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<br />excluding direct measurement of the nuchal hump, <br />is shown in Figure 2. Component I reflects gener- <br />al size; II is especially ~orrelated with numbers <br />of vertebrae, dorsal rays, anal rays, and gill <br />rakers. Figure 3 shows the plot of components II <br />and III of the same analysis. Component III is <br />uncorrelated with II, but is highly correlated <br />with counts of fin rays and gill rakers as well as <br />pectoral length and eye diameter. In the combi- <br />nation of the three axes, which represent summa- <br />ries of the three major trends in the original 34 <br />characters, robusta, elegans, and cypha are dis- <br />criminated with almost no overlap. <br /> <br />Because the status of cypha has been most con- <br />troversial, it was analyzed separately with <br />robusta and elegans. The discrimination of <br />robusta and cypha, when these are considered with- <br />out elegans (Figure 4), and of elegans and cypha <br />alone (Figure 4), shows complete separation for <br />the latter and only a one-specimen overlap between <br />robusta and cypha. This fish (UMMZ 181281, spec. <br />2) falls into the robusta cluster because it has <br />9 anal rays (a character of robusta weighted <br />strongly by this multivariate analysis) and sever- <br />al other traits that lean toward robusta; it is <br />noteworthy that this is the only specimen that <br />did not fall into its own cluster and, when dis- <br />criminated with key characteristics such as <br />nuchal-hump development (which yielded a 9.2 <br />ratio--see Table 1), it is typical of cypha. <br />Moreover, when treated with the other two species <br />together, (Figures 2-3) it also was identified <br />with cypha. We cannot be sure, however, that the <br />several traits aligning it closely with robusta <br />may not indicate that some robusta genes were <br />present in this specimen. <br /> <br />Thus, although some of the individuals falling <br />at or within the borders of clusters (Figures 2- <br />5) could possibly be interpreted as hybrids, the <br />clusters are quite distinct, indicating genetic <br />differentiation and strong reproductive isolation <br />among the three populations in the big-river hab- <br />itat. The differentiation and isolation are <br />probably facilitated by different ecological roles <br />and habitat preferences within the complex big- <br />river habitat, although insufficient data are <br />available to be certain at this time. <br /> <br />\ The characters that contribute most to the <br />ove clustering are dorsal- and anal-ray number, <br />'ll-raker and vertebral-number, and depth and <br />length of the caudal peduncle. Gila robusta and <br />cypha separated primarily on counts of fin rays, <br />vertebrae, lateral-line scales, gill-rakers, and <br />post-anal length (II). Gila cypha and elegans <br />separated on correlation patterns dominated by <br />the same characters, excluding pelvic rays and <br />post-anal length, but including snout, eye, and <br />caudal peduncle dimensions. These characters <br />can be used as key characters, but it is interest- <br />ing that when used alone they do not provide as <br />complete discrimination as do all 34 traits. <br /> <br />Figures 2 and 3 show the position of eleven <br />specimens of Gila robusta from the Virgin River, <br />plotted with the clusters of robusta, elegans, <br />and cypha to demonstrate the nature of overlap <br />of peripheral populations. The Virgin River pop- <br />ulation is clearly a single variable population <br />with individuals that span some of the variation <br />shown by robusta, elegans, and cypha. None of <br />the Virgin River specimens have distinct humps, <br />yet their body proportions may be similar to <br />those of members of the other populations in <br />analyses excluding the hump character. This is <br />interpreted as indicating that environmental con- <br />ditions in the medium-sized tributaries are <br />selecting for some kind of variable average of <br />the three morphotypes present in the large-river <br />habitats, but that the range of heterogeneity of <br />habitat in the medium-sized river is not <br /> <br />sufficient to support three separate species. <br />The populations in the medium-sized rivers, for <br />example, the Virgin and the San Juan, are inter- <br />preted as adapted to their local environments, <br />and not as intergrades (in the introgressed <br />sense), though limited introgression over the past <br />tens of thousands of years cannot be ruled out. <br />When gill-raker number is considered (Table 2)" <br />the Virgin River chub separates well from popula- <br />tions of Gila robusta inhabiting the main river. <br /> <br />The results of the above analyses suggest that <br />three populations in the main Colorado River are <br />morphologically segregated and are behaving as <br />reproductively isolated species. Exceptional <br />circumstances exist in peripheral tributaries, <br />for example, the Virgin River, as mentioned above, <br />and possibly in disturbed habitats, such as arti- <br />ficial Lake powell. These will be mentioned <br />again in the discussion. <br /> <br />The search for key characters to discriminate <br />the above populations involving univariate analy- <br />sis of a larger sample of 261 individuals. The <br />development of the nuchal hump, as expressed by <br />means of a special ratio (see below), was analyzed <br />in 72 individuals. <br /> <br />One of the impediments to key discrimination <br />of these fishes is the variability and lack of <br />complete discriminating power of the nuchal-hump. <br />and caudal-peduncle characters. These traits are <br />obvious and striking and one would like to be <br />able to use them to discriminate the populations, <br />but difficulties in quantification of the nuchal- <br />hump characters have heretofore prevented its <br />effective use. Previous multivariate analyses <br />have included subjective scores for nuchal-hump <br />development, raising the question of subjective <br />influence on the final results, <br /> <br />We have circumvented this dilemma by adapting <br />an instrument described by Eschmeyer and poss <br />(1977). It provides a direct, repeatable measure- <br />ment, accurate to 0.1 mm, of the development of <br />the nuchal 'hump in association with the depressed <br />(often concave) dorsal surface of the skull, <br />features most conspicuous in Gila cypha (Figure 6). <br />A ratio derived by measuring the depth of the <br />frontal depression (the maximum distance between <br />a straight line from highest part of nuchal hump <br />and dorsal tip of snout, and dorsal surface of <br />skull) and dividing this figure into the distance <br />between the insertion of the pectoral and pelvic <br />fins, provides an effective means for distinguish- <br />ing adults of the three chubs of the middle and <br />upper Colorado River basin (Table 1). <br /> <br />Number of precaudal vertebrae (Table 3) proves <br />to be useful, along with gill-raker number <br />(Table 2), in discriminating Gila robusta seminuda <br />Cope (Figure 7), from the typical subspecies, <br />G. r. robusta (Figure 8). Counts of these verte- <br />brae provide a better separation than the total <br />number which (excluding Weberian vertebrae) <br />varies from' 40 to 45, modally 42, in seminuda, <br />and 39 to 44, modally 42, in robusta. Gila r. <br />seminuda is closest to G. elegans (Figure 9) in <br />number of precaudal vertebrae, as well as in gill- <br />raker number, but separates well from that species <br />on the basis of nuchal hump development (Table 1) <br />although more and (especially larger) individuals <br />need to be examined to verify this. Modally, <br />G. robusta has nine dorsal and anal rays; G. cypha <br />usually has nine dorsal and 10 anal rays; and <br />G. elegans has 10 dorsal and 10 or 11 anal rays. <br />G. r. seminuda tends to be intermediate. <br /> <br />DISCUSSION <br /> <br />621 <br /> <br />Our analyses indicate that Gila robusta, cyph~, <br />and elegans coexist as three separate, reproduc- <br />tively isolated species in the main channels of <br />