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Copeia, 1995(3), pp. 636-649 Patterns of Morphological Variation among Endangered Populations of Gila robusta and Gila cypha (Teleostei: Cyprinidae) in the Upper Colorado River Basin i DOUGLAS M. MCELROY AND MICHAEL E. DOUGLAS IX a 9 The native fish fauna of the American southwest is in decline as a result of t habitat destruction, disruption of natural water flows, and introduction of non- native species. The status of several members of the cyprinidgenusl6.Gila-0ccur- ring in the upper Colorado River basin-is-particularly tenuous, in part because._.- of uncertainty regarding their taxonomic status. To examine this uncertainty, we have sampled 363 specimens of G. robusta and G. cypha from eight localities in the upper Colorado River basin and the Grand Canyon and used canonical discriminant and cluster analysis to categorize patterns of morphological vari- ation at three levels of biological organization. At the population level, all sam- pled populations of both species differed significantly, although there was no relationship between morphological similarity and geographic proximity of pop- ulations in either species. At the species level, the two forms were clearly distinct in morphology whether in sympatry or in allopatry. At the generic level, we found two somewhat contradictory results: (1) conspecifics from altopatric lo- calities generally clustered together to the exclusion of heterospecifics; and (2) heterospecific populations at Cataract and Desolation canyons were more similar to one another than to allopatric conspecifics. This locality effect influencing the morphological similarities between species at these sites may be a consequence of either introgressive hybridization and/or convergent local adaptation. In gen- eral, allopatric populations of both G. robusta and G. cypha appear to represent independent evolutionary and conservation units. Populations of Gila should not be considered in isolation of congeners or of the environment in which they occur, and biological foresight and an emphasis on habitat conservation should be used in managing these species. MEMBERS of the western North American cyprinid genus Gila represent both a tax- onomic conundrum and a morphological curi- osity. Extensive (but poorly defined) morpho- metric variation against a background of mor- phological specialization is characteristic of the group (Minckley, 1973; Douglas et al., 1989; Douglas, 1993), a dichotomy that has hampered attempts to elucidate species interrelationships (Douglas et al., 1989). The G. robusta species- complex has proven especially problematic. Several big-river species of the Colorado River basin (e.g., G. cypha, G. elegans) display extreme morphologies, presumably reflecting adapta- tions to life in high current regimes (Miller, 1946; Minckley, 1973; but see Kaeding et al., 1990). However, morphological variation with- in and among populations of these and other members of the G. robusta complex is extensive, and few if any morphometric characters reliably separate species (see Suttkus and Clemmer, 1977; Smith et al., 1979). Evidence (DeMarais et al., 1992; Dowling and DeMarais, 1993) that introgressive hybridization has contributed to the evolutionary history of the group at several levels further clouds the distinction among spe- cies, and with other factors confound the iden- tification of evolutionary units and limit confi- dence in sorting individual specimens to species. Questions of species identity in Gila have con- sequences beyond evolutionary biology and tax- onomy. To develop appropriate conservation strategies for these endangered or threatened taxa, it is desirable to clearly establish species identity and distinctiveness (Valdez and Clem- mer, 1982; Douglas et al., 1989). However, the need to protect these fishes precludes extensive sampling and/or handling of specimens. Con- sequently, detailed quantitative studies on Gila have been limited in both number and scale, and recovery efforts remain stalled largely be- cause of the taxonomic confusion surrounding the G. robusta complex (Douglas et al., 1989). Vanicek and Kramer (1969) found differ- ences in growth and length-weight relationships between G. robusta and G. elegans and suggested that they constituted distinct species (as opposed to subspecies of G. robusta). Holden and Stal- naker (1970) concluded that G. robusta and G. elegans were each morphologically homoge- m 1995 by the American Society of ichthyologists and Herpetologists