Recent genetic studies of asexually reproducing fishes in the genus Poeciliopsis (Poeciliidae) revealed abundant variation in the form of multiple sympatric clones. Recurrent hybridizations between sexual species provides the principal source of clonal variation. The hybrids are spontaneously endowed with a clonal reproductive mechanism that perpetuates a high level of heterozygosity. Migration within and between river systems, and mutations, also contribute to clonal diversity in these fish. Coexistence among different clones and with the sexual ancestors depends in part upon specializations characteristic of individual clones. Clonal reproduction is an efficient mechanism for freezing a portion of the niche-width variation contained in the gene pool of the more broadly adapted, sexual ancestors. Multiclonal populations achieve significantly higher densities relative to the sexual forms than do monoclonal populations. This relationship is a function of the clonal variability upon which natural selection can act and upon the capacity of a multiclonal population to better exploit a heterogeneous environment through niche diversification. In all-female organisms such as Poeciliopsis, which are dependent upon sexual species for insemination, competitive abilities probably are at a premium in the densely populated pools and arroyos of the Sonoran Desert. Competitive abilities are probably less important for truly parthenogenetic clones which rely on colonization abilities to escape from their sexual ancestors and from other clones.