In a number of multicellular organisms, microgametes and macrogametes are produced in the specialized organs of a single individual. This state is termed hermaphroditism. This is a derivative state in most modern multicellular organisms, which emerged secondarily during evolution, for instance as a consequence of the specific ecological requirements of the individual species. For example, adaptation to a parasitic life style is a frequent reason for the emergence of hermaphroditism. Macroparasites, of which human tape worms are a typical prototype, frequently enter the bodies of their hosts or can survive there only as a very few specimens. Thus, as gonochorists, they would be faced by the danger that they would not be able to find an individual of the opposite sex in their vicinity as adults. It is thus advantageous for them if any given individual can function as both a male and a female. Similarly, hermaphroditism is advantageous from an ecological viewpoint in sessile, immobile organisms (e.g. plants).
Some hermaphrodites can use either a uniparental or a biparental mode of reproduction. From the viewpoint of exploitation of the advantages of sexual reproduction, biparental reproduction is preferable, where the microgametes and macrogametes forming the zygote are derived from different individuals. However, sometimes a situation occurs where the hermapahrodite is dependent on uniparental reproduction, i.e. fertilization of the macrogametes by its own microgametes. So far, it seems that a great many hermapahrodites are capable of self-fertilization in such a situation; however, it is mostly not clear how long the individual hermaphroditic species are capable of surviving without biparental reproduction.
The differentiation of organisms into individuals producing microgametes and individuals producing macrogametes, gonochorism, is advantageous for two reasons. To begin with, it prevents uniparental reproduction, i.e. it ensures that the microgametes and macrogametes forming the zygote are derived from two different individuals. In addition, it allows organisms to be differentiated morphologically, physiologically, ecologically and ethologically into males, producing microgametes and females producing macrogametes. The production of microgametes and macrogametes places somewhat different demands on the properties of the organism. The properties of hermaphrodites must necessarily be only a certain evolutionary compromise in this respect. In contrast, the evolution of the properties of gonochoristic organisms can proceed in both males and females separately and can optimize the relevant properties for each sex separately.