Each of these characters champions a type of explanation for sex. The molecular biologist is essentially talking about why sex was invented, which is not necessarily the same question as what sex achieves today, the question the geneticist prefers to address.
The ecologist, meanwhile, is asking a slightly different question:
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Th'e Red Queen
Under what circumstances is sex better than asex? An analogy might be the reasons for the invention of computers. The historian (like the molecular biologist) will insist they were invented to crack the codes used by German submarine commanders. But they are not used for that today. They are used to do repetitive tasks more efficiently and quickly than people can (the geneticist's answer). The ecologist is interested in why computers have replaced telephone operators but not, say, cooks. All three may be
" right" on different levels.
THE MASTER-COPY THEORY
The leader of the molecular biologists is Harris Bernstein of the University of Arizona. His argument is that sex was invented to repair genes. The first hint of this was the discovery that mutant fruit flies unable to 'repair genes are unable to "recombine " them, either. Recombination is the essential procedure in sex, the mixing of genes from the two grandparents of the sperm or egg. Knock out genetic repair, and sex stops, too.
Bernstein noticed that the tools the cell uses for sex are the same as it uses to repair genes. But he has been unable to convince the geneticists or the ecologists that repair is more than the original, long superseded purpose of the machinery sex uses. The geneticists say the machinery of sex did indeed evolve from the machinery of gene repair, but that is not the same thing as saying sex exists today to repair genes. After all, human legs are the descendants of fishes ' fins, but they are designed nowadays for walking, not swimming."
A quick digression into molecules is necessary here. DNA, the stuff of genes, is a long, thin molecule that carries information in a simple alphabet of four chemical "bases," like Morse code with two kinds of dots and two kinds of dashes. Call these bases "letters" : A, C, G, and T. The beauty of DNA is that each letter is complementary to another, meaning that it prefers to align itself opposite that other letter. Thus A pairs with T and vice versa, C
THE ENIGMA
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with G and vice versa. This means there is an automatic way of copying DNA: by going along the strand of the molecule, stitching together another from the complementary letters. The sequence AAGTTC becomes, on the complementary strand, TTCAAG; copy that and you get the original sequence back again. Every gene normally consists of a strand of DNA and its complementary copy closely entwined in the famous double helix. Special enzymes move up and down the strands, and where they find a break, repair it by reference to the complementary strand. DNA is continually being damaged by sunlight and chemicals. If it were not for the repair enzymes, it would quite quickly become meaningless gobbledygook.
But what happens when both strands are damaged at the same place? This can be quite common—for example, when the two strands get fused together like a spot of glue on a closed zipper. The repair enzymes have no way of knowing what to repair the DNA to: They need a template of what the gene used to look like: Sex provides it: It introduces a copy of the same gene from another creature (outcrossing) or from another chromosome (recombination) in the same creature. Repair can now refer to a fresh template: Of course, the fresh template may also be damaged at the same place, but the chances of that are small. A shopkeeper adding up a list of prices makes sure he has it right the first time by simply repeating the task. His reasoning is that he is unlikely to make the same mistake twice.
The repair theory is supported by some good circumstantial evidence. For example, if you expose a creature to damaging ultraviolet light, it generally fares better if it is capable of recombination than if it is not, and it fares better still if it has two chromosomes in its cells. If a mutant strain appears that eschews recombination, it proves to be especially susceptible to damage by ultraviolet light: Moreover, Bernstein can explain details that his rivals cannot—for example, the curious fact that just before dividing its chromosome pairs in two to make an egg, a cell will double the number and then dispose of three-quarters of the proceeds. In the repair theory, this is to find, and convert to a "common currency, " the errors that are to be repaird.''
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The Red Queen
Nonetheless, the repair theory remains inadequate to the task it has set itself: It is silent on outcrossing. Indeed, if sex is about getting spare copies of genes, it would be better to get them'
from relatives rather than seek out unrelated members of the species: Bernstein says outcrossing is a way of masking mutations, but this amounts to no more than a restatement of the reason why inbreeding is a bad thing; and sex is the cause of inbreeding, not the consequence:
Moreover, every argument that the repair people give for recombination is merely an argument for keeping backup copies of genes, and there is a far simpler way of doing that than swapping them at random between chromosomes. It is called "diploidy: "ZB An egg or a sperm is "haploid"—it has one copy of each gene. A bacterium or a primitive plant, such as moss, is the same. But most plants and nearly all animals are diploid, meaning they have two copies of every gene, one from each parent: A few creatures, especially plants that are descended from natural hybrids or have been selected by man for large size, are "polyploid." Most hybrid wheat, for example, is " hexaploid "; it has six copies of each gene: In yams, female plants are "octoploid " or hexaploid, males all "tetraploid "
—a discrepancy that renders yams sterile. Even some strains of rain-bow trout and domestic chicken are "triploid "—plus a single parrot that turned up a few years ago: 1° Ecologists have begun to suspect that polyploidy in plants is a sort of alternative to sex: At high altitudes and high latitudes many plants seem to abandon sex in favor of asexual polyploidy:3°
But by mentioning ecologists we are getting ahead of ourselves: The point at issue is gene repair: If diploid creatures were to indulge in a little recombination between chromosomes every time their cells divided as the body grew, there would be plenty of opportunity for repair: But they do not: They recombine their genes only at the final peculiar division called meiosis that leads to the formation of an egg or a sperm. Bernstein has an answer for this: He says that there is another, more economical way to repair damage to genes during ordinary cell division, which is to allow the fittest cells to survive: There is no need for repair at that stage THE ENIGMA
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because the undamaged cells will soon outgrow the damaged ones.
Only when producing germ cells, which go out to face the world alone, need you check for errors."
The verdict on Bernstein: unproven. Certainly the tools of sex seem to be derived from the tools of repair, and certainly recombination achieves some gene repair. But is it the purpose of sex? Probably not.