Taxonomic Hierarchy: From Species to Kingdom

Once individual species have been recognised, taxonomists do not leave them as an unordered list — they arrange them into a nested series of ranks, each rank grouping together the rank below it on the basis of shared characteristics. This step-wise arrangement, in which each level or step represents a unit of classification, is called the taxonomic hierarchy, and each individual rank within it is called a taxonomic category, or taxon (plural: taxa).

Working from the smallest, most specific unit up to the broadest, the standard taxonomic hierarchy used for both plants and animals runs as follows:

• Species — the basic unit; a group of actually or potentially interbreeding natural populations.
• Genus — a group of closely related species that share a set of similar characters and are believed to share a relatively recent common ancestor. For example, the genus Panthera includes the lion, tiger and leopard as distinct species.
• Family — a group of related genera, based on similarities that are somewhat less close than those linking species within a genus. For example, the family Felidae groups together the genus Panthera along with other cat genera such as Felis.
• Order — a group of related families sharing a broader set of common features.
• Class — a group of related orders.
• Phylum (for animals) / Division (for plants) — a group of related classes sharing the most fundamental body-plan features of that kingdom.
• Kingdom — the highest and broadest traditional category, grouping together all the phyla or divisions that share the most basic features defining that kind of life.

As an example applied to an animal: the domestic cat is classified as Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Carnivora, Family Felidae, Genus Felis, Species catus. Applied to a plant: the mango is classified as Kingdom Plantae, Division Angiospermae, Class Dicotyledonae, Order Sapindales, Family Anacardiaceae, Genus Mangifera, Species indica. …