self-fertilization, fusion of male and female gametes (sex cells) produced by the same individual. Self-fertilization occurs in bisexual organisms, including most flowering plants, numerous protozoans, and many invertebrates. Autogamy, the production of gametes by the division of a single parent cell, is frequently found in unicellular organisms such as the protozoan Paramecium. These organisms, however, may also reproduce by means of conjugation, in which cross-fertilization is achieved by the exchange of genetic material across a cytoplasmic bridge between two individuals. Likewise, among higher plants, most of which are monoecious (i.e., bisexual—male and female gametes being produced by the same individual), most self-pollinating species are also capable of cross-fertilization, and even those that are obligate self-fertilizers are occasionally cross-pollinated by accident. Hermaphroditic animals (those in which both male and female gonads are borne on one individual) are rarely capable of self-fertilization, since many such species have adaptations encouraging cross-fertilization.
As an evolutionary and reproductive mechanism, self-fertilization allows an isolated individual to create a local population and stabilizes desirable genetic strains, but it fails to provide a significant degree of variability within a population and thereby limits the possibilities for adaptation to environmental change.