IB Biology SL

Revision Notes

5.2.1 Classification System

Binomial System

  • The diversity of life on Earth is vast, and is known as global biodiversity
  • The extent of global biodiversity is such that scientists can only estimate the total number of species present on Earth, and it is likely that there are many species yet to be discovered
  • For biologists to make sense of the huge array of species, organising them into logical groups is essential
    • This process of putting organisms into groups is known as classification
    • The science of classification is known as taxonomy, and scientists working in the field of taxonomy are taxonomists
  • Classifying an organism involves deciding which biological group, or taxon (plural taxa), it fits into best, and then naming it according to its taxon
  • Historically an organism’s biological group was determined on the basis of its observable characteristics, and today this information is combined with DNA sequence data for more accurate classification

A Universal Naming System

  • The biological system of naming used to name species according to their taxa is known as the binomial system
  • This system is universal, ensuring that scientists around the world all use the same method of naming species
    • In the past, individual scientists decided on species names; names could be very long, and often one species could have different names in different parts of the world
  • To ensure that all biologists know, and agree on, the criteria for naming species, regular meetings called congresses are held to discuss naming conventions
    • The first International Zoological Congress was held in 1889, during which taxonomists agreed on the rules that should be used for classifying and naming species
    • Congresses have since been held at regular intervals, with separate meetings for scientists who study different groups of organisms e.g. animals, plants, and fungi

The Binomial System

  • The binomial naming system, or system of nomenclature, was introduced by the Swedish taxonomist Carl Linnaeus in his 1758 book, Systema Naturae
  • The system involves giving a species a two-part name, hence binomial
  • Both parts of the name are in Latin, or a latinised version of a non-Latin word
    • e.g. Eriovixia gryffindori is a species of orb spider named after a famous school house
  • The first part of the name is an organism’s genus, and the second is its species name
    • E.g. the binomial name of a wolf is Canis lupus; wolves belong to the genus Canis, and the species lupus
  • There are several conventions, or rules, that should be used when writing binomial names
    • The genus should begin with a capital letter, and the species with a lower-case letter, e.g. the honey bee is Apis mellifera
    • When typed, binomial names should appear in italics, and when written by hand, they should be underlined e.g. a limpet is Patella vulgata when typed, or Patella vulgata by hand
    • The first time a binomial name is used in a text it should appear in full, e.g. wheat is Triticum aestivum, but the genus name can from then on be abbreviated to T. aestivum

NOS: Cooperation and collaboration between groups of scientists; scientists use the binomial system to identify a species rather than the many different local names

  • The work of scientists does not take place within the confines of a laboratory, or even a country, but can have implications for the work of other scientists around the world
  • Because of the international nature of scientific research, it is essential that scientists are able to communicate with each other clearly about their work
  • For scientific communication to be effective and allow collaboration, it is essential that scientific language enables scientists to be sure that they are talking about the same thing
  • The binomial naming system is a good example of a system that enables scientists to communicate clearly with each other about living organisms
  • The binomial system is essential because it ensures that scientists are all talking about the same species
    • Different countries may have different common names for the same species e.g. In English, Felis domesticus is known as a cat, but around the world it is also known as kitte, maow, chat, kissa, bili, and gato
    • Different local or cultural names may arise even within a country e.g. in the UK alone the woodlouse, Onescus asellus, has more than 50 different local names that include woodpigs, cheesy bobs, woodywigs, chuggy pigs, and crunchy bats, and another country may have an equally long list of local, or cultural names for the same species
    • There are also examples of species where one name might be used for different species in different parts of the world, e.g. the robin in the US is a completely different species of bird to the robin in Europe
  • There is nothing wrong with maintaining the use of different common names around the world, but when scientists discuss their work, the binomial system ensures that they are communicating effectively with each other


  • Biological classification involves putting organisms into groups, or taxa (singular taxon)
  • The taxa form a hierarchy
    • A hierarchical system is one in which larger groups contain smaller groups with no overlap between groups
  • The smallest taxon in the taxonomic hierarchy is species
  • The species taxa are grouped within the next biggest taxon in the hierarchy, genus (plural genera)
  • The genera are grouped within the next taxon, family, and so on until the biggest taxon, domain


  • Taxonomy is the practice of biological classification
    • Organisms are grouped into taxa, with the smallest taxon being species
  • The science of taxonomy has frequently changed to match the latest discoveries about the features of organisms
    • Historically, the largest taxonomic groups were the plant and animal kingdoms, then fungi were discovered and added (incorrectly) to the plant kingdom
    • Microscopes led to the discovery of prokaryotes and eukaryotes, and the taxa were later divided into five kingdoms; plants, animals, fungi, protoctists, and prokaryotes
      • The protoctists are eukaryotic, primarily single-celled, organisms
    • RNA analysis has recently shown that there are two distinct groups of prokaryotes, leading to a shift in taxonomic thinking and the beginnings of the three domain system
  • The largest taxonomic group is now known as a domain
  • There are three domains, which are:
    • Archaea (prokaryotes)
    • Eubacteria (prokaryotes)
    • Eukaryotes (eukaryotes)

The three domains, downloadable IB Biology revision notes

The three domains

Classifying the Archaea

  • The archaea were originally classified with the rest of the bacteria in one taxon due to sharing several features with them
    • Prokaryotic cell structure
    • Circular chromosome
    • Presence of a cell wall
    • 70S ribosomes
  • Closer analysis of the archaea however showed that some of their features were distinct from the rest of the prokaryotes
    • Their cell walls are made of a different material from the cell walls of the rest of the bacteria
    • Their cell membranes are made of a distinct type of lipid
    • The small subunit of their ribosomes is more similar to eukaryotic ribosomes than to the ribosomes of the rest of the prokaryotes
  • These discoveries led to a change in the classification of the archaea, which became their own domain

Hierarchy of Taxa for Eukaryotes

  • Eukaryote is the domain of all eukaryotes, distinguishable from the Bacteria and the Archaea which are both prokaryotic domains
  • Just like the other domains, the Eukaryote domain contains a hierarchy of taxa
  • The taxonomic hierarchy contains the following taxonomic groups in descending order of size:
    • Domain
    • Kingdom
    • Phylum
    • Class
    • Order
    • Family
    • Genus
    • Species
  • It can be good to use a mnemonic, or memory aid, to help you remember the different ranks in the taxonomic classification system
    • There are lots out there, or you can make up your own, but here’s an example that you might find helpful:
      • Do Keep Ponds Clean Or Fish Get Sick
      • Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

Classification system, downloadable AS & A Level Biology revision notes

The hierarchy of taxa

  • The wolf, Canis lupus, is an example of an organism in the eukaryote domain
    • A wolf belongs to the following taxonomic groups:
      • Domain: Eukaryote
      • Kingdom: Animalia
      • Phylum: Chordata
      • Class: Mammalia
      • Order: Carnivora
      • Family: Canidae
      • Genus: Canis
      • Species: lupus
  • The flowering plant Hibiscus rosa-sinensis is another example of an organism in the eukaryote domain
    • It belongs to the following taxonomic groups:
      • Domain: Eukaryote
      • Kingdom: Plantae
      • Phylum: Angiospermae
      • Class: Dicotyledonae
      • Order: Malvales
      • Family: Malvaceae
      • Genus: Hibiscus
      • Species: rosa-sinensis

The Classification of the Wolf and the Hibiscus Plant Table

The Classification of the Wolf and the Hibiscus Plant Table, downloadable IB Biology revision notes

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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