Genetic Variation, Selection and Speciation in Thomomys talpoides Pocket Gophers

Allozymic variation in proteins encoded by 31 loci was analyzed electrophoretically in 276 specimens representing 10 populations and 6 chromosome forms of the Thomomys talpoides complex of pocket gophers in the southern Rockies of the U.S. Of the 31 loci examined 8 are monomorphic in all populations. Twenty-one loci display either weak or local polymorphism: the same allele is either dominant or fixed in most populations. Only two loci, Es.-3 and Es.-4 display strong polymorphism in most populations. Alternative fixation, between and within karyotypes, is relatively rare. In no one case is one allele fixed in one species and completely absent from all others. Ldh-1 may be the only diagnostic locus of 2n = 48 NM as opposed to all other species. Mean heterozygosity per locus per individual (H̄) is low, .047 (.008-.085), and is even lower when Es.-3 and Es.-4 are excluded, .037 (.005-.067). H̄ differs significantly among populations, varies geographically in clinal patterns, and is predictable by a combination of ecological variables which differ partly in each of three groups of proteins. Calculations of estimates of genetic variation for the three protein groups (I) glucose metabolizing enzymes, (II) other enzymes, and (III) nonenzymatic proteins shows that group II is more polymorphic and heterozygous than either group I or group III. Genic similarity among populations is high (S̄ = .838, range, .705-.967) but is even higher among species (S̄0 = .874, range, .795-.967). The pattern of genetic variation within and between species suggests that (a) selection for homozygosity as an adaptive strategy is probably operating in the relatively constant subterranean niche, and (b) speciation in Thomomys talpoides occurred from late Pleistocene to recent times involving very few genomic changes.