subdivisions within species of plants and animals that serve to classify individual (intrapopulational), as opposed to geographic (populational), variation, for which there is a special category called the subspecies.
The origin of the differences that determine infrasubspecific categories are extremely varied; they may be based on polymorphism (including dimorphism), ecological, seasonal, or age variation, or insignificant genetic differences between individuals. For plants, two principal coordinative infrasubspecific categories—the variety (varietas) and the form (forma)—are used, along with two complementary categories—the subvariety and the subform. For parasitic plants (especially for fungi) there is a special form (forma specialis), the members of which differ only in the species affiliation of their hosts. The following infrasubspecific categories are used for animals: variety (varietas) and form (forma), usually considered equivalent categories, and, in some groups of animals, morph and aberration are used. In concrete classifications of animals the infrasubspecific category corresponds to the infrasubspecific form, to which a special Latin name is frequently assigned (the abbreviated Latin name of the infrasubspecific category and the name of the infrasubspecific form placed after the Latin name of the species).
In botanical nomenclature the scientific names of varieties and forms are subject to the same rules as for the naming of species. The rules for naming infrasubspecific forms are not specified in the International Code of Zoological Nomenclature.
I. M. KERZHNER