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Classification of life (Classification of organisms)

There are two ways in which organisms are classified; artificial and natural classification systems.

Artificial classification system

Classification of organisms that do not consider evolutionary relationships between organisms is called artificial classification. Here, organisms are classified using two or a few notable features.

Such a classification cannot be used for a scientific study. Because in artificial classification, organisms that do not have evolutionary relations belong to the same group, and organisms that have evolutionary relations belong to different categories.

Until the 18th century, artificial classification systems were used to classify organisms. The development of natural taxonomic systems began in the 19th century, with the introduction of evolution by Charles Darwin.

Naural classification system

In a natural classification system, the evolutionary relationships between organisms are considered.

The benefits of a natural classification are ease of identifying evolutionary relationships between organisms, ease of studying the diversity of organisms, ease to remember, improving the ability to predict organisms, ease of giving a scientific name, etc.

History of classification of organism

YearScientists Contribution
384 B.CAristotleAnimals are classified into vertebrates and invertebrates.
350 B.CTheophrastusPlants are classified into herbs, shrubs, and trees.
40 AD - 90 ADDioscoridesPlants are classified according to their medicinal properties and agricultural benefits.
1200 – 1280Albertus MagnusFlowering plants are classified into Dicotyledonae and Monocotyledonous
 1735Carl LinnaeusBinomial nomenclature was introduced.

About 6000 species were given scientific names.

The taxonomic hierarchy was introduced.

Organisms were classified into two kingdoms as Plantae and Animalia.
1869Ernst HaeckelIntroduction of “Phylum” to the taxonomic hierarchy.

Introducing a new living kingdom called “Protista”
1969R.H. WhittakerFive kingdoms were introduced into the taxonomic system.
(Monera, Protista, Fungi, Animalia, Plantae)
1982Carl WoeseMolecular biological features were used to classify organisms.

Three domain classification was introduced

Three domain classification of life by Carl Woese

Living organisms are divided into three domains based on molecular biological factors. Considering the common ancestral relationships of living things, the evolution of life can be represented as a branched tree. This is called the “Phylogenetic tree”.

 Phylogenetic tree
Phylogenetic tree

The tree domain classification was introduced by Carl Woese in 1990. He used molecular biological features to classify organisms. The molecular biological factors that he used are the Base sequence of important genes, amino acid sequence of proteins, the base sequence of rRNA, and molecular structure of the cell membrane.

Comparison of three domains of life

Three domains of life introduced by Carl Woese are Bacteria, Archaea and Eukarya. Domains are classified further into seven levels. Those are kingdom, phylum, class, order, family, genus, species. Each level is called a taxon. When coming from the top to bottom of the taxonomic hierarchy, the number of species decreases, diversity between the species decreases, and the evolutionary relationship between species decreases.

taxonomic hierarchy
Taxonomic hierarchy

Domains
CharacteristicsBacteriaArchaeaEukarya
Nuclear envelopeAbsentAbsentPresent
Membrane enclosed organellesAbsentAbsentPresent
Peptidoglycan in cell wallPresentAbsentAbsent
Membrane lipids.Unbranched hydrocarbonsSome branched hydrocarbonsUnbranched hydrocarbons
RNA polymeraseOne kindSeveral kindsSeveral kinds
Initiator amino acid for protein synthesis.Formyl-methionineMethionineMethionine
Noncoding parts in genes (introns)RarePresent in some genesPresent
Response to the antibiotics. (streptomycin and chloramphenicol)Growth inhibitedGrowth is not inhibited.Growth not inhibited
Histones associated with DNA.AbsentPresentPresent
Circular chromosomesPresentPresentAbsent
Ability to grow at temperatures above 100℃NoSome speciesNo
Characteristics comparison between the three domains

Classification of living organisms

  • Archaea
  • Eukarya
  • Protista
  • Protozoa
  • Rizopoda
  • Ciliophora
  • Algae
  • Bacillariophyta
  • Phaeophyta
  • Rhodophyta
  • Chlorophyta
  • Fungi
  • Zygomycota
  • Ascomycota
  • Basidiomycota
  • Chytridiomycota
  • Plantae
  • Bryophyta
  • Pterophyta
  • Lycophyta
  • Cycadophyta
  • Coniferophyta
  • Anthophyta
  • Animalia
  • Coelenterata
  • Platyhelminthes
  • Nematoda
  • Annelida
  • Mollusca
  • Arthropoda
  • Echinodermata
  • Chordata

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References and Attributes

Figures:

Image by Gerd Altmann from Pixabay was used in the cover image


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