Classification and Taxonomy
Organising the Diversity of Life
Linnaeus and Binomial Nomenclature
Carl Linnaeus (18th century) developed the system of binomial nomenclature — every species has a two-part Latin name:
- Genus (capitalised) + species epithet (lowercase) — both italicised
- Example: Homo sapiens (genus Homo, species sapiens)
This universal naming system prevents confusion across languages and cultures. Names are Latinised and internationally standardised by the International Code of Nomenclature.
Taxonomic Hierarchy and the Three Domains
The classical hierarchy (mnemonic: Dear King Philip Came Over For Good Soup):<br>Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species
The modern Three Domain System (Woese, based on rRNA sequences):
- Bacteria: most known prokaryotes
- Archaea: extremophile and other prokaryotes, genetically distinct from bacteria
- Eukarya: all eukaryotes (plants, animals, fungi, protists)
Traditional Five Kingdoms (Monera, Protista, Fungi, Plantae, Animalia) have largely been replaced by cladistics-based classification.
Classification of Humans
| Rank | Human classification |
|---|---|
| Domain | Eukarya |
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Mammalia |
| Order | Primates |
| Family | Hominidae |
| Genus | Homo |
| Species | Homo sapiens |
What is the correct format for a species' binomial name?
Binomial nomenclature: Genus species — genus is capitalised, species epithet is lowercase, both are italicised (or underlined in handwriting).
Correct answer: Genus capitalised, species lowercase, both italicised
The Three Domain system is based primarily on:
Carl Woese used 16S rRNA sequence comparisons to reveal the deep evolutionary split between Bacteria, Archaea, and Eukarya.
Correct answer: rRNA (ribosomal RNA) sequence analysis
Explain why rRNA sequences are particularly useful for constructing phylogenetic trees across all life.
Universal presence + conserved but variable sequences = ideal phylogenetic marker.
Correct answer: rRNA genes are present in ALL living organisms (needed for ribosomes), are highly conserved (change slowly), yet have sufficient variation to distinguish major groups. This universality and gradual change makes them ideal molecular clocks for phylogenetics.