9327

• UCREFRP Catalog Number: 9327
• Author: Starnes, W. C.
• USFW Year: 1995.
• Copyright Material: NO

polymorphic in the Gila Basin compared to Mexican populations
which have probably undergone processes similar to those just
discussed and are, in fact, in many cases isolated from one
another in different drainage basins. This further begs the
question as to whether additional sources of variation, again, in
conjunction with constrained gene flow, are superimposed on any
that might be attributable to drift phenomena resulting in
complex patterns seen in the Gila Basin.
Rapid selection in allele frequencies may be imposed by
local adaptation (e.g., Mueller et al., 1985) and may be a source
of genetic variation among related populations (e.g., Vourinen et
al., 1991). Vourinen et al. purported to document shifts in
allele frequencies of up to 31% at some loci in a little less
than one hundred years between donor and founded populations of a
salmonid species with no evidence that drift was a factor.
Shifts of this magnitude could easily result in the range of
frequency variation seen among Gila Basin Gila samples. However,
environmental conditions in the two habitats under consideration
in the salmonid study differed substantially for several
parameters. It is questionable, even in a region of drastic
enviromental conditions such as the Gila Basin, particularly when
conditions are averaged for a given habitat, whether a sufficient
number of distinctive selective regimes are operative among Gila
populations to effect the array of frequency combinations seen in
samples therefrom.
Many cases of introgressive hybridization have been
documented in fishes (summarized in Verspoor and Hammar, 1991).
Introgression among two or more taxa having fixed or allelic
frequencies differences, whereby novel genetic variants are
introduced into respective populations, might result in an array
of allelic combinations if superimposed on a framework of
constrained gene flow among populations. This would be
particularly true if involved taxa occasionally interfaced on
several different fronts and thus introgression was attributable
to geographically and temporally separated multiple hybridization
events. Populations might fix for different allele freq_.."icies
depending on the individual circumstances (e.g., relative
contributions of parental types, post F1 mating patterns, etc.)
of the particular event affecting a given population. This is
precisely the pattern observable at several loci in Gila Basin
populations, including EST-2, GPI-1, MPI-2, and PGM (Figs.
6,7,10,11). Moreover, there are morphological patterns which may
be manifestations of such events (e.g., Rinne, 1976; DeMarais,
1986; Douglas, herewith).
Taxonomic Implications
Under recent morphological concepts (e.g., Minckley, 1973;
Rinne, 1976; DeMarais, 1986), Gila populations which inhabited
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