species, in biology, a category of classification, the original and still the basic unit in the demarcation of plant and animal types. The species marks the boundary between populations of organisms rather than between individuals. Because related species are not absolutely permanent (see evolution), a precise definition of the term is difficult. On the basis of genetics, scientists now include in a species all individuals that are potentially or actually capable of interbreeding and that share the same gene pool. The latter term refers to that collection of characteristics whose combination is unique in the species, although each individual of the group may not display every single one of the characteristics (see genetics). In the few cases where members of different species can interbreed, the offspring are usually sterile (e.g., the mule). Groups distinguished by lesser differences than those marking a species are called variously subspecies, varieties, races, or tribes.