A3.2.5 (HL)—Molecular clock
Molecular clocks estimate divergence time from accumulated DNA or amino acid substitutions, assuming calibrated rates of sequence change over lineages.
- Syllabus
- First assessment 2025
- Objective
- A3.2.5
- Level
- HL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceNovember 2023
Most common paperPaper3
Typical marks1
Common command terms
- Outline
- State
Scoring notes
- Common mistake
- Saying only that the relationship is linear without stating that substitutions increase with divergence time.
Recent exam appearances
November 2023Paper3 ["HL"] · TZ211(a)[ 1 ]A3.2.5 (HL)—Molecular clock
Practice this objective
Coverage 2023–2023 · Updated 15 Jul 2026
Concept essentials
- Sequence differences can be counted to estimate time since divergence.
- More substitutions usually indicate a longer divergence time.
- Molecular clocks require calibration because rates can vary.
- DNA and amino acid sequence data can both be used in molecular-clock reasoning.