Principles of living organisms classification and Taxonomic hierarchy

The cell is the building and functional unit of the living organism , Living organisms may be unicellular or multicellular , Although the similarity of all living organisms in their building and functional unit ( cell ) .

As well as in the features of life ( nutrition , excretion , respiration , reproduction , movement , sensation and growth ) , They differ in many other characteristics such as shape , structure , way of living , nutrition and reproduction .

No one knows how many several kinds of living organisms on earth’s surface , In spite of human success in describing and naming about 1.4 million kinds of these types till now , Biologists are thinking that this number is not representing more 10 % only of the living organisms on earth’s surface .

There are millions of insects , small animals and plants that live in oceans which are not yet discovered till now , Due to the enormous diversity in living organisms , the need for classification process appeared .


Classification is the arranging of living organisms in groups according to similarities and differences between each other to facilitate studying and identifying them , Taxonomy is the science that concerns with classification of living organisms in groups on scientific bases.

Importance of classification

  • It facilitates identifying new organisms and adding them into their similar groups .
  • It benefits many other fields of science .

The modern classification system depends on the definition of the species as a scientific and basic principle in classifying living organisms .

Principles of living organisms classification

Principles of living organisms classification

The species

It is a group of individuals having similar morphological characteristics , mate with each other and produce fertile offspring similar to them .

There are individuals that do not give the term species because they are sterile ( infertile ) individuals, they are unable to mate and produce a new generation of the same kind, such as Tigon and Mute .

Tigon is produced from mating of female lion with a male tiger , Mule is produced from mating of a female horse with a male donkey .

Naming of living organisms

A need appeared among scientists to name living organisms with unified scientific names , where the same organism has many names that differ in various earth’s regions and environment and these names are known as common names .

To overcome this problem , The scientist Carolus Linnaeus developed a system for naming organisms and gave it the name of binomial nomenclature system .

Conditions of writing scientific names of living organisms
  • Be written in Latin language .
  • Be written in italic letters or underlined for distinguishing them from the others .
  • Each living organism in binomial nomenclature is given a binomial name , The first represents the genus and begins with a capital letter , The second represents the species and begins with a small letter .

Example : The scientific name of the cat is Felis domesticus , where , Felis represents the genus of the cat , domesticus represents the species of the cat .

Taxonomic hierarchy

There are seven levels ( or groups ) for classifying living organisms , Each group comprises less numbers of organisms , that have more similar characteristics , than the group preceding it .

  • Kingdom is the highest level in taxonomic hierarchy of living organisms and it includes a number of phyla such as Kingdom Animalia .
  • Phylum includes a number of classes such as phylum Chordata .
  • Class includes a number of orders such as Class Mammalia .
  • Order includes a number of families such as Order Carnivora .
  • Family includes a number of genera such as Family Felidae .
  • Genus includes a number of species such as Genus Felis .
  • Species includes an interbreeding population of organisms that can produce healthy offspring such as Species domesticus  .

There are other groups that intermediate each two successive groups , such as sub-phylum and sun-class .

Dichotomous key

It is a series of descriptions ordered in pairs that lead to identify an unknown living organism , Scientists use it to help them in identifying the unknown living organisms .

Ways of its designing
  • It starts with broad feature , then it gets more specified and more privacy whenever we go through the levels of dichotomous key .
  • Through each step , you can choose one of the two descriptions according to the characteristics of the living organism .
  • At the end , you will reach to a description leads you to the organism’s name or the group to which it belongs to .

An example : shows a bilateral dichotomous key for five species of insects .

Insects are wingless ( ants ) and winged , Winged insects contains one pair of wings and two pairs of wings .

One pair of wings legs are shorter than the body ( Fly ) and legs are longer than the body ( Mosquito ) .

Two pairs of wings Wings covered with bright scales ( Moth ) and Wings are transparent ( Dragonfly ) .

Modern classification of living organisms , Kingdom ( Monera and Protista )

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