Variety, subspecies, race, breed, cultivar, strain, clone, stock are comparable when they mean a group of related plants or animals narrower in scope than a species.
These terms tend to be variable in application and subject to confusing overlap in use, but all can carry distinctive implications.
Variety stresses deviation from a type; historically, it denotes an infraspecific category differing from the typical form of the species in characters that are too trivial or too inconstant to justify its separation as a distinct species. In modern use it is appropriately selected when it is desirable to call attention to such deviation without making any specific taxonomic suggestion or it may be used specifically of any such divergent group developed under human control (as by selective breeding, hybridization, and cultivation) <an early variety of peach> <developing new varieties to meet special conditions>
Subspecies , which stresses subordinate status, is primarily a taxonomic term applicable to a morphologically distinguishable subdivision of a species that is geographically isolated but physiologically capable of interbreeding with other comparable subdivisions of the same species.
Race stresses common ancestry and differentiation based on readily discernible hereditary characters.
As applied to the human species (Homo sapiens ), race is a highly controversial term that basically denotes any of the primary subdivisions of recent man historically native to distinct parts of the world and distinguished by relatively fixed characters in physical type (as skin color, hair form, and skull shape).
In more general use race may apply to either large or small groups within a species. Though often used as if interchangeable race and variety are not exactly correspondent; while they sometimes agree, they more often overlap in their reference, for race emphasizes a common descent, while variety stresses divergence from a type.
Breed can refer to a group within a species of animals or occasionally of plants the members of which presumably share a common ancestry and are visibly similar in most characters. More specifically breed refers to such a group (as Jersey cattle or beagle dogs) that has been developed under human control chiefly through selective breeding and the fixing of desired qualities.
Cultivar applies specifically to a race or breed of plants originated (as by selection or hybridization) under cultivation.
Strain , like variety , stresses difference, but it is more likely to be used of subdivisions of subdivisions (as subspecies, or especially breeds or varieties) than, as variety typically is, of primary subdivisions of the species. It is especially applicable when the distinguishing character is a physiological quality (as vigor, or yield, or virulence); the term may imply human control as a means of gaining this result (as through crossing or inbreeding) or it may imply chance variation or controlled conditions.
Clone is the most precisely delimited term of this set; it denotes all the individuals that constitute the asexually produced progeny of a single parent and are therefore genetically identical. Though applicable to organisms (as bacteria and protozoans) that reproduce asexually in nature, it is used typically of economic plants that are propagated by such means as dividing, budding, or grafting and in such use may come close to variety , race , or strain ; thus, one can speak of the Baldwin variety of apple or the Baldwin clone .
Stock places emphasis upon community of origin and genetically close relationship in the group but its range of reference is not clearly defined. Often also it carries over from other senses of the word the notion of being a source or original.