Levels of fluctuating asymmetry were measured to try to assess the fitness of the hybrids relative to the parentals. Hybrid asymmetry was greater than that of S. mexicanus but tended to be less than the levels of asymmetry of S. tridecemlineatus. The asymmetry data suggested lower fitness of the hybrids relative to that of S. mexicanus, the species that is experiencing introgression.

The evolutionary relationships of the hybridizing ground squirrel species Spermophilus mexicanus and S. tridecemlineatus were examined using chromosome banding techniques, electrophoresis of 42 structural gene loci, and multivariate statistical analyses of 40 skeletal characters. Forty-four of 596 individuals were hybrids between the two taxa. The majority of the hybrid individuals ((TURN)77%) were either F(, 2) or backcross hybrids. All observed hybrids were associated with populations of S. mexicanus, and it appeared that hybridization and introgression were unidirectional and into S. mexicanus.

The genetic and morphologic differences between S. mexicanus and S. tridecemlineatus probably arose during a period of geographic isolation of the species. Divergence of the species probably occurred approximately 30,000 years BP. Secondary contact and hybridization between the species may be the result of range expansion due to habitat alteration by human activities. Speciation of S. mexicanus and S. tridecemlineatus seems to fit best the allo-parapatric model. The morphologic differentiation may be more indicative of the relationships between the species than are the chromosomal and electrophoretic differences. Although reproductive isolation has not been completed, the ground squirrel taxa have maintained their genetic integrity except for a few scattered incidences of hybridization and should be considered separate species.

Chromosomally and electrophoretically S. mexicanus and S. tridecemlineatus are very similar. The G- and C-banding patterns for 13 of the 17 pairs of chromosomes are identical. The remaining three pairs of chromosomes have similar banding patterns but are different in size. The differences appear to be the result of translocations from the long arm of chromosome 10 to chromosomes 15 and 16 in S. mexicanus.

Electrophoretically S. mexicanus was more variable than S. tridecemlineatus with mean observed heterozygosity of 5.7% and 4.0%, respectively. Both species exhibited population subdivision with the higher F(, ST) values found for S. tridecemlineatus. Genetic similarity between the species was high with a mean genic identity (I) of 0.956. In contrast, the species were morphologically distinct. The major morphometric difference was overall size. On the average, S. mexicanus was about 15% larger than S. tridecemlineatus. Additionally, the species have very different pelage patterns.