Population and subspecific genetic differentiation in the foxtail pine (Pinus balfouriana)

We performed an allozyme survey of genetic differentiation in Pinus balfour iana, a subalpine conifer endemic to California that is comprised of two al lopatric subspecies, one in the Klamath Mountains and the other in the sout hern Sierra Nevada. Although the two subspecies are morphologically distinc t and gene flow between them is virtually nonexistent, we observed much hig her levels of differentiation among populations within a subspecies than be tween the two subspecies. Differentiation is particularly strong in the Kla math populations (multilocus F-ST = 0.242), which are small, isolated, and ecologically marginal. We attribute this strong differentiation to the moun tain island effect, in which populations restricted to high elevations beco me isolated from each other on different mountains separated by unsuitable intervening habitat, with consequent reduced gene flow allowing populations to evolve independently. Populations of P. balfouriana in the Klamath regi on only exist scattered on the few highest ridges and peaks that rise above 2000 m, which defines the lower limit of the species elevational distribut ion. This pattern of distribution has allowed genetic drift and allelic sor ting through historical events to produce strong population-level different iation, which was likely in place before the two subspecies were geographic ally separated. Because P. balfouriana occurs on both serpentine soils and nonserpentine soils in the Klamath Mountains, we tested for genetic differe ntiation between populations growing on serpentine versus nonserpentine soi ls and our results were equivocal. Our data, combined with several other st udies of conifers, show that the mountain island effect can produce signifi cant genetic differentiation in conifers whose life-history traits of widel y dispersed pollen, long generation times, and high outcrossing rates would lead us to predict a more homogenous population genetic structure.