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CONSTRAINTS ON UNISEXUAL VERTEBRATESsupports my general thesis that strictly asexual reproductionis an evolutionary dead end. The unisexual lineage that gaverise to this new sexual species was itself drawn from the recombinantvariation in a sexual gene pool. When that unisexuallineage gave rise to something new, it did so because clonalreproduction broke down. The new sexual species is a productof recombination, not cloning. Although mutational changeshave occurred in the hemiclonal genomes that gave rise to thisnew species (Spinella <strong>and</strong> Vrijenhoek, 1982), the critical transitionsin this unusual speciation pathway involved recombination.SUMMARY AND CONCLUSIONSIn general, the origin of new unisexual vertebrates dependson a delicate balance between meiotic disruption <strong>and</strong> developmentalcoordination in interspecific hybrids (Wetherington etal., 1987; Moritz et al., this volume). Once formed, a new unisexuallineage must either invade a sexual population or becomeestablished in habitats that are <strong>ecological</strong>ly suboptimalfor its sexual ancestors. Some partheno<strong>genetic</strong> lizards arethought adept at colonization <strong>and</strong> occupation of marginal habitats,but others coexist with one or more of their sexual relatives.Sperm-dependent unisexual fishes <strong>and</strong> amphibiansmust live with a sexual relative. Furthermore, the conditionsnecessary for a stable sperm-dependent relationship may behighly restrictive (Stenseth et al., 1985). Studies of <strong>ecological</strong>interactions between unisexual hybrids <strong>and</strong> their sexual ancestorscan offer many new insights on factors that promotecoexistence between closely related taxa that are morphologically<strong>and</strong> behaviorally very similar.Discoveries of abundant clonal diversity in many unisexualpopulations should alert us to the danger of erecting simple<strong>ecological</strong> hypotheses that assume the existence of only one"kind" of bisexual form <strong>and</strong> one "kind" of unisexual form instable complexes. Evidence for niche partitioning between coexistingclones <strong>and</strong> their sexual ancestors is accumulatingfrom careful field studies (Schenck <strong>and</strong> Vrijenhoek, 1986, thisvolume). Most of the clonal diversity observed in unisexualvertebrates stems from multiple hybridization events. Experimentalstudies are showing that new clones are likely to differsubstantially in many traits that could affect resource use inspatially <strong>and</strong> temporally heterogeneous environments(Wetherington, 1988). Over the long term, individual clonesprobably are evolutionary dead ends, but the unisexual populationmight persist because of recurrent clonal origins. Thishypothesis needs to be tested. Application of molecular approacheswill help us learn more about the relative ages of individualclones. The limited data currently available suggestthat extant unisexual vertebrates represent relatively recentevolutionary events (Avise <strong>and</strong> Vrijenhoek, 1987; Goddard etal., this volume; Lowcock, this volume; Moritz et al., this volume).Despite the <strong>ecological</strong> success of clonally diverse populationsof unisexual fishes, amphibians, <strong>and</strong> lizards, we lack evidencethat strictly asexual lineages are clado<strong>genetic</strong>, that they cansplit into diversified descendent clonal lineages. Although theapparent example of speciation reported here does involve aunisexual ancestor, the product of this event was a new sexualspecies, not a new clone. Furthermore, the origin of this morphologically<strong>and</strong> <strong>ecological</strong>ly distinct species involved <strong>genetic</strong>recombination, the essence of sexuality. Clado<strong>genetic</strong> speciation<strong>and</strong> sexual recombination may be closely intertwined(Stanley, 1975). The phylo<strong>genetic</strong> rarity of asexual "species"would not appear, therefore, to require any special explanations.A low rate of origin of novel unisexual forms might simplybe balanced by a high rate of extinction, keeping the overallfrequency of unisexual "species" low. lb draw an analogywith events at the genic level, we could imagine all unisexual"species" as mutants <strong>and</strong> all the bisexual species as wild-typealleles at a hypothetical locus controlling meiosis. As such, unisexualityis not even a valid polymorphism. The taxonomicrarity of unisexual "species" could be determined by a higherlevel process comparable to mutation-selection balance at thegenic level.ACKNOWLEDGEMENTSI wish to thank my colleagues, J. Wetherington, R. Schenck,K. Kotora, S. Weeks, J. Quattro, D. Baldwin, L. Vukovich, <strong>and</strong>C. 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