More than 40 years ago, Bill Neaves, then a young Ph.D. student, discovered how an all-female, asexual species of the whiptail lizard came to be. He found that the lizard was a cross between the female species of one type of lizard and the male species of another.

But what has puzzled him for years is how this all-female species maintains its high level of genetic variation, a contribution to evolutionary fitness that typically comes from sexual reproduction.

Despite reproducing without a male partner, this lizard species has a strong presence in the wild. Now, Aracely Lutes, a graduate student at the Stowers Institute for Medical Research in Kansas City, Mo., where Dr. Neaves works, has figured out the missing piece of the puzzle. The findings were reported Sunday in the journal Nature.