The Hierarchy of Classification-Groups

Biologists, such as Ernst Haeckel (1894), Robert Whittaker (1959) and Carl Woese (1977) have tried to classify all living organisms into broad categories, called Kingdoms. The classification Whittaker proposed has five kingdoms : Monera, Protista, Fungi, Plantae and Animalia, and is widely used. These groups are formed on the basis of their cell structure, mode and source of nutrition and body organisation. The modification Woese introduced by dividing the Monera into Archaebacteria (or Archaea) and Eubacteria (or Bacteria) is also in use.

Further classification is done by naming the sub-groups at various levels as given in the following scheme:

Kingdom

Phylum (for animals) / Division (for plants)

Class

Order

Family

Genus

Species

• Species: It is a basic unit for understanding taxonomy as well as evolution. Species is a group of individuals with similar morphological characters, which are able to breed among themselves and produce fertile offsprings of their own kind. Individually of the same species resemble each other closely both structurally and functionally.
• Genus: It is a group of species which are related and have less characters in common as compared to species. Members of a genus have identical reproductive organs.
• Family: It is represented by a group of related genera that are more similar to each other than with the genera of other families.
• Order: It is an assemblage of families resembling one another in a few characters.
• Class: It represents organisms of related orders.
• Phylum: It includes all organisms belonging to different classes having a few common characters.
• kingdom: It includes all organisms who share a set of distinguishing common characters.

Thus, by separating organisms on the basis of a hierarchy of characteristics into smaller and smaller groups, we arrive at the basic unit of classification, which is a ‘species’. Broadly, a species include all organisms that are similar enough to breed and perpetuate.