The relative importance of factors determining genetic drift: mating system, spatial genetic structure, habitat and census size in Arabidopsis lyrata.

• The mating system, dispersal and census size are predicted to determine the magnitude of genetic drift, but little is known about their relative importance in nature. • We estimated the contributions of several population-level features to genetic drift in 18 populations of Arabidopsis lyrata. The factors were outcrossing rate, within-population spatial genetic structure, census size and substrate type. The expected heterozygosity (H(E)) at 10 microsatellite loci was taken to reflect the effective population size (N(e)) and the strength of genetic drift. • The mating system explained most of the variation in H(E) (60%), followed by substrate (10%), genetic structure (9%) and census size (6%). The most outcrossing population had a +0.32 higher predicted H(E) than the most selfing population; the estimated N(e) of selfing populations was less than half that of outcrossing populations. Rocky outcrops supported populations with a +0.14 higher H(E) than did sandy substrates. The most structured population had a +0.24 higher H(E) than the least structured population, and the largest population had a +0.18 higher H(E) than the smallest population. • This study illustrates the importance of outcrossing, genetic structure and the physical environment--together with census size--in maintaining H(E), and suggests that multiple population-level characteristics influence N(e) and the action of genetic drift.