A variety of factors serve as reproductive barriers among plant taxa. These barriers, which can be subdivided into those acting prior to fertilization (prezygotic) or following fertilization (postzygotic), restrict natural hybridization and help maintain species boundaries.
Prezygotic Barriers. The potential for natural hybridization is largely determined by the proximity of potential mates in both space and time. The likelihood of hybridization is therefore governed, to a large extent, by differences in the ecology (spatial isolation) and/or phenology (temporal isolation) of the individuals of interest. Even if ecological and temporal differentiation are absent, pollen transfer may be limited by differences in floral morphology (form). Differences in traits such as floral color, fragrance, and nectar chemistry can influence pollinator behavior and may discourage the transfer of pollen among different species (ethological isolation). Alternatively, the structure of the flower may preclude or limit pollination of one taxon by the pollinator(s) of others (mechanical isolation). Finally, even if pollen transfer is successful, the pollen may not germinate on a foreign stigma; if it does, the pollen tubes may fail to effect fertilization due to slow growth or arrest prior to reaching the ovule (cross-incompatibility).
Postzygotic Barriers. Assuming that fertilization occurs, the resulting hybrid progeny (offspring) may fail to survive to reproductive maturity due to developmental aberrations (hybrid inviability). If the hybrids do survive, their flowers may be unattractive to pollinators, thereby restricting further hybridization (floral isolation). Alternatively, the hybrids may be attractive to pollinators but partially or completely sterile (hybrid sterility). Finally, even if first generation hybrids are viable and fertile, later-generation hybrids may exhibit decreased levels of viability and/or fertility (hybrid breakdown).
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