Ecologists and plant evolutionary biologists have long recognized that plants are not distributed at random within communities but, rather, are clustered in distinct patches. Environmental heterogeneity is usually cited as playing a critical role but colonization patterns and stochastic events affecting establishment and mortality are also important. More recently plant evolutionary biologists have demonstrated that genetic variation in plant populations is also distributed nonrandomly (Antonovics, 1971; Allard et al. 1972; Hamrick and Allard, 1972; Turkington and Harper, 1979). Rather, like the plants themselves, genes and genotypes tend to be clumped, with marked genetic differences occurring over short distances. This nonrandom distribution of genetic variation is often referred to as the genetic structure of a population (Loveless and Hamrick, 1984).