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What is Biological Classification and Taxonomy?

You must be wondering why the word taxonomy is mentioned when a biological classification is being studied; but, taxonomy is also known as biological classification.

It is basically the grouping of all organisms, be it plants, animals, and insects along with human beings under certain categories to help biologists and other scientists as well as those studying them differentiate among the organisms.

All organisms are grouped based on their similarities and differences into groups and sub-groups in a hierarchy.

Who and what led to biological classifications?

Initially started by the ancient Greek Philosopher Aristotle, he invented the multi-system to group organisms into different categories. But, the major effort was put in by a Swedish scientist named Carolus Linnaeus, who created the binomial nomenclature.

Binomial nomenclature is a system where each plant, animal, and every other organism in our ecosystem is given a name of two words from Latin origin; a genus and a species.

This system was soon used by biologists as well as other scientists all around the world and Linnaeus is now known as the ‘Father of modern taxonomy’.

What are the objectives of biological classification of organisms?

There are four main aims for classification:

  • Aid in the easy identification of all organisms
  • Organization of the species in a variety of categories on the basis of their similarities and dissimilarities.
  • Identification and description of all the possible types of species.
  • To evolve a completely accepted system which should indicate origin and evolution of the species.

Types of Biological Classification

  1. The artificial system of classification

  • Use of 1 or more morphological character for grouping organisms.
  • Aristotle first divided the animals into two categories; on the basis of red blood cells (RBCs). The animals were divided into enaima (having RBCs) and anaima (lacking RBCs).
  • Aristotle secondly classified animals based on their habitat; aquatic, terrestrial and aerial.
  • Pliny the Elder was a Roman naturalist who used this system of classification for both plants and animals, dividing them into land, water, and air.
  • This scientist also differentiated animals based on flight and non-flight ones.
  1. The natural system of classification

  • Use of a number of characteristics of the organisms to differentiate between their similarities and differences.
  • Characters used for classification include anatomical, morphological, cytological, physiological, reproduction, cytochemistry, and biochemistry, just to name a few.
  • The above characters used to bring out the similarities of a group and comparing one set of organisms with another.
  • For example, birds are characterized by the presence of wings and feathers while mammals are characterized by the presence of mammae.
  1. Phylogenetic system of classification

  • This classification is based on the evolutionary relationship among organisms.
  • The evolutionary relationship is based on Charles Darwin’s theory of natural selection.
  • This type of classification was first proposed by scientists Adolf Engler and Karl Prantl.

5 Kingdom system of classification

American taxonomist R H Whittaker divided all the organisms into 5 kingdoms.

Since a virus is neither living nor non-living, he didn’t include them in his system.

He used the following 5 criteria for his classification:

  • The complexity of cell structure; if they are prokaryote or eukaryote
  • The complexity of body organization; if they are unicellular or multicellular
  • Type or mode of nutrition; if they are photoautotrophic, absorptive heterotrophic or ingestive heterotrophic.
  • Ecological lifestyle; if they are producers, decomposers or consumers
  • Phylogenetic relationships

The 3 Domains of Life

Also known as the 6 kingdom system of classification, this was proposed by Carl Woese, a microbiology professor at the University of Illinois.

The 3 domain system is the biological classification that differentiates the cellular organisms into archaea, bacteria and eukarya categories.

It is vaguely based on the 5 kingdom classification system, but divides the monera into 2 more categories, while the rest of the eukaryotes remain in the third category.

It separates the prokaryotes into two more groups; eubacteria (bacteria) and archaebacteria (archaea).

It is also known as the 6 kingdom system of classification.

Biological Classification


  • This category or domain consists of those prokaryotic organisms that have distinct nucleotides in their 16S RNA.
  • The prokaryotes live in extreme environments.
    • Methanogens: help in metabolizing hydrogen and carbon dioxide into methane.
    • Halophiles: live in salty environments
    • Thermoacidophiles: their habitat is in acidic and high temperatures.


  • This domain includes those prokaryotes that lack membrane covered cell organelles.


  • This domain contains those eukaryotic organisms which initially originated due to the endosymbiotic association between certain eubacteria and archaebacteria.
  • It consists of 4 kingdoms: Protista, Fungi, Plantae, and Animalia. 

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