IB Biology · Theme A: Unity and diversity · HL only
A3.2 Classification and cladistics
A one-page summary of A3.2 Classification and cladistics, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What tools are used to classify organisms into taxonomic groups?
How do cladistic methods differ from traditional taxonomic methods?
What A3.2 covers
Why and how we classify
- A3.2.1Why we need to classify
- A3.2.2The traditional hierarchy
- A3.2.2Where the hierarchy breaks down
- A3.2.3Classifying by evolution
Clades and cladograms
- A3.2.4Clades: shared ancestry
- A3.2.5The molecular clock
- A3.2.6Building cladograms from sequences
- A3.2.6Choosing a tree: parsimony
- A3.2.7Analysing cladograms
Testing and changing classifications
- A3.2.8Reclassifying the figwort family
- A3.2.9Three domains
A3.2 Classification and cladistics: summary
Why classify
- Immense diversity of species.
- Classification makes further study possible.
The hierarchy
- Kingdom → phylum → class → order → family → genus → species.
- Ranks are arbitrary and do not always match evolution.
Clades
- Ancestor plus all descendants; traits predictable.
- Best evidence: base or amino acid sequences.
Molecular clock
- Differences build up gradually → divergence times.
- Estimates only: rates vary with generation time, population size, selection.
Cladograms
- Root, node (hypothetical ancestor), terminal branch.
- Parsimony: the tree with fewest changes.
Reclassification
- Figwort family split: convergent flowers, not one clade.
- Three domains from rRNA (Woese and Fox, 1977).
Key terms
- Taxon
- a group in a classification, such as a family or genus.
- Clade
- a common ancestor and all of its descendants.
- Cladogram
- a branching diagram of the probable sequence of divergence.
- Node
- a branch point: a hypothetical common ancestor.
- Molecular clock
- estimating divergence times from accumulated sequence differences.
- Parsimony
- choosing the tree that needs the fewest sequence changes.
- Convergent evolution
- similar features evolving separately in unrelated groups.
- Domain
- the highest level: Bacteria, Archaea or Eukaryota.
Sample exam questions
Three of the 28 multiple-choice questions for A3.2. Try each one before opening the answer.
Question 1. Why do biologists use a classification system for living organisms?
- It increases the number of species in a habitat
- It lets organisms be named, grouped and studied
- It changes the physical features of organisms
- It removes the need for scientific names
Show the answer
Answer: B. A shared classification system lets scientists worldwide identify, name and organise the huge diversity of life, and when it is natural it also reflects how organisms are related.
Question 2. The introduction of the three domains was based mainly on differences in the base sequences of:
- messenger RNA only
- the whole nuclear genome
- ribosomal RNA (rRNA)
- transfer RNA molecules
Show the answer
Answer: C. Comparisons of small-subunit rRNA base sequences revealed that Archaea and Bacteria are as different from each other as each is from eukaryotes, leading to the three domains.
Question 3. Living organisms are classified into three domains: Archaea, Bacteria and Eukaryota. Which feature may be found in cells of all three domains?
- a membrane-bound nucleus
- many membrane-bound organelles
- a cell wall
- DNA never bound to proteins
Show the answer
Answer: C. Cell walls occur in members of all three domains — in bacteria, in archaea, and in plant, fungal and some protist eukaryotes — so a cell wall does not distinguish one domain from another. Their chemistry differs, but the structure is present in all three.
Linking questions
Questions that connect A3.2 to other parts of the course, the kind that come up in Paper 2.
- How do differences in DNA base sequences, used to build cladograms, arise in the first place? (see A1.2, D1.3)
- How does classification based on cladistics provide evidence for evolution by natural selection? (see A4.1)
- How does the biological definition of a species relate to placing organisms into a genus and higher taxa? (see A3.1)
- How does the shared structure of ribosomes and rRNA across all cells support the three-domain classification? (see A2.2, A4.1)
Practise A3.2
Study notes, every question and full markschemes for A3.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.