QUESTION 89 (5 marks)
A conserved sequence accumulates substitutions at an average rate sites per million years in each of two lineages. The lineages differ at 12 sites. Assume a constant rate, independent accumulation, and no repeated changes at the same site. Use .
a) Estimate their divergence time. [2 marks]
b) Explain why the model includes a factor of 2. [1 marks]
c) Explain how a different mutation rate could affect a divergence estimate if it were not accounted for. [2 marks]
Practice marking scheme
Answer
a) 15 million years. b) Both lineages accumulate substitutions. c) A faster true rate would cause an overestimate when a slower rate is assumed, and vice versa.
Working
million years. Pairwise difference reflects changes accumulating along both descendant lineages. For a fixed difference count, the estimated time is inversely related to the assumed rate: if the actual rate is faster than the assumed value, the same number of differences accumulates in less time and the model overestimates age.
Marking criteria
- Rearranges/substitutes into t = d/(2r). [1 mark]
- Calculates 15 million years. [1 mark]
- Explains accumulation in both lineages. [1 mark]
- Identifies mutation-rate variation as a limitation. [1 mark]
- Explains an appropriate direction of estimation error. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.