Patterns of Morphological Variation among Endangered Populations of Gila robusta and Gila cypha (Teleostei: Cyprinidae) in the Upper Colorado River Basin

Mcelroy, D; Douglas, Michael

The native fish fauna of the American southwest is in decline as a result of habitat destruction, disruption of natural water flows, and introduction of non-native species. The status of several members of the cyprinid genus Gila occurring 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 variation at three levels of biological organization. At the population level, all sampled populations of both species differed significantly, although there was no relationship between morphological similarity and geographic proximity of populations 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 allopatric localities 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 general, 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.

Patterns of Morphological Variation among Endangered Populations of Gila robusta and Gila cypha (Teleostei: Cyprinidae) in the Upper Colorado River Basin

Series