins. 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

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The Red Queen

lots of different locks; members of an asexual one will all have the same locks. So a 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. i6

The Red Queen 's case is both subtler and stronger than that, though: It is that an individual, by having sex, can produce offspring more likely to survive than an individual that produces clones of itself: The advantage of sex can appear in a single generation: This is because whatever lock is common in one generation will produce among the parasites the key that fits it: So you can be sure that it is the very lock not to have a few generations later, for by then the key that fits it will be common: Rarity is at a premium.

Sexual species can call on a sort of library of locks that is unavailable to asexual species. This library is known by two long words that mean roughly the same thing: heterozygosity and polymorphism: They are the things that animals lose when their lineage becomes inbred. What they mean is that in the population at large (polymorphism) and in each individual as well (heterozygosity) there are different versions of the same gene at any one time. The

' polymorphic' blue and brown eyes of Westerners are a good example: Many brown-eyed people carry the recessive gene for blue eyes as well; they are heterozygous. Such polymorphisms are almost as puzzling as sex to true Darwinists because they imply that one gene is as good as the other. Surely, if brown eyes were marginally better than blue (or, more to the point, if normal genes were better than sickle-cell- anemia genes), then one would gradually have driven the other extinct. So why on earth are we stuffed full of so many different versions of genes? Why is there so much heterozygosity?

In the case of sickle-cell anemia it is because the sickle gene helps to defeat malaria, so the heterozygotes (those with one normal gene and one sickle gene) are better off than those with normal genes where malaria is common, whereas the homozygotes (those with two normal genes or two sickle genes) suffer from malaria and anemia respectively.'

THE POWER OF PARASITES

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This example is so well worn from overuse in biology textbooks that it is hard to realize it is not just another anecdote but an example of a common theme. It transpires that many of the most notoriously polymorphic genes, such as the blood groups, the histocompatibility antigens and the like, are the very genes that affect resistance to disease—the genes for locks: Moreover, some of these polymorphisms are astonishingly ancient; they have persisted for geological eons: For example, there are genes that have several versions in mankind, and the equivalent genes in cows also have several versions. But what is bizarre is that the cows have the very same versions of the genes as mankind. This means that you might have a gene that is more like the gene of a certain cow than it is like the equivalent gene in your spouse: This is considerably more astonishing than it would be to discover that the word for, say, 'meat' was viande in France, fleisch in Germany, viande again in one uncontacted Stone Age village in New Guinea, and fleisch in a neighboring village.

Some very powerful force is at work ensuring that most versions of each gene survive and that no version changes very much. 3B

That force is almost certainly disease: As soon as a lock gene becomes rare, the parasite key gene that fits it becomes rare, so that lock gains an advantage: In a case where rarity is at a premium, the advantage is always swinging from one gene to another, and no gene is ever allowed to become extinct. To be sure, there are other mechanisms that can favor polymorphism: anything that gives rare genes a selective advantage over common genes: Predators often give rare genes a selective advantage by overlooking rare forms and picking out common forms. Give a bird in a cage some concealed pieces of food, most of which are painted red but a few painted green; it will quickly get the idea that red things are edible and will initially overlook green things: J: B. S Haldane was the first to realize that parasitism, even more than predation, could help to maintain polymorphism, especially if the parasite 's increased success in attacking a new variety of host goes with reduced success against an old variety—which would be the case with keys and locks:'°