Creatures that can reproduce without sex have arisen sporadically throughout history, but the fossil record and other evidence show that within several thousand years they become extinct. The female's eggs just hatch. This has happened in many insects, and in some fishes, snakes, and lizards. But once males disappear, even though the species may do better than its sexual progenitors for a while, it eventually becomes extinct. Meselson and Mark Welch studied four genes in four species of bdelloids.
Sexual organisms carry two copies of any given gene, each supplied by one parent.
But in asexual organisms, there is only one parent, which supplies its offspring with both of its own copies. In each subsequent generation, the two copies of the gene are reproduced. After many millions of years, the difference betwen the two copies will have become far greater than it is in sexually-reproducing species. This is exactly what Mark Welch and Meselson found. Using standard rates of mutation in other animals, they estimate that the common female ancestor of all bdelloid rotifers lived 50 million to 100 million years ago.