Genetic diversity and the mating system of a rare Mexican piñon,Pinus pinceana, and a comparison withPinus maximartinezii(Pinaceae)

Pinus pinceana) has a restricted and fragmented range, trees are widely scattered within populations, and reproduction is limited. Nevertheless, genetic diversity was high; based on 27 isozyme loci in 18 enzyme systems, unbiased expected heterozygosity averaged 0.174. Differentiation also was high (FST 5 0.152), reflecting isolation between southern, central, and northern fragments of the range. Among populations in the northern fragment, FST was only 0.056, and the number of migrants per generation (Nm) was 4.21, which should preclude fixation. Nm between central and southern populations or between them and populations in the northern fragment was lower, 0.99‐1.66, indicating a degree of genetic isolation. Multilocus outcrossing rates ( tm) ranged from 0.836 in the south to 0.897 in the north. Therefore, selfing is low but statistically significant. The equilibrium inbreeding coefficient