Sex and its evolutionary significance have received considerable attention from theoretical biologists in recent years (see Ghiselin 1974, Williams 1975, Maynard Smith 1978, Stanley 1979, Bell 1982, Shields 1982). Sexual reproduction is a highly coordinated and potentially costly process that results in the production of genetic diversity among the progeny of an individual. Since the rate of adaptive evolution is believed to be a function of this diversity, the recombination of genes in sexual lineages provides an obvious long-term benefit in a changing environment (Fisher 1930, Muller 1932). However, asexuality provides an immediate “twofold” numerical advantage due to all-female reproduction (Williams 1975, Maynard Smith 1978). Despite this apparent short-term benefit of asexual reproduction, sexuality clearly predominates in all groups of plants and animals. How is the “monopoly of sex” maintained in the presence of this cost? Perhaps an answer to this question requires studies of the rare exceptions, the asexual populations, and the peculiar genetic and ecological conditions under which they sometimes prosper. A surprising result of such studies recently has challenged a common assumption of most theoretical considerations of sex — that asexual populations lack genetic diversity. Natural populations of asexual organisms are often composed of several genetically (and sometimes ecologically) distinct clones (Solbrig 1971, Vrijenhoek 1978, 1979a, Mitter et al.