Populations with reduced gene flow and restricted population size are
expected to show reduced genetic variation. Using starch gel
electrophoresis, we examined allozyme variation at 12 loci in two
species of freshwater, sphaeromatid isopods. Thermosphaeroma
thermophilum, an endangered species, inhabits a single thermal spring in
central New Mexico, USA; and T. milleri, inhabits a more complex
thermal spring system in northern Chihuahua, México. We found no
significant differences in allelic variation between the sexes within
each species. Between species, electromorphs at each locus differed
significantly in both number and moiety on the gel, with T. milleri
showing greater polymorphism and greater heterozygosity than T.
thermophilum. Nei’s unbiased genetic distance, calculated using the nine
loci common to both populations (D = 0.75), was consistent with
morphological classification of T. thermophilum and T. milleri as
separate species, as well as with molecular analyses suggesting that
these populations have been separated since the late Cretaceous
(88 myr). Moreover, consistent with the theoretical expectation that
small, isolated populations will exhibit reduced genetic variation, T.
thermophilum, an endangered species, exhibited significantly less
genetic variation than the more numerous and less confined T. milleri.
We compare our results with other recent studies using this approach to
understand the population genetics of natural populations.