Self-pollination produces clones of the parent plant, which reduces genetic variation within the plant population. The purpose of sexual reproduction is to mix and combine the genetic information of two plants into a single plant. Sexual reproduction increases diversity and allows the plant species to adapt to changing environments over time. Self-pollination stagnates the plant, since the only visible changes in the population must result from genetic mutations, which rarely are beneficial to the species.
The anthers of the flower produce the pollen and are located on the ends of the filaments. These anthers and filaments are the male stamens. The female pistils contain the stigmas that receive the pollen during fertilization. The male and female organs are separated physically on the flower to hinder self-pollination. The pistils are located in the center of most flowers, while the stamens surround the outside edge of the flower's centers. The stamens and the pistils do not touch without outside interference.
Some flowers, such as those on cucumber and pumpkin plants, do not produce male and female organs on the same flower to further inhibit self-fertilization. These flowers are called imperfect. The plants may produce male and female flowers on a single plant, or produce only one sex or the other on a single plant. These plants require cross-pollination between the female and male flowers. The disadvantage of imperfect flowers is that this structure lowers the chances that the plant will successfully reproduce. Pollinators must visit to transfer the pollen between the flowers, and the flowers must grow close enough together to encourage the transfer.
In some flowers, timing of pollen maturity versus stigma maturity prevents self-pollination. Most of these plants, like the pecan tree, produce mature pollen first. When the pollen matures, the stigma of the same flower, or the same plant in the case of imperfect flowers, is immature. The only receptive stigmas are located on other plants. When the stigma finally is receptive and mature, the pollen of the same flower is already spent. The timing of the maturity of the flower's sexual organs can vary as little as a single day to successfully prevent self-fertilization.
Self-incompatibility refers to a flower that cannot reproduce with itself under any circumstances. Many fruit trees, particularly apple trees, fall into this category. Self-fertilization may occur, but the pollen will not fully germinate or will produce non-viable seeds. All flower stigmas chemically recognize compatible pollen, which is the reason cross-pollination of vastly different flower species does not occur. In self-incompatible flowers, the flower rejects the pollen from the same flower, or the pollen incompletely germinates.