The Queen - Matt Ridley

The case of Madame B, as she was described, is tantalizing in the extreme. Did her daughters and nieces have only daughters?

Did her first cousins? Is there still, in Nancy, an ever-growing dynasty of women, so that the city 's sex ratio will soon be unbal-

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The Red Queen

anced? Was the explanation proffered by the French doctors the right one? If so, what was the gene and wherein did it live? It might have been in a parasite or in an organelle. How did it work? We may never know:

THE ALPHABETICAL BATTLE OF THE LEMMINGS

With the exception of some female inhabitants of the city of Nancy, the gender of a human being is determined by his or her sex chromosomes: When you were conceived, your mother 's egg was chased by two kinds of your father 's sperm, one containing an X

chromosome and one containing a Y chromosome: Whichever got there first decided your gender: Among mammals, birds, most other animals, and many plants, this is the usual way of going about things: Gender is determined genetically, by sex chromosomes: Those with an X and a Y are male, those with two Xs are female.

But even the invention of sex chromosomes and their success in largely suppressing the rebellion of cytoplasmic genes did not succeed in making life harmonious in the society of genes. The sex chromosomes themselves began to have an interest in the gender of their owners ' children. In man, for instance, the genes that control gender are on the Y chromosome. Half of a man 's sperm are X carriers and half are Y carriers. To father a daughter, the man must fertilize his mate with an X carrier. In doing so he passes none of the Y's genes to her: From the Y ' s point of view, his daughter is unrelated to him. Therefore, a Y gene that causes the death of all the man 's X-bearing sperm and ensures its own monopoly of the man' s children will thrive at the expense of all other kinds of Y genes: That all those children are sons and the species will therefore go extinct matters not in the least to the Y; he has no foresight.

This phenomenon of the " driving Y " was first predicted by Bill Hamilton in

He saw it as a powerful danger that was

1967."

liable to drive species extinct suddenly and silently. He wondered what prevented it from happening, if anything did: One solution GENETIC MUTINY AND GENDER

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was to gag the Y chromosome, removing all but its gender-determining role: Indeed, Y chromosomes are kept in a kind of house arrest most of the time: Only a few of their genes are expressed, and the rest are entirely silent. In many species gender is determined not by the Y chromosome but by the ratio of the number of X chromosomes to the number of ordinary chromosomes. One X

fails to masculinize a bird, two succeed; and in most birds, the Y

chromosome has withered away altogether.

The Red Queen is at work. Far from settling down to a fair and reasonable way of determining gender, nature has to face an infinite series of rebellions: It suppresses one only to find it has opened the way to another: For this reason gender determination is amechanism full of, in the words of Cosmides and Tooby, "meaningless complexity manifesting unreliability, aberrations, and (from the individual 's point of view) waste." "

But if the Y chromosome can drive, so can the X. The lemming is a fat arctic mouse famous among cartoonists for apocryphally throwing itself off cliffs in hordes: It is famous among biologists for its tendency to explode in numbers and then collapse again when overcrowding has destroyed its food supply. But it is notable for another reason: It has a peculiar way of determining the gender of its babies: It has three sex chromosomes, W, X, and Y. XY

is a male; XX, WX, and WY are all females. YY cannot survive at all: What has happened is that a mutant form of driving X chromosome, W, has appeared that overrules the masculinizing power of the Y The result is an excess of females. Since this puts males at a premium, you might expect that males would soon evolve the ability to produce more Y-bearing than X-bearing sperm, but they have not done so. Why? At first biologists thought it had something to do with population explosions during which an excess of daughters is a good idea, but recently they have determined that this is unnecessary: The female-biased sex ratio is stable for genetic, not ecological, reasons:"

A male that produces only Y sperm can mate with an XX

female and produce all sons (XY) or with a WX female and produce half sons and half daughters or with a WY female. In the last

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The Red Queen

case he has only WY daughters because YY sons die: The net result, therefore, is that if he mates with one of each, he will have as many daughters as sons, and all his daughters will be WY

females, who can have only daughters: So, far from restoring the sex ratio to equality by producing only Y sperm, he has kept it unbalanced toward females: The case of the lemming demonstrates that even the invention of sex chromosomes did not prevent mutinous chromosomes from altering the sex ratio."

LOTTERY OR CHOICE?

Not all animals have sex chromosomes: Indeed, it is hard to see why so many do. They make gender a pure lottery, governed by an arbitrary convention with the sole advantage of (usually) keeping the sex ratio at fifty-fifty: If the first sperm to reach your mother 's egg carried a Y chromosome, you are a male; if it carried an X chromosome, you are a female. There are at least three different and better ways to determine your gender.

The first, for sedentary creatures, is to choose the gender appropriate to your sexual opportunities. For example, be a different gender from your neighbor because he or she will probably turn out to be your mate. A slipper limpet, which delights in the Latin name Crepidula fornicata, begins life as a male and becomes a female when it ceases peregrinating and settles on a rock; another male lands on it, and gradually it, too, turns female; a third male lands, and so on, until there is a tower of ten or more slipper limpets, the bottom ones being female, the top ones male. A similar method of gender determination is employed by certain reef fish: The shoal consists of lots of females and a single large male: When he dies, the largest female simply changes gender. The blue-headed wrasse changes gender from female to male when it reaches a certain size.'"

This sex change makes good sense from the fish' s point of view because there is a basic difference between the risks and rewards of being male or female: A large female fish can lay only a few more eggs than a small one, but a large male fish, by fighting GENETIC MUTINY AND GENDER

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for and winning a harem of females, can have a great many more offspring than a small male. Conversely, a small male does worse than a small female because he fails to win a mate at all. Therefore, among polygamists the following strategy often appears: If small, be female; if large, be male:"

There is a lot to be said for such stratagems. It is profitable to be a female while growing up and get some breeding done, and then change sex and hit the jackpot as a polygamist male once you are big enough to command a harem: Indeed, the surprise is that more mammals and birds do not adopt this system: Half-grown male deer spend years in a state of celibacy awaiting the chance to breed, while their sisters produce a fawn a year.

A second way of determining gender is to leave it to the environment. In some fish, shrimp, and reptiles, gender is determined by the temperature at which the egg is incubated. Among turtles, warm eggs hatch into females; among alligators, warm eggs hatch into males; among crocodiles, warm and cool eggs hatch into females, intermediate ones into males: (Reptiles are the most adventurous sex determiners of all: Many lizards and snakes use genetic means, but whereas XY iguanas become male and XX