Use DNA sequences to infer evolutionary relationships
Compare DNA sequence similarities and differences as evidence of shared ancestry.
Section 17.3 · Evolution
A Level; Cambridge International Biology 9700, 2028–2030. Written answers and calculations are self-checked; this is not fieldwork or practical-skills certification.
Before you begin
- Populations contain inherited variation, and alleles can be transmitted from parents to offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does lower sequence difference generally support?
What you will learn
- Explain how DNA-sequence data can show evolutionary relationships
- Interpret sequence similarity while distinguishing evidence from certainty
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Reference | AGCTTAGC |
| Same-length comparison | AGCTTAAC: position 7 G → A |
| Stated insertion | AGCTATAGC: nine bases |
| Missing context | Protein function, chromosome number and transmission |
Build the evolutionary explanation
DNA sequences can be aligned and compared between species. More similar sequences are generally consistent with a more recent common ancestor, while accumulated substitutions can indicate divergence over time. Multiple genes or independent evidence can strengthen an inference. Sequence differences should be interpreted in context, including mutation rates, gene function and sampling.
Interpret data with care
Similarity is evidence of relationship, not a complete timeline or proof from a single sequence. Use the supplied data, identify the measure compared and avoid assuming that every nucleotide change accumulates at a constant rate.
Pause and explain
An alignment shows species X and Y share fewer differences than either shares with Z. State a cautious conclusion.
Worked example
Species A and B differ at 4 bases in a sequence; A and C differ at 22. Which pair is more similar in this sequence?
Show the worked solution
- Compare the number of differences.
- Fewer differences mean greater sequence similarity.
Answer A and B are more similar in the supplied sequence.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why closely related species may have more similar DNA sequences.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- They share a more recent common ancestor.
- Fewer substitutions may have accumulated since divergence.
Closely related species generally share a more recent common ancestor, so fewer sequence differences may have accumulated since their lineages separated.
Look for these in your work
- Population process identified
- Cause and consequence linked
A single short sequence differs by one base between two species. Why is that not enough to establish the full evolutionary tree?
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- A short sequence gives limited evidence.
- Other genes and evidence can strengthen or revise the inference.
One short sequence may not represent the whole genome or lineage history; use longer alignments, multiple genes and independent evidence before inferring a robust evolutionary relationship.
Look for these in your work
- Evidence interpreted
- Assumptions stated
An alignment shows species X and Y share fewer differences than either shares with Z. State a cautious conclusion.
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- Use the pattern in the supplied sequence.
- Do not assert direct ancestry.
The supplied sequence supports a closer relationship between X and Y than either has with Z, but it does not show that one extant species is the direct ancestor of another.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Treating sequence similarity as proof of direct ancestry
- Ignoring sequence length or alignment quality
What can you explain now?
What does lower sequence difference generally support?
Compare with the explanation
Greater similarity and a closer inferred relationship
It is consistent with more recent shared ancestry, subject to evidence limits.
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Make it stick
How can sequence differences inform, but not prove, evolutionary relationships?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.