In many species of plants and animals, females can reproduce without mating. Specifically, they produce only daughters, which are genetically identical (clones) of the mother. One of the major questions in Biology asks why this method of clonal reproduction is not more common, especially given the two-fold reproductive advantage of clonal reproduction. In this talk, I present the results of studies on a freshwater New Zealand snail, where clonal and sexual females coexist. The results of field and laboratory studies suggest that coevolving parasites are an important factor that favors the genetically diverse offspring produced through sexual reproduction (the Red Queen Hypothesis). The results are relevant to agriculture and the spread of diseases in human populations.