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Taxonomy and biological classification

The biological classification system is used to categorise organisms in a way that clarifies their relationship to other organisms.

The system uses taxonomic ranks to name individual species. The system of ranks used today was invented by the 18th century Swedish botanist Carolus Linnaeus.

The order of taxonomic ranks (from most general to most specific, leaving out the domain) can be remembered using the mnemonic King prawn curry or fat greasy sausages.

Main taxonomic ranks Mnemonic Example
Domain Eukarya
Kingdom King Animalia
Phylum (plural phyla) Prawn Chordata
Class Curry Mammalia
Order Or Primates
Family Fat Hominidae
Genus Greasy Homo
Species Sausages Homo sapiens

Binomial nomenclature is the naming system for species. It is a part of the biological classification system.

Each species name is composed of two words (bi = two, nomen = word). The first word refers to the genus and the second word specifies the species within the genus.

The two words of the species name are always italicised (or underlined when handwritten).

The part of the name that refers to the genus is always capitalised.

The species name of humans is Homo sapiens. Homo refers to the genus and sapiens refers to the species within that genus.

The genus name is sometimes abbreviated to just the first letter.

The name Escherichia coli (a bacterium often used as a model organism in experiments) is often abbreviated to E. coli.

The Scarlet and the American White Ibis are both members of the genus Eudocimus, but are different species.
The Scarlet and the American White Ibis are both members of the genus Eudocimus, but are different species.

The biological classification system today is based on evolutionary relationships.

Before Charles Darwin's theory of evolution (1859) became widely accepted, biologists had used physical similarities to classify organisms (into domains, kingdoms, phyla, ...).

Animals are now classified according to the degree of similarity between DNA sequences, rather than physical characteristics.

Some animals with different evolutionary backgrounds living in similar environments can evolve very similar characteristics (convergent evolution) which could cause them to be incorrectly classified.

Many historical groupings have been altered, and with ongoing genetic research, classifications continue to change.

The long-tailed tit was initially classed in the genus Parus. It now has the binomial name Aegithalos caudatus.
The long-tailed tit was initially classed in the genus Parus. It now has the binomial name Aegithalos caudatus.

Phylogeny is the evolutionary history of organisms.

The phylogeny of an organism can be analysed by comparing the physical characteristics of an organism with that of others. The more characteristics the two organisms share, the more likely they are to be closely related.

Convergent evolution can cause false conclusions to be made, so it is important to study a large number of traits.

Genome sequencing can also be used to analyse the phylogeny of organisms. The more similar the DNA sequence, the closer the relation.

The results of these studies can be used to construct phylogenetic trees showing the history of different species.

Phylogenetic representation of vertebrates
Phylogenetic representation of vertebrates