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
fleas are asexual: They are all female, they give birth to other females,
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,
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.
