Effects of population disjunction on isozyme variation in the widespread Pilgerodendron uviferum

Geographical range is considered a good predictor of the levels of isozyme variation in plants. Widespread species, often consisting of historically larger and more continuous populations, maintain higher polymorphism and are less affected by drift, which tends to erode genetic variation in more geographically restricted species. However, widespread species occurring in small and disjunct populations may not fit this pattern. In this study we examined genetic variation in Pilgerodendron uviferum, a conifer endemic to temperate forests of southern South America, and is such a widespread and habitat-restricted species. Twenty populations along the whole range of Pilgerodendron were analysed by isozyme electrophoresis to resolve 14 putative genetic loci. Eleven were polymorphic in at least one population although only six of them were polymorphic in more than one population. We found reduced within-population levels of isozyme variation, with only 11% polymorphic loci (0.95 criterion), 1.2 mean number of alleles per locus, and mean observed and expected heterozygosities of 0.024 and 0.033, respectively. Most genetic diversity was found within populations (H(T)=0.039, H(S)=0.033, F(ST) 15%). Greater polymorphism and lower divergence was estimated in the more geographically restricted and closely related Fitzroya. Thus, total range, in combination with information on the degree of among-population isolation, may be a better predictor of the levels of polymorphism than range size alone.