Are males necessary? Not if you’re an aquatic snail, a crayfish, a python or a host of other species of insects, crustaceans and reptiles. In every major group of vertebrates and many invertebrates, females occasionally or always manage to reproduce without an assist from males—every major group except, that is, mammals and so-called cartilaginous fish, which include sharks. Now the roster of animals able to produce offspring by virgin birth has a new addition. According to a genetic analysis released this week, a bonnethead (a small species of hammerhead shark) born in December 2001 at a Nebraska aquarium contained only female DNA. Her mother was her father, too, so to speak.

In 1861 Charles Darwin wrote, “We do not even in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements, instead of by a process of parthenogenesis.” Ever since, biologists have been asking, who needs sex, anyway?

The standard answer has been that sex is nearly ubiquitous in both the plant and animal kingdom because it creates genetic diversity. When DNA from two parents mix it up inside a fertilized egg, the resulting offspring are more likely to have novel genetic combinations that will stand them in good stead in a variety of conditions (it’s like packing for a trip with snow boots as well as sandals—you never know what you’ll find out there). But there is a competing argument. Evolution is all about getting your genes into the next generation—the “selfish gene,” as British biologist Richard Dawkins called it. And what better way to do that than to get all your genes into your offspring rather than half of them, which is what happens when you let a mate contribute 50 percent of the DNA to junior? Offspring that come from just an egg, or perhaps two eggs, would be all Mom.