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A better (or older?) marker possibility is the 11111 allele at the PGM locus (Fig. 11). That allele is, on the average, quite common in samples from throughout the Gila Basin and adjacent populations in the lower Colorado Basin but is rare to absent in upper basin samples and therefore is strongly correlated with the range of intermedia. It is absent in samples from populations in! the lower Colorado Basin that may have been effectively isolated for many millenia (Chevelon Cr. in Little Colorado syst. and Pahranagat Valley). It is also absent from one Gila Basin sample, from Cienega Creek (morphologically assignable to intermedia), which however has considerable evidence of bottlenecking as previously discussed and has become fixed for the alternate allele. The distribution of this PGM allele is consistent with having an ancestral distribution in the Gila Basin (associated with the genome of intermedia?) followed by introgression with an alternate allele associated with another taxon (possibly robusta) entering that basin, perhaps after the integration of the lower Colorado and Gila basins (as has been postulated by several workers, e.g., Minckley et al, 1986), followed by diffusion into other portions of the Colorado diminishing with distance from the source basin. An alternate hypothesis, whereby the 112" allele was first present in the Gila Basin followed by entry of the "ill allele from the east may not supported, as the 11111 allele is not present in samples from basins (e.g., Guzman) to the east (but see below). Again the possibility of a novel allele, as opposed to a former marker, arising through hybridizaton must be considered. Intuitively, because of the high degree of morphological similarity between the intermedia morphotype and forms in the Guzman and Rio Grand basins to the east, as well as the genetic similarity between Gila and Guzman basin samples (e.g., Fig. 13), one might search among alleles in the Guzman sample for possible markers for intermedia. However no alleles have distributions (Figs. 2-12) which unambigously qualify as candidates. It is also notable that niether of the candidate marker alleles for intermedia discussed above are present in the small nigrescens sample incorporated into this study. However, DeMarais (1992) scored larger samples of nigrescens and pandora homozygous for a slow allele at "PGM-A" which appears analagous to the slow allele scored by him in Gila Basin samples and, by comparison, to the 111" allele discussed herein above. - DeMarais (1992), DeMarais et al. (1992), and Dowling and DeMarais (1993) pointed out, based on studies focused primarily in the lower Colorado Basin, that fixed or nearly fixed allelic differences occur between Lake Mohave bonytail (G. elegans) stock and extant chub populations (robusta, intermedia, or other) in the Gila Basin and Bill Williams systems at the CBP-1 and CK-A loci. These findings are generally corroborated herein (Figs. 4,5), though such differences do not exist at these loci between Lake Mohave bonytail stock and upper Colorado Basin samples 21