GENETIC CONSEQUENCES OF SEED DISPERSAL IN THREE SYMPATRIC FOREST HERBS. II. MICROSPATIAL GENETIC STRUCTURE WITHIN POPULATIONS.

Significant spatial genetic differentiation over short distances was detected by F-statistics and spatial autocorrelation within populations of the temperate forest herbs Cryptotaenia canadensis, Osmorhiza claytonii and Sanicula odorata (Apiaceae). Differences among the three species were consistent with estimates of their seed-dispersal abilities. Populations of Cryptotaenia, with the most limited seed dispersal, are characterized by genetic structure at smaller spatial scales than those of Osmorhiza or Sanicula, as indicated by higher estimates of θ(F ), larger autocorrelation coefficients, and correlograms with more 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, patchy populations than in a high-density site. Observed patterns of spatial autocorrelation, particularly for Cryptotaenia, were in agreement with expectations based on simulations of isolation by distance. The magnitude of observed autocorrelations was less than those typically produced in computer-simulation studies, but this discrepancy between empirical and theoretical results probably is derived from a lack of genetic and demographic equilibrium in natural populations. Isolation by distance can be an important evolutionary force organizing spatial genetic structure in plant populations, particularly in predominantly self-fertilizing species such as those studied here.