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Long-distance seed dispersal influences many key aspects of the biology of plants, including spread of invasive species, metapopulation dynamics, and diversity and dynamics in plant communities. However, because long-distance seed dispersal is inherently hard to measure, there are few data sets that characterize the tails of seed dispersal curves. This paper is structured around two lines of argument. First, we argue that long-distance seed dispersal is of critical importance and, hence, that we must collect better data from the tails of seed dispersal curves. To make the case for the importance of long-distance seed dispersal, we review existing data and models of long-distance seed dispersal, focusing on situations in which seeds that travel long distances have a critical impact (colonization of islands, Holocene migrations, response to global change, metapopulation biology). Second, we argue that genetic methods provide a broadly applicable way to monitor long-distance seed dispersal; to place this argument in context, we review genetic estimates of plant migration rates. At present, several promising genetic approaches for estimating long-distance seed dispersal are under active development, including assignment methods, likelihood methods, genealogical methods, and genealogical/demographic methods. We close the paper by discussing important but as yet largely unexplored areas for future research.

In their classic study, Alston and Turner (American Journal of Botany, vol. 50, 159-173, 1963) documented extensive hybridization among four morphologically distinct Baptisia species native to East Texas. While Alston and Turner found putative F1 hybrids in great numbers, they found no evidence of backcrossing. In this study prezygotic and postzygotic reproductive barriers between two of these species, B. leucophaea and B. sphaerocarpa, were investigated and found to be quite weak. Flowering times overlap and bumble bees were observed visiting both species and intermediate hybrids. While pollinator constancy in flights between B. leucophaea and B. sphaerocarpa was moderately strong, significant levels of constancy were not observed in flights involving hybrids and either parental species. Thus, backcrossing was not impeded by pollinator behavior. Further, hybrid pollen was highly stainable (93.5%) and able to effectively set seed in crossing experiments with both parental species. Pollinator behavior was compared in experimental populations with and without hybrid ramets and found to differ between these two treatments. Hybrids were found to facilitate pollinator movement between species. In total, these results suggest that reproductive isolation is not responsible for the rarity of backcrossing in naturally hybridizing B. leucophaea and B. sphaerocarpa populations.

Although numerous exceptions are well known, organelle DNA is often exclusively or predominantly maternally inherited. In such cases, maternal inheritance is often documented by the study of progeny derived from crosses between parents with distinguishable organelle genotypes. These studies generally detect a single type of progeny, those containing organelles derived from only one of the parents; no progeny containing organelles derived either from the opposite parent or from both parents are found. However, in most cases the number of progeny examined is quite small. I present a simple binomial model of organelle inheritance to determine the power of such experiments to distinguish between the hypothesis of strict maternal inheritance and a more complex hypothesis involving the presence of both organelle types within a progeny array. Extremely large sample sizes of progeny are required to distinguish these two hypotheses with reasonable confidence when organelle transmission is not strictly maternal. As a result, studies involving simple progeny testing may be misleading. Larger samples, more complex breeding designs, or more sensitive molecular methods are required to document adequately strict maternal inheritance of organelles.