Evolution seems to favor (and be favored by) genetic variability. Genetic variability is promoted by outbreeding - sexual reproduction between genetically dissimilar parents. Just why sexual reproduction is so popular throughout the world of living things is still a hotly-debated question, but the fact remains. Plants, being anchored in position, have a special problem in this regard. Many employ the services of animals (e.g., insects, birds, bats) to transfer pollen from plant to plant. But if the flowers have both sex organs:what is to prevent the pollen from fertilizing its own eggs?
A variety of solutions have been tried in the plant kingdom. These include:
Having imperfect flowers; that is, flowers that are either male or female.
Dioecy. The imperfect flowers are present on separate plants. Dioecy is the equivalent of the separate sexes of most animals. But it is rather rare. Some examples include poplars and hollies.
Monoecy. The imperfect flowers are present on the same plant. But if they mature at different times, self-fertilization is avoided. Corn (maize) is a common example.
But the vast majority of angiosperms have perfect flowers; that is containing both male and female sex organs. So how do they avoid self-fertilization?
The flowers are perfect but come in two structural types; for example
long stamens with a short style
short stamens with a long style
Homomorphic flowers. All flowers have exactly the same structure. Avoidance of self-fertilization depends on genetic/biochemical mechanisms. There are two quite different types of self-incompatibility.
Sporophytic self-incompatibility (SSI)
Gametophytic self-incompatibility (GSI)
Sporophytic Self-Incompatibility (SSI)
This form of self-incompatibility has been studied intensively in members of the mustard family (Brassica), including turnips, rape, cabbage, broccoli, and cauliflower.