'' 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.
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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 sexual: 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
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
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.
Graham Bell, an English biologist working in Montreal, has THE ENIGMA
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dubbed this traditional theory the 'Vicar of Bray ' hypothesis after a fictional sixteenth-century cleric who was quick to adapt to the prevailing religious winds, switching between Protestant and Catholic rites as the ruling monarch changed. Like the flexible vic-ar, sexual animals are said to be adaptable and quick to change. The Vicar of Bray orthodoxy survived for almost a century; it still survives in biology textbooks. The precise moment when it was first questioned is hard to pin down for sure. There were doubts as far back as the 1920s. Only gradually did it dawn on modern biologists that the Weismann logic was profoundly flawed. It seems to treat evolution as some kind of imperative, as if evolving were what species exist to do—as if evolving were a goal imposed on existence.'
This is, of course, nonsense. Evolution is something that happens to organisms. It is a directionless process that sometimes makes an animal 's descendants more complicated, sometimes simpler, and sometimes changes them not at all: We are so steeped in notions of progress and self-improvement that we find it strangely hard to accept this. But nobody has told the coelacanth, a fish that lives off Madagascar and looks exactly like its ancestors of 300
million years ago, that it has broken some law by not 'evolving. '
The notion that evolution simply cannot go fast enough, and its corollary that a coelacanth is a failure because it did not become a human being, is easily refuted. As Darwin noticed, mankind has intervened dramatically to speed up evolution, producing hundreds of breeds of dogs, from chihuahuas to St. Bernards, in an evolutionary eye blink: That alone is evidence that evolution was not going as fast as it could. Indeed, the coelacanth, far from being a flop, is rather a success: It has stayed the same—a design that persists without innovation, like a Volkswagen beetle. Evolving is not a goal but a means to solving a problem.
