GEOGRAPHIC PATTERNS OF ALLOZYME VARIATION IN JEFFREY PINE

Genetic structure of seven Jeffrey pine (Pinus jeffreyi Grev. & Balf.) populations restricted to ultramafic soils in the Klamath Mountains of Oregon and California was compared to that of seven populations occurring on a broader range of soils in the rest of the range (Sierra Nevada‐southern California). Patterns of variation at 20 allozyme loci were examined. Eighteen loci were polymorphic in at least one population. Despite discontinuous distribution in the Klamath Mountains, differentiation was limited among the seven populations. Average expected heterozygosity, however, was lower there (mean He = 0.185) than in the Sierra Nevada‐southern California region (mean He = 0.255). Significant differences (P < 0.05) in allele frequencies between the two regions were found at 11 loci. Of Sierra Nevada‐southern California populations, the one on ultramafic soil was most similar to Klamath populations in allele frequencies, which suggests genetic adaptation to such soils. Lower expected heterozygosity in the Klamath region may reflect strong directional selection under harsh soil conditions, genetic drift in an ancestral population, or both.