THE INFLUENCE OF SEED DISPERSAL MECHANISMS ON THE GENETIC-STRUCTURE OF TROPICAL TREE POPULATIONS

Seed dispersal mechanisms should have a direct impact on the genetic s tructure of populations. Species whose seeds are dispersed near the ma ternal plant (e.g. gravity or wind dispersal) or species whose seeds a re deposited in clumps or patches should have more fine-scale genetic structure than species whose seeds are dispersed singly by mobile anim als. Furthermore, due to the overlap of seed shadows, species with hig h adult densities should have less genetic structure than species with lower densities. Allozyme analyses of three tropical tree species bel onging to the moist tropical forest of Barro Colorado Island, Republic of Panama, were used to describe variation in the scale and intensity of genetic structure within their populations. The genetic structure of seedlings and immature trees in the low-density, wind-dispersed spe cies (Platypodium elegans) was the coarsest and strongest whereas gene tic structure in a population of Swartzia simplex var. ochnacea (high density, bird-dispersed) was both the finest and the weakest. The gene tic structure of Alseis blackiana, a high-density, wind-dispersed spec ies was intermediate in both degree and scale. In P. elegans and A. bl ackiana, which had 'J' shaped size distributions, the significant gene tic structure seen in the smaller and intermediate diameter classes di sappeared in the largest diameter class. The loss of genetic structure was not observed in S. simplex, a species with a more even size distr ibution.