Many plants reproduce by self-pollinated. They are either hermaphroditic, or monoecious. Many lower animals are also hermaphroditic such as earthworms or parasitic species which copulate and fertilize each other.

Monoecious means that the protandrous or protogynous gametes of the two sexes are produced in the same organism, sometimes in the same gonad, at the same time. The specimen is thus able to self-fertilize. The oyster and the sage plant can be  simultaneously both male and female.

In the hermaphroditic case, the protandrous or protogynous gametes of the two sexes are produced in the same organism, sometimes in the same gonad, but at different times. To reproduce, the specimen needs another specimen but with no discrimination about its sex.