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Generally, a class of individuals with common qualities and a common name. But there is also a specific biological meaning with the species classification. A species in biology is a category of biological classification ranking below the genus.

During the era before genetic information morphological changes where used to define a species. Therefore a plant or animal species was defined to be:

  • A population of individuals able to interbreed under natural conditions
  • Able to produce fertile offspring
  • Reproductively isolated from other populations

Species concept of prokaryotes

The older (above mentioned) classification is not relevant for prokaryotes because they

{Guerrero, 2002 #47}

Current patterns of biodiversity are the results of past events reflected in the genetic code. Therefore prokaryotes and eukaryotes are classified as species on genetic bases:

  • Homologous genetic sequences (“molecular clocks”) are compared. Classification is based on the number of amino acid substitutions between the sequences.
  • A new prokaryote species is defined to have a 16S ribosomal RNA sequence that differs more than 3% from all other organisms.

In addition to the species concept, microbes are usually also classified according to the kind of environment where they thrive.


A species is a category of biological classification ranking below the genus. Therefore a species is designated by a binomial that consists of the name of the genus followed by a Latin or latinized uncapitalized noun or adjective agreeing grammatically with the genus name, for example Acidithiobacillus ferrooxidans.

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