Classification of living organisms

Classification is how biologists arrange the millions of species into ordered groups. It helps us identify organisms, communicate clearly, and understand evolutionary relationships.

Linnaean system (Carl Linnaeus, 18th century)

Linnaeus arranged organisms by physical similarities into a strict hierarchy:

Kingdom → Phylum → Class → Order → Family → Genus → Species

A useful mnemonic: King Penguins Classify Organisms For Good Service.

Each species is given a two-part Latin name (binomial nomenclature):

• Genus species, e.g. Homo sapiens, Panthera leo (lion).
• The genus is capitalised; the species name is lower-case; both are italicised (or underlined when handwritten).

The five kingdoms were: animals, plants, fungi, protists and prokaryotes (bacteria).

Why classification has changed

The Linnaean system was based on visible features. Modern microscopy and DNA analysis have revealed that some organisms grouped together by appearance are not closely related, and others that look very different share much of their DNA. So the system has been updated.

The three-domain system (Carl Woese, 1990)

Using analysis of DNA and ribosomal RNA, Woese proposed three domains above the kingdom level:

1. Bacteria — typical bacteria you'd recognise.
2. Archaea — primitive single-celled organisms living in extreme environments (hot springs, salt lakes, deep ocean).
3. Eukaryota — all organisms with cells that have a nucleus: protists, fungi, plants and animals.

This was a big change: it showed that what used to be called "prokaryotes" actually contain two very different domains — bacteria and archaea differ in their cell membranes, RNA and metabolism.

Evolutionary trees

An evolutionary tree (or phylogenetic tree) shows how organisms are related through common ancestors.

• The trunk of the tree is the common ancestor.
• Branches split where new species diverged.
• The tips are species alive today (or fossils).
• Two species sharing a recent branching point are more closely related than two species whose branches split long ago.

Evolutionary trees are constructed using:

• Anatomical similarities.
• Fossil evidence (with rough dates from rock layers).
• Molecular data — comparing DNA / protein sequences. The more similar the sequences, the more closely related the organisms.

Why use evolutionary trees?

• Easier than long Linnaean lists for showing relationships.
• Show time and the order of branching, which Linnaean lists cannot.
• Continually updated as new DNA evidence comes in.

Links

Builds on B6.6 (speciation, formation of new species) and B6.9 (evidence for evolution from DNA and fossils).