And so the arms race escalated: Sometimes when he ran the
::: 70 :::
The Red Queen
computer, Ray was confronted with spontaneously appearing hyperparasites, social hyperparasites, and cheating hyper-hyperparasites—all within an evolving system of (initially) ridiculous simplicity. He had discovered that the notion of a host-parasite arms race is one of the most basic and unavoidable consequences of evolution."
Arms race analogies are flawed, though. In a real arms race, an old weapon rarely regains its advantage. The day of the longbow will not come again. In the contest between a parasite and its host, it is the old weapons, against which the antagonist has forgotten how to defend, that may well be the most effective. So the Red Queen may not stay in the same place so much as end up where she started from, like Sisyphus, the fellow condemned to spend eternity rolling a stone up a hill in Hades only to see it roll down again.
There are three ways for animals to defend their bodies against parasites. One is to grow and divide fast enough to leave them behind. This is well known to plant breeders, for example: The tip of the growing shoot into which the plant is putting all its resources is generally free of parasites. Indeed, one ingenious theory holds that sperm are small specifically so they have no room to carry bacteria with them to infect eggs." A human embryo indulges in a frenzy of cell division soon after it is fertilized, perhaps to leave behind any viruses and bacteria stuck in one of the compart-ments. The second defense is sex, of which more anon. The third is an immune system, used only by the descendants of reptiles. Plants and many insects and amphibians have an additional method: chemical defense. They produce chemicals that are toxic to their pests.
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
::: 71 :::
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
::: 72 :::
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
::: 73
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.