Classification of Biodiversity

Taxonomy. The science of identifying, naming and classifying organisms.

Linnaean hierarchy (most inclusive to most specific):

Level	Example (humans)
Domain	Eukarya
Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Primates
Family	Hominidae
Genus	Homo
Species	sapiens

Mnemonic: Dear King Philip Came Over For Good Soup.

Three domains (Woese, 1990). Originally Linnaeus had only two kingdoms (plants and animals); microorganisms forced expansion. Carl Woese's ribosomal RNA studies revealed three deep branches of life:

1. Bacteria — prokaryotes; cell walls of peptidoglycan; ester-linked membrane lipids.
2. Archaea — prokaryotes; cell walls without peptidoglycan; ether-linked lipids; many live in extreme environments (hot springs, salt lakes); more closely related to Eukarya than to Bacteria.
3. Eukarya — eukaryotic cells; subdivided into kingdoms Animalia, Plantae, Fungi, Protista (and others).

Why classification matters:

• Helps identify and communicate about species precisely.
• Reveals evolutionary relationships.
• Identifies conservation priorities (related species share threats).
• Predicts characteristics of new species based on relatives.

Binomial nomenclature (Linnaeus, 1753):

• Each species has a two-part Latin name: Genus species.
• Genus name is capitalised; species name is lower-case; both are italicised (underlined when handwritten).
• After first use, the genus is often abbreviated (e.g. E. coli for Escherichia coli).

Examples:

• Human: Homo sapiens.
• Domestic cat: Felis catus.
• Common reed: Phragmites australis.

Binomial names are stable across languages — a researcher in Tokyo and one in Berlin can communicate precisely about the same species.

Dichotomous key. A tool for identifying organisms via a sequence of paired choices.

Example key for some plants:

1. a) Leaves needle-shaped → go to 2. b) Leaves broad → go to 3.
2. a) Needles in clusters of 3 → Pinus. b) Needles attached singly → Picea.
3. a) Leaves opposite each other → Acer. b) Leaves alternating → Quercus.

Each step splits the remaining possibilities in two. Well-constructed keys use unambiguous, easily observable characteristics.

Phylogeny. Modern classification uses DNA evidence to construct phylogenetic trees that reflect evolutionary relationships. Sometimes traditional Linnaean groupings have been revised when molecular evidence revealed surprising relationships (e.g. whales are nested within Artiodactyla, the even-toed ungulates).