There are now eight people in the cave. Next, the three young girls each have two children and the first generation dies off. Now there are ten people in the cave, but five of them are parthenogens. In two generations the gene for parthenogenesis has spread from one-quarter to one-half of the population. It will not be long before men are extinct.
This is what Williams called the cost of meiosis and Maynard Smith called the cost of males. For what dooms the sexual cave people is simply that half of them are men, and men do not produce babies. It is true that men do occasionally help in child rearing, killing woolly rhinos for dinner or whatever, but even that does not really explain why men are necessary. Suppose that the asexual women at first gave birth only when they had intercourse.
Again there are precedents. There are grasses that only set seed when fertilized by pollen from a related species, but the seed inherits no genes from the pollen. It is called " pseudogamy. "'° In this case the men in the cave would have no idea that they are being genetically excluded and would treat the asexual babies as their own, serving woolly rhino meat to them just as they would to their own children.
This thought-experiment illustrates the numerically huge advantage a gene that makes its owner asexual has. Logic such as this set Maynard Smith, Ghiselin, and Williams to wondering what compensating advantage of sex there must be, given that every mammal and bird, most invertebrate animals, most plants and fungi, and many protozoa are sexual.
For those who think that to talk about the "cost of sex " is merely to illustrate how absurdly pecuniary we have become, and who reject the whole logic of this argument as specious, I offer the THE ENIGMA
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following challenge. Explain hummingbirds—not how they work but why they exist at all. If sex had no cost, hummingbirds would not exist. Hummingbirds eat nectar, which is produced by flowers to lure pollinating insects and birds. Nectar is a pure gift by the plant of its hard-won sugar to the hummingbird, a gift given only because the hummingbird will then carry pollen to another plant.
To have sex with another plant, the first plant must bribe the pollen carrier with nectar. Nectar is therefore a pure, unadulterated cost incurred by the plant in its quest for sex. If sex had no cost, there would be no hummingbirds. 2'
Williams was inclined to conclude that perhaps his logic was good, but for animals like us the practical problems were simply insurmountable. In other words, getting from being sexual to being asexual would indeed confer advantages, but it would be just too difficult to achieve. About this time sociobiologists were beginning to fall into a trap of being too readily enamored of
" adaptationist" arguments—just-so stories, as Stephen Jay Gould of Harvard called them. Sometimes, he pointed out, things were the way they were for accidental reasons. Gould 's own example is of the triangular space between two cathedral arches at right angles, known as a spandrel, which has no function but is simply the by-product of putting a dome on four arches. The spandrels between the arches on St. Mark 's Basilica in Venice were not there because somebody wanted spandrels. They were there because there is no way to put two arches next to each other without producing a space in between. The human chin may be such a spandrel; it has no function but is the inevitable result of having jaws. Likewise the fact that blood: is red is surely a photochemical accident, not a design feature. Perhaps sex was a spandrel, an evolutionary relic of a time when it served a purpose. Like chins or little toes or appendix-es, it no longer served a purpose but was not easily got rid of. 31
Yet this argument for sex is pretty unconvincing because quite a few animals and plants have abandoned sex or have it only occasionally. Take the average lawn. The grass in it never has sex—
unless you forget to cut it, at which point it grows flower heads.
And what about water fleas? For many generations in a row water
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The Red Queen
fleas are asexuaclass="underline" They are all female, they give birth to other females, they never mate. Then as the pond fills up with water fleas, some start to give birth to males, which mate with other females to produce "winter " eggs that lie on the bottom of the pond and regenerate when the pond is flooded again. Water fleas can turn sex on and off again, which seems to prove that it has some immediate purpose beyond helping evolution to happen. It is worth an individual water flea 's while to have sex at least in certain seasons.
So we are left with an enigma. Sex serves the species but at the expense of the individual. Individuals could abandon sex and rapidly outcompete their sexual rivals. But they do not. Sex must therefore in some mysterious manner "pay its way" for the individual as well as for the species. How?
PROVOCATION BY IGNORANCE
Until the mid-1970s the debate that Williams had started remained an arcane and obscure one: And the protagonists sounded fairly confident in their attempts to resolve the dilemma. But in the mid-1970s two crucial books changed that forever by throwing down a gauntlet that other biologists could not resist picking up.
One book was by Williams himself, the other by Maynard Smith. 23
" There is a kind of crisis at hand in evolutionary biology, " wrote Williams melodramatically. But whereas Williams 's book, Sex and
Evolution, was an ingenious account of several possible theories of sex—an attempt to defuse the crisis—Maynard Smith 's book, The
Evolution of Sex, was very different: It was a counsel of despair and
,
bafflement. Again and again Maynard Smith came back to the enormous price of sex: the twofold disadvantage—two parthenogenetic virgins can have twice as many babies as one woman and one man: Again and again he declared it insurmountable by current theories.
" I fear the reader may find these models insubstantial and unsatisfactory," he wrote. "But they are the best we have. " And in a separate paper: "One is left with the feeling that some essential feature THE ENIGMA
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of the situation is being overlooked."" By insisting that the problem was emphatically not solved, Maynard Smith 's book had an electrifying impact: It was an unusually humble and honest gesture.
Attempts to explain sex have since proliferated like libidi-nous rabbits: They present an unusual spectacle to the observer of science: Most of the time scientists are groping around in a barrel of ignorance trying to find a fact or a theory or to discern a pattern where none had been seen before. But this was a rather different game. The fact—sex—was well known. To explain it—to give sex an advantage—was not sufficient. The proffered explanation had to be better than others: It is like the gazelle running faster than other gazelles rather than running faster than cheetahs. Theories of sex are a dime a dozen, and most are "right" in the sense of making logical sense: But which is most right?"
In the pages that follow you will meet three kinds of scientists. The first is a molecular biologist, muttering about enzymes and exonucleolytic degradation. He wants to know what happens to the DNA of which genes are made. His conviction is that sex is all about repairing DNA or some such molecular engineering. He does not understand equations, but he loves long words, usually ones he and his colleagues have invented: The second is a geneticist, all mutations and Mendelism. He will be obsessed with describing what happens to genes during sex. He will demand experiments, such as depriving organisms of sex for many generations to see what happens: Unless you stop him, he will start writing equations and talking of "linkage disequilibria: " The third is an ecologist, all parasites and polyploidy: He loves comparative evidence: which species has sex and which does not: He knows a plethora of extraneous facts about the arctic and the tropics. His thinking is a little less rigorous than others, his language a little more colorfuclass="underline" His natural habitat is the graph, his occupation the computer simulation.