Taxonomic Categories and the Taxonomic Hierarchy

Classification is not a single step but a series of taxonomic categories (ranks). All the categories together constitute the taxonomic hierarchy. Each category, being a rank, is part of the overall taxonomic arrangement; the actual group of organisms placed at a rank is a taxon.

There are seven obligate (main) categories. In ascending order they are:

Species -> Genus -> Family -> Order -> Class -> Phylum (Division) -> Kingdom

  • For animals the category above Class is Phylum.
  • For plants the corresponding category is Division.

Golden rule: As we move up the hierarchy (species -> kingdom), the number of common characteristics decreases and it becomes harder to establish relationships. As we move down (kingdom -> species), organisms share more common features.

The categories, defined

  • Species - the lowest category. A group of individuals with fundamental similarities, capable of interbreeding (Mayr's biological concept). The specific epithet denotes the species: sapiens, indica, tuberosum, leo.
  • Genus - a group of related species that have more characters in common with one another than with species of other genera. Example: genus Panthera (lion P. leo, tiger P. tigris, leopard P. pardus); genus Solanum (potato, brinjal, makoi).
  • Family - a group of related genera with still fewer similarities than species and genera. Plant families use both vegetative and reproductive features. Example: Solanaceae (Solanum, Petunia, Datura); Felidae (Panthera, Felis); Hominidae (Homo).

Higher Categories and Worked Examples

  • Order - an assemblage of related families sharing a few similar characters. Example: order Primata (comprising families such as Hominidae); order Carnivora (Felidae + Canidae); order Polymoniales (Solanaceae + Convolvulaceae).
  • Class - a group of related orders. Example: class Mammalia (orders Primata + Carnivora and others).
  • Phylum / Division - a group of related classes. For animals it is Phylum (e.g., Chordata - fishes, amphibians, reptiles, birds, mammals); for plants it is Division (e.g., Angiospermae).
  • Kingdom - the highest category. All animals belong to kingdom Animalia; all plants to kingdom Plantae.

NCERT taxonomic-hierarchy table (memorise this)

Category Man Housefly Mango Wheat
Species sapiens domestica indica aestivum
Genus Homo Musca Mangifera Triticum
Family Hominidae Muscidae Anacardiaceae Poaceae
Order Primata Diptera Sapindales Poales
Class Mammalia Insecta Dicotyledonae Monocotyledonae
Phylum/Division Chordata Arthropoda Angiospermae Angiospermae
Kingdom Animalia Animalia Plantae Plantae

Common exam traps

  • Plant family, class and division names have typical endings: family often ends in -aceae (Poaceae, Solanaceae); animal family in -idae (Hominidae, Muscidae, Felidae); plant order in -ales (Sapindales, Poales); plant class in -ae (Dicotyledonae).
  • The category between Class and Kingdom is Phylum in animals but Division in plants.
  • The higher the category, the fewer the shared characters (kingdom = fewest; species = most).

Visual - The Taxonomic Hierarchy

Vertical hierarchy of taxonomic categories from species up to kingdom

Reading up (Species -> Kingdom) the number of shared characters decreases and relationships get harder to establish; reading down, organisms share more characters.