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<br /> <br /> <br />Fig. 11. Crenichthys b. moapae: above, holotype (47.8); below, allotype (41.9). <br /> <br />. <br /> <br />DISCUSSION. Like many native fishes of the Great Basin, Crenichthys baileyi exhibits <br />a relict distribution. The obligate spring existence of Crenichthys and the drying of the <br />pluvial White :\.'liver system have resulted in geographic isolation among the many <br />populations of C. baileyi. Geographic isolation has been demonstrated to be a potent <br />force in the evolution of desert fishes (Hubbs and Miller 1948b, Miller 1948). Miller (1948) <br />found geographic isolation to be the most important factor allowing for speciation in <br />Death Valley Cyprinodon populations. Populations of Crenichthys baileyi have probably <br />been isolated for many thousands of years. Geographic isolation persists at present time <br />due to the following: 1) usually dry condition of the White River channel, 2) obligatory <br />spring existence of Crenichthys, 3) extreme distances among some spring populations, <br />and 4) location of the springs a small distance from the river bed. The last three <br />characteristics provide continued isolation among spring populations during flood <br />events. An exception to the isolation occurs in Pahranagat Valley, where Crystal Spring <br />and the outflow creek of Ash Spring occur in the river bed. This permits some <br />contamination of the Ash Spring outflow creek fish by individuals from Crystal Spring <br />during floods. The head pool of Ash Spring is isolated, however, from its outflow creek <br />by precipitous topography, insuring the genetic purity of the head pool fish. <br />Arid regions of the western United States have provided numerous examples of <br />remnant fish populations in isolated springs evolving relatively rapidly (Hubbs 1941, <br />Hubbs et al. 1974). Much of the rapid evolution in Crenichthys baileyi populations can <br />be attributed to differences in temperature and oxygen values among the isolated spring <br />habitats occupied by this species. As the populations became isolated, a divergence of <br />morphological features among the fish populations occurred as they adapted to the <br />unique qualities of their environment. Sumner and Sargent (1940) illustrated this concept <br />by finding that C. baileyi from cool, well oxygenated waters of Preston Spring could not <br />survive in the warm, poorly oxygenated waters of Mormon Spring. We attempted to <br /> <br />SOl <br />