Ae. Metcalf et al., Geographic patterns of genetic differentiation within the restricted rangeof the endangered Stephens' kangaroo rat Dipodomys stephensi, EVOLUTION, 55(6), 2001, pp. 1233-1244
Using mtDNA variation in the kangaroo rat Dipodomys stephensi, we found no
support for the hypothesis that a species with an historically restricted r
ange will exhibit low levels of genetic polymorphism and little genetic str
ucture. Dipodomys stephensi has long been restricted to a few interior coas
tal valleys in southern California encompassing an area of approximately 70
x 40 km; however, we found high levels of genetic variation over much of i
ts range and significant genetic structure both within and between regions.
We also found evidence for a recent range expansion. Dipodomys stephensi i
s a federally endangered species that is separated from D. panamintinus, it
s presumed sister taxon, by a mountain range to the north. We assessed gene
tic variation by sequencing 645 bases of the mitochondrial d-loop from 61 i
ndividuals sampled from 16 locations across the species range and rooted th
eir relationship using two D. panamintinus individuals. Despite its limited
geographic range, the level of mtDNA variation in D. stephensi is comparab
le to that of other rodents, including that of the more widely distributed
D. panamintinus. This variation revealed significant regional differentiati
on. The northern, central, and southern regions of the range differ in both
the level and the distribution of genetic variation. Phylogenetic analysis
revealed that the center of the range contains the most diversity of linea
ges, including the most basal. In this region and in the north, most haplot
ypes were found at only a single location (25/29), or at a pair of nearby l
ocations (3/29). In addition, related haplotypes clustered geographically.
These results are consistent with long-term demographic stability character
ized by limited dispersal and high local effective population size. Further
support for this conclusion is the finding of unique diversity in two nort
hern peripheral populations, Norco and Potrero Creek (PC). However, in shar
p contrast, one haplotype (CC) was found at five of ii central and northern
locations and comprised 18% of individuals sampled. The atypical distribut
ion of the CC haplotype reflected a pattern seen more strongly in the south
ern region. Here the CC haplotype comprised 69% of the sample and was found
at all five sampling locations. Consequently, the southern region had very
low genetic variability. We propose that this dominance of CC was probably
due to a local population bottleneck that occurred during a recent range e
xpansion into the southern region.