Classification systems

Aristotle’s classification

• Classification system was first designed by Sir Aristotle [384- 322 B.C.]. He classified all living organisms as plants and animals. Plants were further classified on the basis of their size and animals were classified on the basis of their habitat.

Limitations of Aristotle’s Classification

• Aristotle’s classification was based on habitat and same habitat can share different types of organisms.

Example:

• Marine habitat has different type of animals like fish, mammals, corals etc. same is true for terrestrial and aerial habitat too.

Two kingdom System

• It was proposed by Sir Carolus Linnaeus in 1758. He classified organisms on the basis of their mode of nutrition into two categories.

• Plants are autotrophic and stationary whereas animals are heterotrophic and can move from one place to another. They are further sub-classified depending on their complexity.

Limitations of two kingdom system:

• There are many unicellular lower forms of organisms that have characters of both plants and animals.

Example:

• Euglena: A unicellular organism having certain features of animals and certain features of plants. It has a flagellum which is used for locomotion and food capturing. However, it has chloroplasts like plant cells.

• Chlamydomonas: A unicellular alga purely autotrophic but has a flagella which helps in locomotion like protozoans.

• Fungi: They have cell wall like plants but lack chlorophyll therefore heterotrophic like animals.
• Slime moulds: A type of fungi. They do not have a cell wall in the vegetative phase and ingest particulate matter like animals. However, they develop a cell wall in the reproductive phase which is similar to other fungi. Thus they resemble animals in one phase and plants in the other.

Note:

• Since there was no clear distinction between lower plants and animals there was a need to revise the two kingdom system.

Three kingdom System:

• Ernest Haeckel in 1860s raised a third kingdom Protista which included all unicellular organisms both from plant and animal origin.

Four Kingdom System:

• Later with discovery of prokaryotic and eukaryotic cell a new kingdom was introduced that included all unicellular prokaryotic organisms.

Five-kingdom system of classification:

• In1959, Robert Whittaker proposed five kingdom system which is accepted worldwide and a base for modern classification system. This classification is based on following criteria:
• Prokaryotic cell versus eukaryotic cell
• Unicellular versus multicellular body organization
• Mode of nutrition

• Later in 1977, Carl Woese sub classified kingdom monera into Eubacteria and Archaebacteria.