In: J.W. Wright and L.J. Vitt (eds), Biology of Whiptail Lizards (Genus Cnemidophorus), The Oklahoma Museum of Natural History, Norman. pp. 371-410. 1993.   The ecology and evolutionary implications of competition and parthenogenesis in Cnemidophorus. Price, A.H., J.L. LaPointe, and J.W. Atmar The evidence collected during this experiment, along with that from other studies, does not support the unequivocal allocation of ecological competitive superiority to unisexual Cnemidophorus based upon either the attributes associated with (1) the possession of hybrid genotypes, (2) the reproductive potential of all-female populations, or (3) the fidelity in transmission of genotypes from generation to generation. An alternative explanation in four parts of the evolutionary significance of parthenogenetic Cnemidophorus is suggested: (1) They are quite young, formed in an evolutionary "instant" from hybridization between congeneric bisexual species resulting from the breakdown of previous ecogeographic premating isolating mechanisms (Wright and Lowe, 1968; Neaves and Gerald, 1968,1969; Neaves, 1969). (2) They are an accident; not every hybridization event between bisexual species of Cnemidophorus can possibly result in a successful parthenospecies (Wetherington et al., 1987; Moritz et al, 1992). Interspecific hybridizations probably occur frequently, as male Cnemidophorus are indiscriminate and may attempt to mate with anything slightly resembling a conspecific female (so-called "rape strategy"; Bogart, 1980). These events will produce some unknown frequency of reproductively competent parthenogens, a smaller fraction of which may persist as independent lineages for indeterminate periods of time depending on the synergy between genetics and the locally available niche. The mechanisms by which a hybrid individual develops reproductive competency are poorly understood (Lowe and Wright, 1966; Wright and Lowe, 1967b; Neaves, 1971; Cuellar, 1977; Wright, 1978). (3) They are obligate parthenospecies (Cuellar, 1971; Cole, 1979, 1984) and as such, they essentially cannot evolve through natural selection. Genetic change within a lineage depends solely on the accumulation of non-deleterious, functionally adaptive mutations. They can neither track environmental changes nor novel selective demands with any precision or appropriateness as compared to their gonochoristic congeners (Uzzell, 1970; Weinshall, 1986). They are ecologically unresponsive; the genetic complexion of each parthenogenetic lineage of Cnemidophorus was "fixed-in" at the time of its origin and therefore so was its phenotypic expression. Selection cannot act to "fine-tune" (Jacob, 1983) the responses of a parthenospecies to ecologically competitive factors in its environment. The ecological performance of each phenotype will thus be functionally invariant. It cannot competitively adjust its responses and will behave as if biotic competitors are not there. (4) They are not well-adapted; the integration of two or more differently optimized genotypes into one rigid, relatively immutable genotype cannot be expected to produce an implementation that is as fit as any of the originals if all exist together in the same selective environment. This does not imply that parthenogenetic Cnemidophorus cannot exist nor coexist with sympatric bisexual congeners (Case and Taper, 1986; Schenck and Vrijenhoek, 1986). The environmental instability that permitted the hybrid origin of parthenospecies of Cemidophorus in the first place suggests that the localized fitness optima in the adaptive topography (Wright, 1960; Templeton, 1982) are shallow. Episodic environmental instability relaxes the precision of the selective threshold for all phenotypes. A relatively broad spectrum of possible phenotypes are allowed to pass through the selective filter until the nature of the adaptive landscape itself again changes. (2.4MB)