Homozygosity and patch structure in plant populations as a result of nearest-neighbor pollination

The population genetic consequences of nearest-neighbor pollination in an outcrossing plant species were investigated through computer simulations. The genetic system consisted of two alleles at a single locus in a self-incompatible plant that mates by random pollen transfer from a neighboring individual. Beginning with a random distribution of genotypes, restricted pollen and seed dispersal were applied each generation to 10,000 individuals spaced uniformly on a square grid. This restricted gene flow caused inbreeding, a rapid increase in homozygosity, and striking microgeographic differentiation of the populations. Patches of homozygotes bordered by heterozygotes formed quickly and persisted for many generations. Thus, high levels of inbreeding, homozygosity, and patchiness in the spatial distribution of genotypes are expected in plant populations with breeding systems based on nearest-neighbor pollination, and such observations require no explanation by natural selection or other deterministic forces.