ESTIMATION OF GENETIC NEIGHBORHOOD PARAMETERS FROM POLLEN AND SEED DISPERSAL IN THE MARINE ANGIOSPERM ZOSTERA MARINA L.

The relative importance of random genetic drift and local adaptation in causing population substructuring in plant species remains an important empirical question. Here I estimate the effective size of the genetic neighborhood, N , as a means of evaluating the likely role of genetic drift in creating genetic differentiation within a population of a marine plant, Zostera marina L. (eelgrass). Calculations of effective neighborhood size are based on field estimates of pollen and seed-dispersal distributions, an electrophoretic estimate of the mating system using open-pollinated progeny arrays, and determination of the effective density of reproductive individuals in the population. Neighborhood area calculated from the parent-offspring dispersal variances was equal to N = 524 m ; variance in the seed-dispersal distribution contributes more than twice as much as variance in pollen dispersal to N . Including an outcrossing rate slightly different from random, estimated neighborhood size for Z. marina is N = 6255. This estimate is one of the largest reported for plants or animals and indicates that genetic drift in small neighborhoods is highly unlikely to cause genetic substructuring in the study population. High gene-flow levels provided by the marine environment appear to prevent genetic isolation by distance among eelgrass patches, but the importance of drift through founder events in this population characterized by high patch turnover cannot be discounted and is the subject of ongoing study.