B39: Classification
Classification systems and the three-domain system
Key Definitions
Classification: The organisation of living organisms into groups based on their shared characteristics and evolutionary relationships. Classification helps scientists identify, study, and communicate about organisms.
Binomial naming system: A two-part Latin name for each species: the Genus name (capitalised) followed by the species name (lowercase). Written in italics or underlined if handwritten. E.g. Homo sapiens.
Evolutionary trees: Diagrams that show the evolutionary relationships between organisms based on DNA/protein similarities. Closely related species share a more recent common ancestor.
The Linnaeus System
Carl Linnaeus developed a classification system in the 1700s that groups organisms into a hierarchy of increasingly specific groups: Kingdom → Phylum → Class → Order → Family → Genus → Species.
Kingdom • Phylum • Class • Order • Family • Genus • Species
(Mnemonic: King Philip Came Over For Good Soup)
(Mnemonic: King Philip Came Over For Good Soup)
Example 1: Classification of a Human
| Taxonomic Rank | Group |
|---|---|
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Mammalia |
| Order | Primates |
| Family | Hominidae |
| Genus | Homo |
| Species | sapiens |
Binomial name: Homo sapiens
Example 2: Classification of a House Cat
Kingdom: Animalia → Phylum: Chordata → Class: Mammalia → Order: Carnivora → Family: Felidae → Genus: Felis → Species: catus
Binomial name: Felis catus
The Binomial Naming System
Linnaeus also developed the binomial system of naming species. Each species has a two-part Latin name: the Genus name (capitalised) followed by the species name (lowercase). The entire name is written in italics or underlined if handwritten.
Example 3: Binomial Names
- Homo sapiens — humans (Genus: Homo, species: sapiens)
- Panthera leo — lion (Genus: Panthera, species: leo)
- Panthera tigris — tiger (Genus: Panthera, species: tigris)
Lions and tigers share the same genus (Panthera) but are different species. This shows they are closely related but distinct.
The Three-Domain System
Based on new evidence from molecular biology (especially DNA and RNA analysis), Carl Woese proposed the three-domain system, which reclassifies organisms into three domains at the highest level above kingdoms.
| Domain | Description | Examples |
|---|---|---|
| Archaea | Primitive prokaryotes — often live in extreme conditions (high temperature, high salt, no oxygen). Cell chemistry differs from bacteria | Methanogens, thermophiles, halophiles |
| Bacteria | True bacteria — prokaryotic organisms found in all environments. Cell chemistry differs from archaea | E. coli, Salmonella, Staphylococcus |
| Eukarya | Eukaryotic organisms — cells have a nucleus and membrane-bound organelles | Animals, plants, fungi, protists |
Example 4: Why Classification Systems Change
New technologies have led to changes in classification:
- DNA sequencing revealed that Archaea and Bacteria are fundamentally different at the molecular level, leading to the three-domain system
- Microscopy advances (electron microscopy) revealed detailed cell structures, showing some organisms were incorrectly grouped
- Biochemical analysis of proteins and enzymes showed evolutionary relationships not visible from appearance alone
Evolutionary Trees
Evolutionary trees (phylogenetic trees) are diagrams that show the evolutionary relationships between organisms. Closely related species share a more recent common ancestor (branches closer together). The trees are constructed using evidence from DNA sequences, protein structures, and fossil records.
| Evidence Source | How It Is Used |
|---|---|
| DNA sequences | More similar DNA = more closely related (more recent common ancestor) |
| Protein structure | Similar proteins (e.g. cytochrome c) indicate close evolutionary relationships |
| Fossil record | Fossils show when organisms existed and how they changed over time |
Comparison: Linnaeus vs Three-Domain System
| Feature | Linnaeus System | Three-Domain System |
|---|---|---|
| Developed by | Carl Linnaeus (1700s) | Carl Woese (1990s) |
| Based on | Observable characteristics (appearance, structure) | Molecular evidence (DNA, RNA, protein sequences) |
| Highest group | Kingdom (5 kingdoms) | Domain (3 domains) |
| Prokaryotes classified as | One kingdom (Monera/Prokaryota) | Two separate domains (Archaea and Bacteria) |
| Accuracy | Less accurate — some unrelated organisms grouped by appearance | More accurate — based on molecular evidence |
Practice Questions
Q1: Foundation List the seven taxonomic groups in the Linnaeus system from largest to smallest.
Q2: Foundation What is the binomial naming system? Write the binomial name for humans correctly.
Q3: Foundation Describe the three-domain system of classification. Who proposed it and what evidence was it based on?
Q4: Higher Explain why classification systems change over time.
Q5: Higher Two species share 99% of their DNA. Another species shares only 75%. Explain what this tells you about their evolutionary relationships.
Answers
- Kingdom → Phylum → Class → Order → Family → Genus → Species
- The binomial naming system gives each species a two-part Latin name: the Genus (capitalised) followed by the species (lowercase). The human binomial name is Homo sapiens (written in italics or underlined if handwritten).
- The three-domain system was proposed by Carl Woese. It classifies organisms into three domains: Archaea (primitive prokaryotes, often extreme environments), Bacteria (true bacteria), and Eukarya (eukaryotic organisms with nuclei). It was based on molecular evidence — analysis of RNA sequences showed that organisms previously grouped as “Prokaryota” actually fell into two very different groups (Archaea and Bacteria), which are as different from each other as from eukaryotes.
- Classification systems change because new evidence becomes available through advances in technology. DNA sequencing reveals previously unknown evolutionary relationships and shows that some organisms grouped by appearance are not closely related. Electron microscopy reveals detailed cell structures that show organisms are more different than previously thought. Biochemical analysis of proteins provides additional evidence for relationships. As new data accumulates, classification systems are revised to better reflect evolutionary relationships.
- The two species sharing 99% of their DNA are very closely related — they share a recent common ancestor. On an evolutionary tree, their branches would be close together. The species sharing only 75% DNA is much more distantly related — their common ancestor was much further back in time, so more mutations have accumulated. On an evolutionary tree, this species would be on a more distant branch. DNA similarity is the most reliable evidence for evolutionary relationships.
Exam Tips
- The binomial name must be written with the Genus capitalised and species lowercase, in italics: Homo sapiens. If handwritten, underline both parts. Never write just the species name alone
- Know why classification systems change: new evidence from DNA analysis, microscopy, and biochemistry. The three-domain system replaced the five-kingdom system because molecular evidence showed Archaea and Bacteria are fundamentally different
- When reading evolutionary trees, remember: branches that split closer to the present = more recent common ancestor = more closely related
- The key difference between Linnaeus and Woese is the evidence used: Linnaeus used observable characteristics; Woese used molecular (DNA/RNA) evidence
🔢 Maths Skills
Mathematical Skills
Interpreting DNA similarity data: species sharing 99% of DNA are closely related with a recent common ancestor; 75% similarity indicates a distant relationship. Percentage difference = higher similarity minus lower similarity.
⚠️ Common Misconceptions
Watch Out!
Students often think the three-domain system means the five-kingdom system was wrong. Wrong: The five-kingdom system was completely wrong Correct: It was refined based on new molecular evidence — it was a reasonable attempt with the evidence available at the time
Students often think the definition of a species is always clear-cut. Wrong: Species boundaries are always clear Correct: Hybrids (e.g. ligers from lions and tigers) and asexual reproduction in bacteria complicate the definition of a species
✍️ 6-Mark Question
Extended Answer
6 marks: Compare the Linnaean and three-domain systems and explain why classification systems change.
The Linnaean system (1700s) classifies organisms into Kingdom, Phylum, Class, Order, Family, Genus and Species, based on observable characteristics. The three-domain system (Woese, 1990s) classifies into Archaea, Bacteria and Eukarya based on molecular evidence (DNA/RNA). The key difference: the Linnaean system grouped all prokaryotes into one kingdom; the three-domain system separates them into Archaea and Bacteria because molecular evidence showed they are fundamentally different. Classification systems change because new technologies (DNA sequencing, electron microscopy, biochemical analysis) reveal evolutionary relationships not visible from appearance alone.
Mark scheme: 1 mark per system described (up to 2), 1 mark per difference (up to 2), 1 mark per reason classification changes (up to 2)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Two species of bacteria share 65% of their DNA and look very similar. One lives in hot springs at 80°C and the other lives in human intestines at 37°C. Should they be in the same domain or different domains? Which type of evidence is more reliable — molecular or morphological — and why?
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