The observation that women should hit a “wall of death” after the menopause renders them infertile was made by Bill Hamilton, a British evolutionary biologist, in 1966. Clearly they don't. But Hamilton failed to understand why.
Hamilton's approach was purely abstract. He reasoned that harmful mutations which have their effect before the menopause (or, in the more sophisticated version, before the last child has fled the nest) will be eliminated by natural selection. Those that have their effect afterwards will not. They will thus accumulate without being winnowed out and will eventually be present in overwhelmingly lethal numbers.
Since then, what is known as the grandmother hypothesis has become popular. This suggests that women's encounter with the grim reaper is postponed because they can perform for their grandchildren services similar to those they performed for their children. This is plausible. Indeed, it is backed by data. But it may not be the only cause, and Dr Tuljapurkar has come up with a purely genetic explanation of a sort that would have been dear to Hamilton's heart.
Unlike women, men do not have their fertility cut off suddenly. Viewed as a sex alone, then, they should not face a wall of death. Rather, they should face a “slope of death” that rises as their fertility falls. Except, of course, that it takes two to tango. A man partnered with an infertile woman is, infidelity aside, as evolutionarily irrelevant as if he were sterile himself. He, too, should therefore hit the wall.
But—and this is crucial—because men almost always marry women younger than themselves, such reproductive irrelevance comes to them later in life. Indeed, if they remarry they can postpone it almost indefinitely. That means harmful mutations whose effects appear in old age can still be eliminated. And since a gene passing down the generations spends half its time in women, they get the benefit of this elimination as well. Hence no wall of death for either sex.
SEXY. (?) Also covered on FuturePundit, where you can enjoy the typically weird double-header of Randall quoting Jello Biafra and advocating eugenics in the same post!