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Two points are made, however, which reduce the liklihood of sampling artifacts. First, it seems unlikely that alterations in molecular structure caused by sample deterioration would coincidentally result in one of the same mobilities as that observed for the two EST-2 alleles present in all other samples which are not suspect. Secondly, it is doubtful that deterioration would uniformly pervade a whole sample and alter all molecules prior to stablization by freezing. Therefore, the sample should yield a combination of bands reflecting both the original and altered mobilities which is apparently (??) not the case. If methodological artifacts continue to be implicated, then, the second observation might point the finger back to sample preparation in the laboratory as the most likely source of pervasive molecular alteration of a given sample. However the first further reduces that possibility in addition to observations made above. If scoring interpretations are capable of standing under further scrutiny, then niether the lab artifact nor null allele explanation can be totally rejected, though there is some possibility that the first may be verifiable by experimentation with preparation procedures. Returning to observations on allelic variation, the extent of variation in frequencies among samples of Gila from populations of such geographic proximity in the Gila Basin is evidence of constrained gene flow, probably acting in concert with one or more additional factors. Factors that might be hypothesized are genetic drift from bottleneck or founder effects, local adaptation, and introgression. As noted above, several population samples exhibit low heterozygosity at several loci with extreme cases in the Fossil (which is homozygous for a single allele at all loci) and Cienega creek samples. Such levels are generally considered strong indicators of one or more population reductions below effective breeding size (Wright, 1931; Nei et al., 1975; Motro and Thompson, 1982; Maruyama and Fuerst, 1985; Echelle, 1991). These processes might be expected in populations inhabiting small habitats in arid regions. Long standing separations of these populations from others due to habitat discontinuity or behavioral constraints (i.e., high fidelity to small home ranges) coincident with decreasing heterogeneity in some of them could result in fixation at varied frequencies among them. Populations in isolated habitats may have been founded, or refounded after extirpation, by relatively small numbers of dispersing individuals which can cause fixation at frequencies different from those of the donor population (e.g., Selander, 1970). Such a pattern is exemplified at some loci studied in the Gila Basin, such as EST-2 (Fig. 6), LDH-B (regionally in upper Gila, Fig. 9), MPI-2 (Fig. 10), and, especially, PGM (Fig. 11). However, a question arises as to why so many combinations of allele frequencies among those Gila populations which remain 16