ScienceDaily has a brief article about how plants reject self-pollination or pollination by other closely (genetically) related plants of the same species.  Once pollen, representing the plant's male reproductive organs, reaches the pistil of the plant, it implants itself and grows there as a pollen tube, with the goal of reaching the ovule, the female side of things.  If the pollen is recognized en route as belonging to a near cousin, some plants release a toxin which effectively stops the pollen tube in its tracks, preventing the too-similar genetic code from producing serious mutations in its offspring.  Enemies to evolution, those plants.