CARL WOESE proposed the THREE-DOMAIN SYSTEM in 1977 based on analysis of ribosomal RNA (rRNA).
This replaced the five-kingdom system with three domains:
1. ARCHAEA — primitive prokaryotes, often found in extreme environments (hot springs, salt lakes).
DNA differs significantly from bacteria despite looking similar under a microscope.
2. BACTERIA — true bacteria, the most numerous organisms on Earth.
Cells without membrane-bound nuclei; different cell wall chemistry from Archaea.
3. EUKARYA — all organisms with membrane-bound nuclei.
Includes all animals, plants, fungi, and protists.
WHY THREE DOMAINS?
RNA sequencing showed Archaea are more closely related to Eukaryotes than to Bacteria — despite looking like bacteria under the microscope.
This illustrates that observable characteristics can be misleading — molecular evidence is more reliable.
Evolutionary Trees
EVOLUTIONARY TREES (phylogenetic trees) show the evolutionary relationships between organisms.
How to read an evolutionary tree:
Branching points (NODES) = common ancestors.
The LENGTH of branches may represent evolutionary time.
Organisms that share a more RECENT common ancestor are more closely related.
EVIDENCE used to build evolutionary trees:
Anatomical similarities — homologous structures.
Fossil record — order of appearance of species.
DNA and RNA sequences — the more similar the sequences, the more closely related.
Protein similarities — amino acid sequences in key proteins (e.g. cytochrome c).
IMPORTANCE:
Evolutionary trees help us understand how species are related.
Can identify the most likely common ancestor of a group.
Help in medicine — understanding which organisms are closely related to pathogens.
LINNAEAN vs EVOLUTIONARY:
Linnaeus classified by appearance/structure (morphology).
Modern classification uses both morphology AND molecular evidence (DNA, RNA).
Molecular evidence has revised some traditional classifications.
⚠️ Common Mistake
A species is defined by the ability to interbreed and produce FERTILE offspring — not just by looking similar. Horses and donkeys can breed but produce sterile mules → they are different species. The three-domain system has Archaea as a SEPARATE domain from Bacteria — they are not the same despite both being prokaryotes.
📌 Key Note
Linnaeus: binomial naming (Genus species), Kingdom→Phylum→Class→Order→Family→Genus→Species. Three-domain system (Woese): Archaea, Bacteria, Eukarya — based on rRNA. Archaea more related to Eukaryotes than Bacteria. Evolutionary trees: nodes = common ancestors; molecular evidence now used alongside morphology.
🎯 Matching Activity — Classification Concepts
Match each term to its correct description. — drag the symbols on the right to match the component names on the left.
Binomial name
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Species
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Three-domain system
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Node on evolutionary tree
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Two-part Latin name: Genus species — e.g. Homo sapiens
Represents a common ancestor from which two lineages diverged
Archaea, Bacteria, Eukarya — based on rRNA sequences (Woese, 1977)
Organisms that can interbreed and produce fertile offspring
⭐ Higher Tier Only
Explain how molecular evidence (rRNA sequences, DNA base sequences, amino acid sequences) has revised traditional morphology-based classification. Interpret evolutionary trees built from molecular data and explain why Woese's three-domain system replaced the five-kingdom system. Evaluate the impact of new molecular data on classification — explain that classification systems are working hypotheses, revised as evidence changes.
🔬 Triple Science Only
Classification of living organisms (4.6.5) is biology-only — not in Combined Science. Covers Linnaean classification, the three-domain system (Archaea, Bacteria, Eukarya), and evolutionary trees as evidence for relationships between organisms.
🎯 Test Yourself
Question 1 of 2
1. Why did Carl Woese's three-domain system replace the five-kingdom classification?
2. Two species share a more recent common ancestor on an evolutionary tree than two other species. What does this indicate?
⭐ How Well Do You Understand This Topic?
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