Describe the system of biological classification and explain how organisms are named and grouped.

What this dot point is asking

Why we classify

There are millions of species, so biologists need a consistent system to name them, group them, and communicate clearly across languages and countries. Classification also reveals patterns: organisms grouped together usually share features because they share a common ancestor. A good classification therefore does two jobs at once, it organises diversity and it tells us about evolutionary history.

The Linnaean hierarchy

Carl Linnaeus developed the ranked system still used today. Each organism is placed in a series of nested groups, from broad to specific:

• Domain
• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species

As you move down the list, the groups get smaller and the members share more features. For example, all animals share the kingdom Animalia, but only humans share the species level. A common memory aid is to take the first letter of each rank and build a sentence.

Binomial nomenclature

Every species is given a two-part Latin name, a system called binomial nomenclature. The first part is the genus (capitalised) and the second is the species descriptor (lowercase). The whole name is italicised or underlined.

This system gives every organism a unique, universal name, avoiding the confusion of common names, which differ between languages and regions and can refer to several species.

The three domains

Older systems used five kingdoms, but molecular evidence (especially comparing ribosomal RNA) showed that life splits most fundamentally into three domains:

• Bacteria: prokaryotic, no nucleus, found almost everywhere.
• Archaea: prokaryotic, superficially like bacteria but biochemically distinct, often found in extreme environments.
• Eukarya: all organisms with eukaryotic cells, including protists, fungi, plants, and animals.

The three-domain system shows that archaea, despite looking like bacteria, are more closely related to eukaryotes in some molecular features. This is a good example of classification changing as new evidence appears.

Classification reflects evolution

Modern classification is phylogenetic, meaning groups are built to reflect evolutionary descent. Organisms placed in the same group share a common ancestor and inherited features from it. Biologists use many lines of evidence to decide groupings, including:

• Structural features (anatomy and morphology).
• Embryological development.
• Biochemical and DNA similarities.

DNA sequencing has become the most powerful tool, because the more similar two organisms' DNA, the more recently they shared an ancestor. This sometimes overturns groupings based only on appearance.

Dichotomous keys

To identify an unknown organism, biologists use a dichotomous key, a series of paired statements that each split the possibilities in two. At each step you choose the option that matches the organism, which leads to the next pair until you reach a name. Keys are practical tools for fieldwork and rely on observable features such as body shape, number of legs, or leaf arrangement.