GENETIC CONSEQUENCES OF SEED DISPERSAL IN 3 SYMPATRIC FOREST HERBS .2. MICROSPATIAL GENETIC-STRUCTURE WITHIN POPULATIONS

Significant spatial genetic differentiation over short distances was d etected by F-statistics and spatial autocorrelation within populations of the temperate forest herbs Cryptotaenia canadensis, Osmorhiza clay tonii and Sanicula odorata (Apiaceae). Differences among the three spe cies were consistent with estimates of their seed-dispersal abilities. Populations of Cryptotaenia, with the most limited seed dispersal, ar e characterized by genetic structure at smaller spatial scales than th ose of Osmorhiza or Sanicula, as indicated by higher estimates of thet a(F-st), larger autocorrelation coefficients, and correlograms with mo re distant x-intercepts. Although spatial autocorrelation was somewhat more sensitive to the distribution of rare alleles than F-statistics, the two methods were generally concordant. Genetic structure was more pronounced, and inbreeding coefficients larger, in low-density, patch y populations than in a high-density site. Observed patterns of spatia l autocorrelation, particularly for Cryptotaenia, were in agreement wi th expectations based on simulations of isolation by distance. The mag nitude of observed autocorrelations was less than those typically prod uced in computer-simulation studies, but this discrepancy between empi rical and theoretical results probably is derived from a lack of genet ic and demographic equilibrium in natural populations. Isolation by di stance can be an important evolutionary force organizing spatial genet ic structure in plant populations, particularly in predominantly self- fertilizing species such as those studied here.