In the years since Hamilton's simulations, empirical support for his hypothesis has been growing. There is, first, the fact that asexuality is more common in species that are little troubled by disease: boom-and-bust microscopic creatures, arctic or high-altitude plants and insects. The best test of the Red Queen hypothesis, though, was a study by Curtis Lively and Robert Vrijenhoek, then of Rutgers University in New Jersey, of a little fish in Mexico called the topminnow.

The topminnow, which sometimes crossbreeds with another similar fish to produce an asexual hybrid, is under constant attack by a parasite, a worm that causes "black-spot disease." The researchers found that the asexually reproducing topminnows harbored many more black-spot worms than did those producing sexually. That fit the Red Queen hypothesis: The sexual topminnows could devise new defenses faster by recombination than the asexually producing ones.