RECENT EVOLUTION AND DIVERGENCE AMONG POPULATIONS OF A RARE MEXICAN ENDEMIC, CHIHUAHUA SPRUCE, FOLLOWING HOLOCENE CLIMATIC WARMING

Fragmentation and reduction in population size are expected to reduce genetic diversity. However, examples from natural populations of forest trees are scarce. The range of Chihuahua spruce retreated northward and fragmented coincident with the warming climate that marked the early Holocene. The isolated populations vary from 15 to 2441 trees, which provided an opportunity to test whether census number is a good predictor of genetic diversity. Mean expected heterozygosity, H_e, based on 24 loci in 16 enzyme systems, was 0.093 for 10 sampled populations, which is within the range reported for conifers. However, estimates varied more than twofold among populations and H_e was closely related to the logarithm of the number of mature trees in the population (r_He,N = 0.93). Diversity among populations, F_ST, was 24.8% of the total diversity, which is higher than that observed in almost all conifer species studied. Nei's genetic distance, D, was not related to geographic distance between populations, and D̄ was 0.033, which is higher than estimates for most wide-ranging species. Most populations had excess homozygosity and the fixation index, F_IS, was higher than that reported for all but one species of conifer. Nm, the number of migrants per generation, was 0.43 to 0.76, depending on estimation procedure, and is the smallest observed in conifers. The data suggest that populations of Chihuahua spruce have differentiated by drift and that they are effectively isolated. The results illustrate how a combination of paleontological observation and molecular markers can be used to illuminate recent evolutionary events. Multilocus estimates of outcrossing for two small populations were zero (complete selfing) and 0.153, respectively, which are in striking contrast to the near complete outcrossing observed in most conifers. The high fixation index and a high proportion of empty seeds (45%) suggest that inbreeding may be a serious problem for conservation of Chihuahua spruce.