Low levels of genetic variation within and high levels of genetic differentiation among populations of species of Abies from southern Mexico and Guatemala.

Populations of Abies in southern Mexico and Guatemala (A. flinckii, A. guatemalensis, A. hickeli, and A. religiosa) have a patchy distribution. This pattern is particularly clear in A. guatemalensis. Genetic diversity within populations, measured by average heterozygosity at 16 isozyme loci, is lower than the range reported for most conifers (mean H(o) ranging from 0.069 in A. guatemalensis to 0.113 in A. flinckii), while differentiation among populations is higher than that observed in most conifer species studied (θ = F(st) ranging from 0.073 in A. hickeli to 0.271 in A. flinckii). Estimated levels of gene flow are low (ranging from 0.672 in A. flinckii to 3.17 in A. hickeli). Populations in most cases had an excess of homozygosity over that expected under Hardy-Weinberg equilibrium, suggesting some inbreeding (F(is) ranging from 0.074 in A. flinckii to 0.235 in A. guatemalensis). A significant relationship between gene flow and geographic distance was observed in A. religiosa, but not in the other three taxa studied. The patterns of genetic variation appear to have been influenced by the distributions and histories of these species. Paleoclimatic evidence suggests that the ranges of these species retreated upwards during the Pleistocene glaciation and became fragmented during the warming period that followed. The populations could have passed through genetic bottlenecks that reduced genetic variation and led to interpopulation differentiation.