Gene flow in forest trees.

Abstract Pollen and seed immigration into forest tree populations promotes genetic continuity by transferring genetic variation among populations. The homogenizing effects of gene flow, therefore, counter the disruptive effects of selection and genetic drift. Gene flow rates are typically estimated in two ways: indirectly from the distribution of genetic variation among populations or directly via the detection of immigrant pollen and seed. Gene flow rates estimated indirectly from spatial genetic structure measure effective rates of immigration pooled over several populations and generations. Reviews of the distribution of allozyme genetic variation demonstrate that tree species typically possess more genetic diversity than herbaceous species and that more of this variation occurs within individual populations. These data support the conclusion that tree species generally experience more gene flow than most herbaceous species. In only a few studies have the relative contributions of pollen and seed flow been distinguished. These indirect estimates indicate that pollen dispersal is responsible for much higher levels of gene migration than seed dispersal. Genetic markers can be used to directly estimate gene flow by detecting immigrant pollen and seed genotypes. The relatively few direct measurements of gene flow into tree populations demonstrate that gene immigration via pollen is often more than 25% over distances of several hundred metres. Direct estimates of seed movement are rare but indicate that seed flow is more restricted than pollen flow over comparable distances. This conclusion is tentative, however, as seed movement has not been characterized for tree species with light, wind-borne seeds or with strong-flying animal vectors.