Units of Classification

Working from the lowest rank upward, the taxonomic units of classification are:

• Species — the principal, lowest natural taxonomic unit, ranking below genus and denoted by a Latin binomial. It is considered the basic unit of classification: a group of organisms that can interbreed under natural conditions to produce fertile offspring. Species was once thought to be an indivisible, stable, static unit, but modern taxonomy recognises further subdivisions such as sub-species, varieties and populations.
• Genus — a rank higher than species, used to group species that closely resemble one another morphologically but that usually do not interbreed. For example, tiger, leopard and lion are all placed in the genus Panthera even though they are clearly different species; brinjal and potato both belong to the genus Solanum.
• Family — one of the major hierarchical ranks, representing a group of closely related genera. For example, the genera Hibiscus, Gossypium, Sida and Bombax are all placed in the family Malvaceae. Among animals, cats, tigers and lions belong to family Felidae, while dogs belong to the separate family Canidae.
• Cohort / Order — a rank recognised by the nomenclature codes, one step above family, grouping closely related families that share definite affinities. For example, the families Papavaraceae, Brassicaceae and Capparidaceae — all sharing parietal placentation — are grouped in the order Parietales. Similarly, the families of dogs and cats, though different, both belong to the order Carnivora.
• Class — a rank higher than order, grouping closely related or allied orders. For example, the orders Carnivora and Primates are both placed in class Mammalia, so monkeys, gorillas and gibbons (Primates) share a class with dogs, cats and tigers (Carnivora).
• Division / Phylum — a category composed of closely related classes. For example, division Angiospermae includes the two classes Dicotyledonae and Monocotyledonae. (In animal classification, "division" is instead used as a sub-unit within phylum.)
• Sub-kingdom — formed when different divisions sharing some similarities are grouped together. For example, the divisions Angiospermae and Gymnospermae together form the sub-kingdom Phanerogams (Spermatophyta).
• Kingdom — the highest taxonomic category, composed of different sub-kingdoms. For example, the sub-kingdoms Phanerogams and Cryptogams together form kingdom Plantae, while all animals are placed in kingdom Animalia. …