Increased seed dispersal potential towards geographic range limits in a Pacific coast dune plant

Dispersal may be favoured at geographic range edges by unstable population and metapopulation dynamics. However, dispersal may also evolve in response to geographic variation in other life-history traits, especially the mating system. Here, increased dispersal at range margins was tested for with a range-wide analysis of seed dispersal and mating system traits in Abronia umbellata, a plant endemic to Pacific coastal dunes of North America. Seeds disperse within winged anthocarps. Anthocarps from 34 populations varied widely in wing size (mass-corrected wing index). Wing index correlated negatively with threshold wind velocity for dispersal in wind tunnel tests, suggesting that wings facilitate tumbling over open sandy substrate. As predicted, wing index increased and threshold velocity decreased towards both range limits. Flower size, herkogamy and self-incompatibility declined towards range limits, indicating a shift to self-fertilization, and flower size and wing index correlated negatively. However, the increase in wing index towards range limits remained after statistically controlling flower size. These results are consistent with selection favouring dispersal at range margins. The evolutionary lability of dispersal across the range may affect the interaction between selection and gene flow in the establishment and maintenance of geographic range limits.