A species is a group of closely related, physically similar beings that can interbreed freely. In practice, the dividing lines between species or between species and subspecies are sometimes unclear.

In sorting out species, biologists search for easily recognized diagnostic characteristics. For example, it is easier to recognize a giraffe by its long neck than by its blood proteins. However, species differ from one another not just by conspicuous features. Members of a species share a common gene pool and have a common geographic range, habitat, and similar characteristics ranging from the biochemical and morphological to the behavioral.

In taxonomy, the hierarchy of biological classification, species is the category just below genus. The major groups in the classification hierarchy are kingdom, phylum (for animals) or division (plants), class, order, family, genus, and species. Taxonomically, a species is designated in italics by the genus name followed by its specific...