Theories developed to explain the processes that govern the assembly and composition of natural plant communities can be divided into two broad categories. Niche-based theories propose that coevolutionary changes among species lead to character divergence (displacement), which allows for coexistence by partitioning resources among species. In contrast, ecological-drift theories propose that species diversity results from a balance of migration, speciation, and extinction, with little microevolutionary change. We use a game-theory model to reconcile drift and niche perspectives by developing a theory of species "marriage." Initially, ecological drift may determine which species encounter each other in a competitive arena. Once species come into contact, historical niche development as a result of prior coevolutionary molding of competitive ability determines which species may coexist. The model shows that only species that display the well-known tradeoff between seed size and competitive ability as a result of past competitive histories comply with the requisite for mutual evolutionary stability. Mutual evolutionary stability of competitive ability tends to make reproductive outputs more similar between species, increasing the chances of ecological equilibrium, i.e., the coexistence of species competing for a single resource. Moreover, mutual evolutionary stability guarantees that such an ecological equilibrium will be stable. The species-marriage model predicts that two or more plant species will coexist indefinitely (i.e., "marry") when their difference in seed size, their densities, and the resource availability obey a specific quantitative relation. For example, when resource availability is high, married species should be characterized by a greater asymmetry in seed size than when resource availability is low. Thus, in the species-marriage model, competition can shape the detailed properties of communities without violating the postulates of ecological-drift theory.
