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The reliability of paternity analysis in estimating mean effective pollen dispersal was evaluated. Pollen parents were inferred based on the degree of genetic relationship with the offspring alone or combined with data on the probability of mating with mother plants. Computer simulation showed that the mean distance between inferred males and mother plants could give reliable estimates of mean effective pollen dispersal under specified circumstances. Some conditions that affect the reliability of paternity analysis were discussed. The results have applications in studies on the role of gene dispersal through pollen on plant population genetic structure.

The role of gene establishment in gene flow was investigated in a population of the annual legume Chamaecrista fasciculata by determining the effect of interparent distance on progeny fitness throughout the entire life history. A decelerating gain in progeny fitness with increasing interparent distance was observed. Selfed progeny suffered a 2-fold fitness disadvantage compared to progeny derived from mating events between individuals in the same neighborhood. Progency derived from within neighborhood crosses had lower fitness than progeny from crosses between neighborhoods. Coupling the effect of interparent distance on gene establishment with information on gene dispersal resulted in a considerable increase in estimated gene flow. However, gene flow was still limited, as the average neighborhood area corresponded to a circle with radius of approximately 3.0 m. Yearly fluctuations in population size and variation in reproductive output lowered the estimate of [N.sub.e] below the census estimate to approximately 100 individuals. The role of a seed bank in increasing the estimate of [N.sub.e] was found to be insignificant. It is likely that genetic drift plays a major role in determining the distribution of genetic variation in this population.

Pollen dispersal is a major component of gene flow in plant populations. It can influence microevolution within and among populations as well as the evolution of floral characters that affect dispersal. Most previous studies have relied on point estimates to characterize dispersal distances, even though there is likely to be substantial intrapopulational and interpopulational variation. We measured variation in pollen dispersal for the hummingbird-pollinated herb Ipomopsis aggregata (Polemoniaceae), using powdered fluorescent dyes to estimate pollen movement. Analysis of 5-6 natural populations in each of three years indicated that mean and mean squared distances of pollen dispersal, measured over the reproductive lifespan of individual plants, varied more than threefold among populations and years. Dispersal distances also shifted over the season within a given population. Unlike the variation among populations, these seasonal changes were associated in part with changes in flower density. The mean distance of pollen dispersal from an individual plant was unrelated to the date of first flowering, but did reflect two floral characters. Plants with higher variance in stamen length across flowers delivered pollen farther on average, as predicted by computer simulations of pollen carryover. Plants with lower mean stamen lengths also delivered pollen farther. Such effects of plant characters on pollen dispersal are a critical prerequisite for dispersal to evolve in response to its effects on fitness.

Arrays of experimental populations were used to examine the patterns of gene flow in the perennial herb Silene alba. Analysis of the experimental data indicates that the factors that affect gene flow at one spatial scale do not necessarily work in the same manner at another. Specifically, it was discovered that the gene flow patterns of newly colonized populations were complex and tended to vary within a regional metapopulation at different times in a growing season.