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and keel size) possessed flannelmouth mtDNA. While some of these individuals may simply <br />have been razorback or flannelmouth suckers with unusual morphologies, hybridization has <br />also been indicated by allozyme analysis (Buth and Haglund, unpublis`~ed data). <br />MtDNA variation within natural populations.- Levels of variation were examined in all <br />samples except for those from the CRIT canals (because there were too few individuals) or <br />the Little Colorado River (due to suspect morphology of all specimens). Estimates of within- <br />population variation were dramatically different among samples (Table 3). Haplotype <br />diversity was highest in Lake Mohave (h=0.97) and lowest in the upper Colorado River <br />(h=0.00). Geographically intermediate samples exhibited intermediate h values, varying from <br />0.73 to 0.82. Because of difficulties in using h as an estimator of population variability <br />(discussed in detail below), we computed the number of haplotypes per sample corrected for <br />sample size (nh, Table 3). The value for nh was greatest in the Lake Mohave sample (0.67 <br />per indivi.lual), considerably larger than for any other population, and declined progressively <br />in samples upstream from Lake Mohave with the lowest value of 0.04 in the upper Colorado <br />River. <br />MtDNA variation among natural populations.- Analysis of geographic heterogeneity using <br />Monte Carlo simulation (Roff and Bentzen, 1989) yielded high x2 values for both 4+6 by and <br />only 6 by composite haplotypes (x2=239.5 and x2=104.7, respectively), significantly greater <br />than values obtained in 1000 simulated replicates (for 4+6 by datz, maximum x2=194.5; for 6 <br />by data, maximum x2=63.5). <br />Due to concerns over the nature of the upper Colorado River sample, a second set of <br />anal; ses were performed without that sample. x2 values were not as large as those obtained <br />from analysis of all populations (for 4+6 by data, x2=160.5; for 6 by data, x2=53.2); however, <br />valves obtained from the original matrix were still significantly greater than thosg obtained <br />8 <br />