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646
<br />COPEIA, 1995, NO. 3
<br />rather than anthropogenic phenomenon (Dowl-
<br />ing and DeMarais, 1993). Second, the two spe-
<br />cies remain distinct despite their putative con-
<br />vergence; at both Desolation and Cataract can-
<br />yons, G. robusta and G. cypha can be clearly dis-
<br />criminated. These results may suggest that
<br />selection is of sufficient magnitude to prevent
<br />genetic homogenization or swamping (Haldane,
<br />1948; Endler, 1977; Grant and Grant, 1989).
<br />Gene flow among Gila species may facilitate lo-
<br />cal adaptation by providing additional variation
<br />upon which selection can act (Ehrlich and Ra-
<br />ven, 1969; Slatkin, 1987; Dowling and De-
<br />Marais, 1993), as has been suggested previously
<br />for a range of organisms (Lewontin and Birch,
<br />1966; Gill, 1980; Grant and Grant, 1989).
<br />The potential for introgressive hybridization
<br />and/or local adaptation suggests that popula-
<br />tions of Gila cannot be considered in isolation
<br />of congeners or of the environment in which
<br />they occur. We echo the view espoused by Grant
<br />and Grant (1989) that "maintaining commu-
<br />nities intact could be essential for the long-term
<br />persistence of their members. It is not enough
<br />to concentrate on one or two species of interest
<br />unless, by conserving them, all others are con-
<br />served as well." Clearly, such an approach will
<br />require an increased emphasis on habitat con-
<br />servation.
<br />Future directions.-Our data point to the need
<br />for additional studies of Gila on at least two
<br />fronts. First, genetic studies are required to
<br />evaluate the potential importance of introgres-
<br />sive hybridization and local adaptation to mor-
<br />phological variation within and among popu-
<br />lations. A better understanding of the relation-
<br />ship between genotype and phenotype in these
<br />fishes is crucial to resolving the status of inter-
<br />mediate specimens and to quantifying the ex-
<br />tent of human intervention (Kaeding et al.,
<br />1990). Second, the nature and range of mor-
<br />phological variation among upper Colorado ba-
<br />sin populations (particularly those of G. robusta)
<br />should be compared to that present in the lower
<br />basin. Only by considering additional species
<br />and populations can the taxonomic confusion
<br />surrounding upper basin Gila be resolved and
<br />appropriate management strategies then be im-
<br />plemented.
<br />ACKNOWLEDGMENTS
<br />The authors recognize numerous individuals
<br />from the following agencies, organizations, and/
<br />or institutions for assistance with organizing riv-
<br />er trips, obtaining specimens, identifying and
<br />filming fishes, and recording data: Arizona
<br />Game and Fish Department (Phoenix); Arizona
<br />State University (Tempe); Bio/West, Inc. (Lo-
<br />gan, UT); Brigham Young University (Provo,
<br />UT); Bureau of Reclamation (Salt Lake City,
<br />UT); Colorado Department of Wildlife (Den-
<br />ver; Grand Junction); National Museum of Nat-
<br />ural History (Washington, DC); U.S. Fish and
<br />Wildlife Service (Parker, AZ; Grand Junction,
<br />CO; Salt Lake City, UT); and Utah Department
<br />of Wildlife Resources (Moab; Price; Salt Lake
<br />City; Vernal). Without such assistance, this pro-
<br />ject could not have been implemented. Scien-
<br />tific collecting permits are also acknowledged
<br />from the states of Arizona, Colorado, Utah, and
<br />from the U.S. National Park Service, as is a
<br />USFWS Endangered Species subpermit (PRT-
<br />676811). This research was supported by Bu-
<br />reau of Reclamation (1-CS-40-0970A).
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