The common assumption that species coexistence in nature can be generally explained by processes of natural selection for niche differentiation does not have strong empirical support for plants. The meaning of competitive ability has traditionally been embedded within an Eltonian view of the niche. In particular reference to plants, however, a true distinction between the meanings of niche and competitive ability is established based on the premise that for one species to competitively exclude another, two distinct conditions must be satisfied: (1) There must be a sufficient degree of overlap in the niche requirements of the two species; and (2) one species must be sufficiently superior at reducing the availability of resources to the other. Following these considerations an evolutionary theory of coexistence is proposed involving two alternative mechanisms based on a broader interpretation of the operation of natural selection in systems of competition than previously recognized. Competitive exclusion ma...