5.1 Essential ideas

5.1.3 Classification and biodiversity

Taxonomy is the science concerned with finding relationships between organisms by putting them in groups. Carolus Linnaeus is the founder of this field of classification. He grouped thousands of species of plants and animals based on their external characteristics and invented the principle of taxonomic hierarchy. 

Today, scientists all over the world adhere to a modern version of Linnaeus’s original taxonomic system when naming species. Having a universally recognised taxonomic system allows scientists to:

  • communicate easily, even if they are working from different academic disciplines in different environments, e.g. university research, government bodies, public interest/conservation groups, or the private sector
  • make predictions about characteristics shared within a group to guide further research, e.g. scientists may look for an internal structure or function that exists in one species of a taxon in another species of the same taxon.

The hierarchy of taxa and binomial nomenclature

The main levels of taxonomy (from largest group to smallest group) are: Kingdom, Phylum, Class, Order, Family, Genus and Species. As you move down each level of the hierarchy, the organisms within a group share fewer and fewer characteristics. At the end of the line there is one unique organism (i.e. not a group). In this system of classification, the genus and higher taxa are understood to have evolved from a common ancestral species.

Figure 5.1.3a – Taxonomy of a grizzly bear
Understanding the principle of grouping by taxa: each taxon represents a group of organisms with shared characteristics

In practice, it is not necessary to go through the entire taxonomy in order to identify an organism. Instead, it is enough to identify an organism by its binomial, or genus and species, name. As a convention started by Carolus Linnaeus, the binomial name is written in italics or underlined. This binomial system of nomenclature is universal and does away with confusing common names.

Other examples of classification using taxonomy:

Taxonomic level

Common name: Killer whale (Figure 5.1.3e)

Common name: Western yellow pine  (Figure 5.1.3f)






















Binomial name

O. orca

P. ponderosa

Note: The genus name is always capitalised, and both the genus and species name are always italicised or underlined. The genus name may also be abbreviated as shown above.

The three-domain system of classification

Until about 1970, all bacteria were grouped together in a single Kingdom called ‘Bacteria’. More recent biochemical evidence has demonstrated that there are very large differences in the ribosomal structure of bacteria. This discovery led to the development of the most current model of taxonomy, which is a three-domain system. The three domains group organisms based on the type of ribosomal RNA they have. The three domains are Archaea (Figure 5.1.3g), Eubacteria and Eukaryotes. This means that all organisms can now be classified using an eight-taxa system, starting at Domain.

The three-domain system of classification:



(ancient bacteria)

The oldest living things on Earth, able to withstand extremes of heat and salinity (e.g. thermophiles, halophiles)
This domain contains the Kingdom Archaea.

(true bacteria)

Modern bacteria, including aerobic and anaerobic heterotrophic bacteria, as well as cyanobacteria.
This domain contains the Kingdom Bacteria.


(true nucleus)

All organisms having nucleated cells.
This domain contains four kingdoms: Animalia, Plantae, Fungi and Protista.

Carl von Linné

Figure 5.1.3b – Carl von Linné

Did you know?

Carolus Linnaeus changed his name to Carl von Linné after he was enobled by the Swedish King Alfred in 1757. Good science will get you places!


Prominent biologists, such as Ernst Mayr (1904–2005) who defined the biological species concept, and Salvador Luria (1912–91) who discovered the genetic basis of bacterial disease-resistance, advocated against the division of Kingdom Bacteria. To what extent is scepticism useful in science?

Ernst Mayr

Figure 5.1.3c – Ernst Mayr

Salvador Luria

Figure 5.1.3d – Salvador Luria

Language tools

Most students find it easier to remember the sequence of taxa by creating a mnemonic using the first letter of the group – the more nonsensical the better! Here’s an example: King Philip Came Over For Green Soup.

killer whale

Figure 5.1.3e – Killer whale

western yellow pine

Figure 5.1.3f – Western yellow pine

volcanic vent

Figure 5.1.3g – Archaea
Archaea such as M. fumariolicum are found in volcanic vents, where they metabolise methane and heavy metals. Archaeans are thought to be the oldest organisms on Earth.