Imagine a Stone Age cave inhabited by two men and two women, one of them a virgin. One day the virgin gives birth “asexually” to a baby girl that is essentially her identical twin. (She becomes, in the jargon, a “parthenogen.”) It could happen in several ways—for example, by a process called “automixis,” in which an egg is, roughly speaking, fertilized by another egg. The cave woman has another daughter two years later by the same méans. Her sister, meanwhile, has had a son and a daughter by the normal method. There are now eight people in the cave. Next, the three young girls each have two
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