In biology, taxonomy refers to the scientific study of naming, defining, and classifying groups of biological organisms on the basis of shared characteristics.

It is the process of classification based on the external features as well as internal structures, development process, and ecological information of organisms, etc.

Classical Taxonomy

The concept of classical or old taxonomy has existed since the time of Aristotle and Theophrastus and continued up to Linnaeus. According to classical taxonomy:

  • Species is the basic unit of taxonomy, that can be described on the basis of one or few preserved specimens.
  • Species are fixed and do not change with time.
  • A species is delimited based on morphological features.
  • Organisms are classified on the basis of some limited features such as root modification, leaf venation, floral structures, and number of cotyledons in the case of plants.

The classical taxonomy, the limited number of groups due to which many organisms could not be classified correctly. This finally led to an artificial system of classification.

Modern Taxonomy

Julian Huxley (1940) gave the concept of modern taxonomy. It uses evidence from all areas of biology like morphology, anatomy, biochemistry, cell biology, physiology, genetics, evolution, etc.

Modern taxonomy is  based on the following factors:

  • Studies are done on a huge number of organisms based on all the variations.
  • The study is also focused on sub-species, varieties, races, and populations.
  • The species are related by common descent and vary from them due to the accumulation of variations.
  • Species are considered dynamic, and ever-changing entities.
  • Biological delimitation includes different branches of systematics, e.g., cytotaxonomy, experimental taxonomy, numerical taxonomy, chemotaxonomy, etc. This led to the development of a phylogenetic system or cladistics of classification.

Some Common Branches of Taxonomy:

  1. Alpha Taxonomy: In this type of taxonomy the morphological characters for the identification and classification of plant is studied.
  2. Beta Taxonomy: Study of anatomical, cytological, genetical, palynological, physiological, and other characteristics.
  3. Omega Taxonomy: Analysis and synthesis of all information and types of data to develop a classification system based on phylogenetic relationships.
  4. Cytotaxonomy: Use of cytological characters in classification.
  5. Chemotaxonomy: Use of chemical compounds for classification.
  6. Karyotaxomy: Based on the characteristics of the nucleus and chromosomes.

Taxonomic Categories

The classification of organisms is a multi-step process where organisms are divided at various levels into many sub-groups. Each sub-group or step represents a rank or a category that collectively forms the taxonomic arrangement. Each category is called a taxonomic category and all categories together make the taxonomic hierarchy. Further, each category is referred to as a unit of classification and is termed a taxon (plural: taxa).

Taxonomic Ranks:

  • Species
  • Genus
  • family
  • Order
  • Class
  • Phulum
  • Kingdom
  • Domain, etc.


A species is a group of organisms that can interbreed and produce fertile offspring. Thus, two species are reproductively isolated from each other.

  • It is the last category of the biological classification. Species are highly diversified in nature.
  • Two species can be distinguished from each other by their distinct morphological differences but with basic similarities.
  • Each genus may have one or more than one specific epithet representing different organisms, but having morphological similarities.


The genus comprises a group of related species which has more characteristics in common in comparison to species of other genera. Thus are aggregates of closely related species.

For example, potato and brinjal are two different species but both belong to the same genus Solanum.


The family comprises a group of related genera with still less number of similarities as compared to the genus and species.

  • Families are characterized on the basis of both vegetative and reproductive features of the species in plants. For example, three different genera Solanum, Petunia, and Datura are placed in the family Solanaceae.
  • Among animals, the genus Panthera, comprising lion, tiger, and leopard is put along with the genus Felis (cats) in the family Felidae.


Order is the assemblage of families that exhibit a few similar characters which are fewer in number as compared to the different genera included in a family.

  • Plant families like Convolvulaceae, and Solanaceae, are included in order the order Polymoniales mainly based on their floral characteristics.
  • The animal order, Carnivora, includes families like Felidae, and Canidae based on their elongated and sharp canines; and carnivorous diet.


Class is the third category of the biological classification, which comprises one or more related orders.

For example: the Mammalia class comprises related orders such as

  • Marsupialia: Kangaroos, bandicoots, opposums, etc.
  • Cetacea: Whale, dolphins, porpoises, etc.
  • Carnivora: lion, tiger, dog, cat, etc.
  • Primata: Monkey, gorilla, and gibbon; human, etc.

Phylum / Division:

Phylum is a group of closely related animal classes with few similar characters. In plants, this category has been named ‘Division’.

  • Example: Classes comprising animals like fishes, amphibians, reptiles, birds, and mammals are placed under the next higher category called phylum Chordata. All these are considered chordates based on common features like the presence of notochord and dorsal hollow neural system.


Kingdom is the highest category of biological classification composed of Animal phyla or plant divisions.

  • The Kingdom Animalia includes all animals belonging to various phyla.
  • The Kingdom Plantae includes all plants from various Divisions.

Important note:

  • As we go higher from Species to Kingdom, the number of common characteristics decreases.
  • The lower the taxa, the more the characteristics that the members within the taxon share.
  • The higher the category, the greater the difficulty of determining the relationship with other taxa at the same level.

Taxonomic Categories Recognised by ICBN

Category Standard Suffix
Kingdom -bionta
Division -phyta
Sub-division -phytina
Class -opsida
Sub-class -idea
Super-Order -anae
Order -aless
Sub-order -ineae
Super Family -ariae
Family -aceae
Sub-family -oideae
Tribe -eae
Sub-tribe -inae
Genus None
Species None


Some Organisms with their Taxonomic Categories:

Organisms with Taxonomic Categories - Taxonomy

Organisms with Taxonomic Categories


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