CB4.2 — Classification (Edexcel 1SC0)

Linnaean classification

Organisms are grouped in a hierarchy from broadest to most specific: Kingdom → Phylum → Class → Order → Family → Genus → Species

Mnemonic: King Philip Came Over For Good Soup.

A species is a group of organisms that can interbreed to produce fertile offspring.

Organisms are given a two-part Latin name (binomial nomenclature): Genus species (e.g. Homo sapiens). The genus is capitalised; the species is lower case; both are italicised or underlined.

Three-domain system (Woese)

Carl Woese proposed a classification based on ribosomal RNA sequences:

• Archaea: single-celled prokaryotes found in extreme environments.
• Bacteria: single-celled prokaryotes; most common.
• Eukarya: all eukaryotic organisms (animals, plants, fungi, protists).

This replaced the five-kingdom system because molecular evidence showed Archaea and Bacteria are fundamentally different despite both being prokaryotes.

Evolutionary trees

Evolutionary trees (phylogenetic trees) show how species are related based on shared ancestry. Species that share a more recent common ancestor are more closely related.