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<br />VICARIANCE AND WESTERN NORTH AMERICAN FISHES <br /> <br />243 <br /> <br />TABLE 1. Matrix comparisons between 10 taxonomic-distance matrices (each matrix represents a different characteristic measured or <br />estimated for the 33 study sites in the Gila River Basin). H, a phenotypic-distance matrix based on sheared principal component scores <br />for fish samples; SIZ, a matrix of stream size (discharge) at each site. ELE, sample-site elevations above mean sea level; BAR, sample- <br />site isolation. aLl, EMI, MMI, PLI, PLE, and REC, respectively, paleodrainage matrices for: Oligocene, Early Miocene, mid-Miocene, <br />Pliocene, Pleistocene, and Recent. The upper triangle of the table presents values for two-way Mantel tests and the lower triangle contains <br />matrix correlations. Significance (3.06, P < 0.001) is represented by an asterisk. <br /> <br />Matrix H BAR SIZ ELE OLl EMI MMI PLI PLE REC <br />H -1.27 -0.06 0.55 0.33 -0.08 3.69* 4.36* -0.18 0.84 <br />BAR -0.10 4.01* -1.28 2.63 2.37 2.41 -0.22 3.47* 0.16 <br />SIZ -0.01 0.30 0.07 1.97 1.50 4.46* 1.73 1.39 2.01 <br />ELE 0.05 -0.10 0.01 0.10 0.10 -0.82 -0.37 0.63 0.32 <br />aLl 0.02 0.17 0.14 0.01 15.74* 13.31 * 7.45* 10.86* 3.96* <br />EMI -0.01 0.14 0.10 0.01 0.87 15.45* 6.98* 10.50* 5.90* <br />MMI 0.15 0.10 0.19 0.03 0.56 0.66 14.89* 9.57* 16.15* <br />PLI 0.29 -0.01 0.12 -0.03 0.43 0.37 0.63 4.42* 7.91* <br />PLE -0.02 0.26 0.12 0.05 0.76 0.66 0.40 0.27 1.30 <br />REC 0.06 0.01 0.14 0.02 0.23 0.31 0.68 0.44 0.09 <br /> <br />rived from paleohydrographic maps (i.e., OLG, EMI, MMI, <br />PLI, PLE, REC). <br />Models I and II offered little explanatory power (Table 1). <br />Instead, historical distributions of physiographic features (re- <br />lated to Model III), particularly those of mid-Miocene and <br />Pliocene, appear paramount in explaining the existing mosaic <br />of variation. A three-way Mantel analysis was performed to <br />unravel interrelations between mid-Miocene and Pliocene hy- <br />drography and phenotype. The resulting correlations and par- <br />tial correlations (Table 2) indicate that although fish body <br />shape has both mid-Miocene and Pliocene components (e.g., <br />both X-matrices are useful predictors of the Y-matrix), shape <br />is more highly correlated with Pliocene (rY 2 = 0.29, P < <br />0.0005) than with mid-Miocene hydrography (rY 1 = 0.15, P <br />< 0.004). The partial correlation of shape and Pliocene hy- <br />drography is significant (rY 2 = 0.26, P < 0.008), whereas <br />that of shape and mid-Miocene hydrography is not (rY2 = <br />-0.04, P > 0.76). The partial correlation analysis (Table 2) <br />can be more succinctly understood through an evaluation of <br />coefficients of multiple determination (R2). Once having fit <br />Pliocene hydrography to the model (top, Table 2). there was <br />virtually nothing gained by adding mid-Miocene, because R2 <br />only increased from 0.085 to 0.087. However, once having <br />fit mid-Miocene hydrography to the model (R2 = 0.023), <br />considerable information was derivable by adding the Plio- <br />cene component (R2 = 0.087). <br />Other significant correlations in Table 1 are intuitively ex- <br /> <br />TABLE 2. Results of three-way Mantel tests between body shape, <br />mid-Miocene, and Pliocene distance matrices compiled for 33 study <br />sites in the Gila River Basin; formulations per Smouse et al. (1986). <br /> <br /> Body shape = Mid-Miocene occurrence, <br /> Pliocene occurrence <br />Model (Y) (XI) (X2) <br />Comparison Formulation Correlation Probability <br />Shape, mid-Miocene rYI 0.154 < 0.004 <br />Shape, Pliocene r Y2 0.293 < 0.0005 <br />Mid-Miocene, Pliocene r YI.Z 0.636 < 0.00001 <br />Shape, mid-Miocene <br />(Pliocene) r YI.2 -0.044 > 0.76 <br />Shape, Pliocene <br />(mid-Miocene) r YZ,1 0.255 < 0.0008 <br /> <br />plicable and do not detract from these interpretations. Those <br />between epochs demonstrate regional continuity of major <br />structural development through later Tertiary apparent in Fig- <br />ure 2. That between SIZ and MMI may relate to MMI basins <br />being dominantly large geographic features that, for that rea- <br />son alone, are associated with watersheds of higher water <br />yield. The same explanation likely exists for the significant <br />relationship between BAR and SIZ because a greater prob- <br />ability exists that larger (higher-discharge) streams will cross <br />geologic formations of differing erodability, thus promoting <br />barrier (e.g., waterfall) formation. That between BAR and <br />PLI, more speculatively, may result from younger basin de- <br />posits still experiencing the process of rapid erosion. Places <br />where downcutting channels encounter parent material at the <br />margin of an incising basin fill could result in barriers, either <br />due to differential durability of substrate (thus creating rapids <br />or waterfalls, as above) or through water flowing over im- <br />pervious parent rock passing onto and percolating into porous <br />alluvium to create dry reaches. <br /> <br />CONCLUSIONS <br /> <br />It is not surprising that vicariant events are correlated with <br />patterns of morphological variation in minnows of the genus <br />Gila. Vicariance has been implicated as an important factor <br />in generating orderly patterns of diversity and variability in <br />a variety of taxa (Signor 1990, p. 529), and hypotheses of a <br />vicariant nature have already been generated for both aquatic <br />and terrestrial components of the western American aquatic <br />and semiaquatic fauna (Hendrickson 1986; Minckley et al. <br />1986). <br />Our study is nonetheless novel in several ways. First, we <br />are able to statistically assess existing phenotypic diversity <br />in relation to patterns of ancient hydrography controlled by <br />tectonism. To our knowledge, this has not before been done. <br />Second, the resolution of our analysis is noteworthy. Riddle <br />(1995) performed a relatively fine-grained analysis by using <br />molecular data to demonstrate arid-land rodent divergence <br />and distribution were attributable to Late Miocene devel- <br />opment of western North American Cordillera. His study <br />would be more appropriately described as "regional," where- <br />as ours would be "local." Third, our results argue strongly <br />for the importance of deep rather than shallow history in the <br />