The Origin of Reproductive Isolating Mechanisms in Flowering Plants

Summary Sympatric congeneric species of flowering plants are precluded from freely exchanging genes by reproductive barriers which are operative prior to pollination, between pollination and fertilization and after fertilization. The barriers may be the by‐products of divergent evolution, or may have been selected for their ability to reduce hybridization and disruptive gene flow, or competition for similar elements of the pollinator fauna. Autogamy and hybrid floral isolation are included in a classification of reproductive isolating barriers. The origin of reproductive barriers is discussed in light of new information and insight on pollinator behavior and perception, the S‐gene complex, pollen‐tube competition, and population level selection. It is suggested that reproductive barriers are not inherent properties of species, but reside in the interface between species, and arise from the incongruities in the reproductive formats of species paired in nature or by the systematist.