The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [55]
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 the 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 which overrules the masculinizing power of the Y. The result is an excess of females (this, incidentally, is one possible explanation of Madame B’s family). Since this puts males at a premium, you might expect that males would soon evolve the ability to produce more Y-bearing sperm than X-bearing, but they have not done so. Why? At first, biologists thought it was something to do with population explosions, during which an excess of daughters is a good idea, but recently they have worked out that this is unnecessary. The female-biased sex ratio is stable for genetic, not ecological reasons.37
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 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 are WY females, who can only have daughters. So, far from restoring the sex ratio to equality by producing only Y sperm, he has kept it imbalanced towards females. The case of the lemming demonstrates that even the invention of sex chromosomes did not prevent mutinous chromosomes from altering the sex ratio.38
Ways of Choosing Gender
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 even, at fifty-fifty. If the first sperm to reach your mother’s egg carried a Y chromosome, you are a man; if it carried an X chromosome, you are a woman. 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 neighbour, 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 upon 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 topmost 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.39
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 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.40
There is a lot to be said for such stratagems.