Origin and Evolution of Life: Classification and Taxonomic Categories

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Classification means identify similarities and arrangement of plants and animals according to their systematic division into groups or classes and different plants and animals into different groups or classes. Taxonomy is the study of the general principle of scientific classification living organisms and organized them according to their similarities.

Taxonomic Categories

A taxonomic category was first proposed by Linnaeus. Each level or categories termed taxon (plural-taxa). Taxonomic categories show its evolutionary relationship with other groups of organisms. The lowermost category is species. The taxonomic categories are.

  • Species - population of individuals or group of population interbreed freely and produce fertile offspring.

  • Genus – group of relate species which resemble one another in certain correlated characters.

  • Family - contains one or more related genera. All the genera of a family have some common features or correlated characters.

  • Order - includes one or more related families.

  • Class - made of one or more relates order.

  • Phylum/division - phylum is used for animals while division is used for plants. A division or phylum is formed of one or more classes. The various phyla belong to their respective kingdoms.

Image of Classification of Human and animal species

Image of Classification of Human Animal Species

Scientific Naming of Organisms

In different languages, states and country plants and animals have different common names. Binomial nomenclature is introduced by Carolus Linnaeus (Swedish biologist), simple system of naming organisms. The name of every kind of organism has two parts, that of the genus followed by that of species. The generic name is written with a capital letter and the specific name with a small letter, e.g. Homo sapiens (Human) and Mangifera indica (mango). Three main features of naming organisms as follows; scientific name is printed in italics or underlined when handwritten, scientific name is according to rules of nomenclature and scientific name are in Greek and Latin.

Prokaryotes and Eukaryotes

Prokaryotes and Eukaryotes are suggested by Hans Ris in 1960. The prokaryotes (pro - primitive, karyon - nucleus) consist of the bacteria and blue green algae while eukaryotic cells (eu - true; karyon - nucleus) are found in all multicellular animals, plants and fungi. There are differences between them are given below.

Image of Differences between Prokaryotes and Eukaryotes
Image of differences between Prokaryotes and Eukaryotes




  1. Size

0.1 m

10-100 m (greater volume)

  1. Genetic material

Circular DNA, no linear DNA, histones associated with DNA, nucleoid form, no nuclear membrane

Histone present on which DNA molecule wrapped, well defined chromosomes, nuclear membrane present

  1. Nuclear material site

DNA in cytoplasm

DNA inside distinct nucleus

  1. Organelles

No membrane bound organelles

Mitochondria, Golgi body, lysosomes present in the cell

  1. Cell wall

Always present, contains peptidoglycan

None (animals) or made of cellulose/chitin (plants and fungi)

  1. Respiration

By mesosomes

By mitochondria

  1. Reproduction

Mostly asexual e.g. bacteria, cyanobacteria and blue green algae

Asexual and sexual e.g. Protoctista, fungi, plants, animals

  1. Flagella

Smaller, distinction of axoneme and sheath is absent in the flagellum

Longer and flagellum shows distinction of axoneme and sheath.

  1. Plasmids



  1. Endocytosis and exocytosis



The Five Kingdoms of Organisms

Five kingdom classifications are proposed by R.H Whittaker in 1969, which is based on; complexity of cell structure (prokaryotes and eukaryotes), complexity of organisms (unicellular and multicellular), mode of nutrition, lifestyle and phylogenetic relationship.

Image of the Five Kingdom Classification of Organisms
Image of the five kingdom classification of organisms

Name of kingdom

Nature of nucleus

Whether unicells or multicells

Kind of nutrition

  1. Monera (Blue green algae and bacteria)



Diverse type of nutrition

  1. Protoctista (Algae and Protozoa)



Diverse kind of nutrition

  1. Fungi (Molds, etc.)



Saprophytic (feed on head, decaying matter)

  1. Plantae (All green plants)



Autotrophic (Synthesize food by photosynthesis)

  1. Animalae



Heterotrophic (Depend on others for food)

Image of the Five Kingdom of Life

Image of the Five Kingdom of Life

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