A model for the origin of parthenogenesis in hermaphrodites is developed. If a dominant mutation causing parthenogenetic development of eggs without affecting meiotic production of sperm arises, the parthenogens will increase in frequency to fixation. Concomitantly, there is selection for reallocation of resources from male to female-related functions in both parthenogenetic and sexual individuals. Occasional fertilization of unreduced eggs may produce polyploid clones. Both the loss of male-related structures and polyploidy are common in parthenogenetic earthworms. Parthenogenesis should be favored in patchy and temporally unstable habitats, in which γ-selection may be expected, because it facilitates colonization and rapid population growth, and because selection by the biotic component of the environment presumably is reduced. Parthenogenetic earthworms commonly occur in decaying logs, leaf litter, and the upper, organic layers of the soil, whereas sexual species more often inhabit the deeper, more stable soil horizons. Long-term persistence of clones depends on their ability to survive and reproduce under a variety of environmental conditions. It is proposed that successful clones possess “general purpose” genotypes that allow persistence in spite of temporal changes and facilitate active dispersal through heterogeneous environments between patches of prime habitat. Two common clones of the parthenogenetic earthworm Octolasion tyrtaeum seem to possess general purpose genotypes, as they occur in a wide variety of soil and habitat types and are geographically widespread.