Evolution's immediate popularity (and it was popular) owed much to the fact that it was misunderstood as a theory of steady progress from amoeba to man, a ladder of self-improvement.
As the end of the second millennium approaches, mankind is in a different mood. Progress, we think, is about to hit the buffers of overpopulation, the greenhouse effect, and the exhaus-tion of resources. However fast we run, we never seem to get anywhere: Has the industrial revolution made the average inhabitant of the world healthier, wealthier, and wiser? Yes, if he is German. No, if he is Bangladeshi. Uncannily (or, a philosopher would have us believe, predictably), evolutionary science is ready to suit the mood. The fashion in evolutionary science now is to scoff at progress; evolution is a treadmill, not a ladder.
PREGNANT VIRGINS
For people, sex is the only way to have babies, and that, plainly enough, is its purpose. It was only in the last half of the nineteenth century that anybody saw a problem with this. The problem was that there seemed to be all sorts of better ways of reproducing.
Microscopic animals split in two. Willow trees grow from cuttings.
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The Red Queen
Dandelions produce seeds that are clones of themselves. Virgin greenfly give birth to virgin young that are already pregnant with other virgins. August Weismann saw this clearly in 1889. "The significance of amphimixis [sex], he wrote, "cannot be that of making multiplication possible, for multiplication may be effected without amphimixis in the most diverse ways—by division of the organism into two or more, by budding, and even by the production of unicellular germs. "'
Weismann started a grand tradition. From that day to this, at regular intervals, the evolutionists have declared that sex is a
" problem," a luxury that should not exist. There is a story about an early meeting of the Royal Society in London, attended by the king, at which an earnest discussion began about why a bowl of water weighed the same with a goldfish in it as it did without. All sorts of explanations were proffered and rejected. The debate became quite heated. Then the king suddenly said, " I doubt your premise." He sent for a bowl of water and a fish and a balance: The experiment was done. The bowl was put on the balance, and the fish was added; the bowl 's weight increased by exactly the weight of the fish: Of course.
The tale is no doubt apocryphal, and it is not fair to suggest that the scientists you will meet in these pages are quite such idiots as to assume a problem exists when it does not. But there is a small similarity. When a group of scientists suddenly said that they could not explain why sex existed and they found the existing explanations unsatisfactory, other scientists found this intellectual sensitivity absurd. Sex exists, they pointed out; it must confer some kind of advantage. Like engineers telling bumblebees they could not fly, biologists were telling animals and plants they would be better off breeding asexually. "A problem for this argument, "
wrote Lisa Brooks of Brown University, "is that many sexual organisms seem to be unaware of the conclusion. "' There might be a few holes in existing theories, said the cynics, but do not expect is to give you a Nobel Prize for plugging them. Besides, why must sex have a purpose? Maybe it is just an evolutionary accident that reproduction happens that way, like driving on one side of the road.
THE ENIGMA
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Yet lots of creatures do not have sex at all or do it in some generations and not others: The virgin greenfly ' s great-great-granddaughter, at the end of the summer, will be sexuaclass="underline" She will mate with a male greenfly and have young that are mixtures of their parents. Why does she bother? For an accident, sex seems to have hung on with remarkable tenacity: The debate has refused to die.
Every year produces a new crop of explanations, a new collection of essays, experiments, and simulations. Survey the scientists involved now and virtually all will agree that the problem has been solved; but none will agree on the solution: One man insists on hypothesis A, another on hypothesis B, a third on C, a fourth on all of the above. Could there be a different explanation altogether? I asked John Maynard Smith, one of the first people to pose the question
" Why sex?," whether he still thought some new explanation was needed. "No. We have the answers. We cannot agree on them, that is all. "'
OF SEX AND FREE TRADE
A brief genetic glossary is necessary before we proceed. Genes are biochemical recipes written in a four-letter alphabet called DNA, recipes for how to make and run a body. A normal human being has two copies of each of 30,000 genes in every cell in his or her body.
The total complement of 6o,000 human genes is called the
" genome," and the genes live on twenty-three pairs of ribbonlike objects called "chromosomes? ' When a man impregnates a woman, each one of his sperm contains one copy of each gene, 30,000 in all, on twenty-three chromosomes. These are added to the 30,000
single genes on twenty-three chromosomes in the woman 's egg to make a complete human embryo with 30,000 pairs of genes and twenty-three pairs of chromosomes.
A few more technical terms are essential, and then we can discard the whole jargon-ridden dictionary of genetics. The first word is "meiosis," which is simply the procedure by which the male selects the genes that will go into a sperm or the female selects the
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The Rtd Queen
genes that will go into an egg. The man may choose either the 30,000 genes he inherited from his father or the seventy-five thousand he inherited from his mother or more likely, a mixture. During meiosis something peculiar happens. Each of the 23 pairs of chromosomes is laid alongside it opposite number. Chunks of one set are swapped with chunks of the other in a procedure called
" recombination." One whole set is then passed on to the offspring to be married with a set from the other parent—a procedure known as "outcrossing."
Sex is recombination plus outcrossing; this mixing of genes is its principal feature. The consequence is that the baby gets a thorough mixture of its four grandparents ' genes (because of recombination) and its two parents' genes (because of outcrossing). Between them, recombination and outcrossing are the essential procedures of sex. Everything else about it—gender, mate choice, incest avoidance, polygamy, love, jealousy—are ways of doing outcrossing and recombination more effectively or carefully.
Put this way, sex immediately becomes detached from reproduction. A creature could borrow another 's genes at any stage in its life. Indeed, that is exactly what bacteria do. They simply hook up with each other like refueling bombers, pass a few genes through the pipe, and go their separate ways. Reproduction they do later, by splitting in half.'
So sex equals genetic mixing. The disagreement comes when you try to understand why genetic mixing is a good idea. For the past century or so, traditional orthodoxy held that genetic mixing is good for evolution because it helps create variety, from which natural selection can choose. It does not change genes—even Weis-manri, who did not know about genes and referred vaguely to "ids,"
realized that—but it throws together new combinations of genes.
Sex is a sort of free trade in good genetic inventions and thus greatly increases the chances that they will spread through a species and the species will evolve. "A source of individual variability fur-nishing material for the operation of natural selection, " Weismann called sex.' It speeds up evolution.