 They genetically manipulated the mating system of "C. elegans", causing populations to mate either sexually, by self-fertilization, or a mixture of both within the same population.Then they exposed those populations to the "S. marcescens" parasite.It was found that the self-fertilizing populations of "C. elegans" were rapidly driven extinct by the coevolving parasites, while sex allowed populations to keep pace with their parasites, a result consistent with the Red Queen hypothesis.