We used data from 20 enzyme loci to test hypotheses concerning the population genetic structure of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) in the Great Basin relative to the southern Rocky Mountains of Utah. Detailed macrofossil data from wood rat (Neotoma) middens indicate that P. menziesii was absent from the central and northern Great Basin during the last glacial (20 000–12 000 years before present), but has recolonized several of the "island" mountain ranges of that region during the past 10 000 years by long-distance dispersal from populations on the southern Rocky Mountain "mainland". The genetic consequences of rare, chance dispersal events should be a reduction in levels of genetic diversity on Great Basin montane islands and more diversity among island populations relative to the Rocky Mountain mainland. We found moderate overall reductions in the level of polymorphism (65 vs. 85%), numbers of alleles per polymorphic locus (2.69 vs. 2.82), and gene diversity (0.113 vs. 0.141) in Great Basin P. menziesii relative to P. menziesii from the Rocky Mountain mainland. Within-population estimates of allozyme diversity, as well as relative partitioning of that diversity among populations of each region, differed to a lesser extent between island and mainland regions. Founder effects and genetic drift thus appear to have had a minor role in shaping the present-day genetic structure of Great Basin P. menziesii populations.
Allozyme diversity and genetic structure in South-Western populations of heather, Calluna vulgaris