Classification of Living Organisms (Taxonomy)

Concept sheet | Science and Technology

Secondary

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Definition

Taxonomy is the science that studies the classification of living organisms into different categories, according to strict rules that take into account physical and genetic characteristics.

Formerly, living organisms were classified into 2 categories: animals and plants. Today, this classification has evolved. There are now 5 categories called kingdoms.

The kingdoms of living organisms are:

Kingdom	Cellular organization	Number of known species
Moneras	Unicellular organisms No nucleus	Around 3 000 species
Protists	Unicellular organisms Presence of a nucleus and other organelles	Around 31 000 species
Fungi	Majority of multicellular organisms Certain unicellular organisms (e.g. yeasts)	About 150 000 species
Plants	Multicellular organisms	Around 300 000 species
Animals	Multicellular organisms	Around 1 200 000 species

Each kingdom is divided into categories, which, in turn, are divided into other categories and so on. In all, there are 7 levels of subdivisions called taxa. These taxa are, from the most general to the most specific, the kingdom, phylum, class, order, family, genus and species.

The classification system of living organisms (taxonomy)

Taxa

The 7 levels to which a living being belongs can be shown in what is called a taxonomic key.

Example

Iris versicolor

Kingdom: Plant
Phylum: Magnoliophyta
Class: Liliopsida
Order: Asparagales
Family: Iridaceae
Genus: Iris
Species: Iris versicolor

Example

E. coli

Kingdom: Monera
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order: Enterobacteriales
Family: Enterobacteriaceae
Genus: Escherichia
Species: Escherichia coli

Example

The common starfish

Kingdom: Animal
Phylum: Echinoderms
Class: Asterias
Order: Forcipulatida
Family: Asteriidae
Genus: Asterias
Species: Asterias rubens

For each of the 7 taxa, there are sub-categories: subclass, superclass, subphylum, etc.

Example

The domestic cat is a subspecies of cats.

The domestic cat

Kingdom: Animal
Phylum: Chordates
Subphylum: Vertebrates
Class: Mammals
Order: Carnivores
Suborder: Feliformia
Family: Felidae
Subfamily: Felinae
Genus: Felis
Species: Felis sylvestris
Subspecies: Felis sylvestris catus