Classification of Living Organisms (B6.4)

Why classify?

Classification groups organisms based on shared characteristics and evolutionary relationships. It enables scientists to communicate precisely about organisms and understand biodiversity.

Linnaean system

Carl Linnaeus (18th century) devised a hierarchical classification:

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

Mnemonic: King Phillip Came Over For Good Soup.

The species is the basic unit. Members of the same species can interbreed to produce fertile offspring.

Binomial nomenclature: two-part Latin name: Genus species (italicised; genus capitalised). e.g. Homo sapiens, Felis catus.

Five kingdoms (traditional)

1. Prokaryotae (Monera) — bacteria; no nucleus
2. Protoctista — mostly unicellular eukaryotes (amoeba, algae)
3. Fungi — cell walls of chitin; decomposers
4. Plantae — cell walls of cellulose; photosynthesise
5. Animalia — no cell walls; heterotrophic

Three-domain system (Woese)

Carl Woese (1990) used ribosomal RNA sequence analysis to propose three domains:

This reclassification showed that Archaea are more closely related to Eukarya than to Bacteria, which the old five-kingdom system did not capture. The three-domain system is now preferred.

Evolutionary trees (phylogenetic trees)

Evolutionary trees show evolutionary relationships. Branches represent common ancestors. The more recent the branch point, the more closely related the organisms.

New molecular evidence (DNA/RNA sequencing, protein comparison) has allowed scientists to revise classification when traditional morphology was misleading.

Common exam errors

1. Saying all bacteria are in Archaea — Bacteria and Archaea are two separate domains.
2. Saying species in different genera cannot interbreed — actually two members of the same species can interbreed (same genus does not guarantee interbreeding).
3. Forgetting binomial nomenclature format — genus capitalised, species lower case, both italicised.