Read the logic as a chain of cause and effect: the longer two species have been apart, the more their sequences differ.
From one ancestor to many differences
Shared ancestor
When two species share a common ancestor, they start with identical sequences.
They split
After they split, each line accumulates its own mutations independently.
Time passes
The longer they have been separate, the more mutations pile up.
Compare today
So you find more differences when you compare their sequences today.
More time apart, more mutations, more differences.
The cause-and-effect chain
- Two species split from a common ancestor — at that moment their sequences are the same.
- After the split, mutations occur independently in each separate line.
- Mutations accumulate over time — the more time passes, the more pile up.
- So more sequence differences today = the split happened longer ago = more distantly related.
- And fewer differences = a more recent split = more closely related.
| Number of sequence differences | How long ago they diverged | How closely related |
|---|---|---|
| FEW differences | Their lines split RECENTLY (little time for mutations to build up) | CLOSELY related |
| MANY differences | Their lines split LONG AGO (lots of time for mutations to build up) | DISTANTLY related |
The molecular clock: Many mutations are neutral: they don't change how the organism survives, so natural selection neither keeps nor removes them. These neutral changes build up at a roughly constant rate over millions of years. That steady ticking is the molecular clock.
Reading the clock
- Because the rate is roughly constant, the number of differences between two species can be used to estimate the time since they shared a common ancestor.
- More differences means more time.
- It is like growth rings on a tree: count the changes, and you can read off how long the two lines have been separate.
Worked example — ranking relatives: Suppose we compare the same protein in three species against a test fish and count the amino-acid differences. Fewest differences = most recent split. The bigger the count, the further back the common ancestor.
| Species | Amino-acid differences | Relationship to the test fish |
|---|---|---|
| P | 3 | Closest relative |
| Q | 11 | Next closest |
| R | 27 | Most distant |
| Species compared with the test fish | Amino-acid differences in protein X | Interpretation |
|---|---|---|
| Species P | 3 | Fewest differences → split most recently → closest relative |
| Species Q | 11 | Intermediate → split earlier than P |
| Species R | 27 | Most differences → split longest ago → most distant relative |
Strengths: Molecular evidence is objective and quantitative (you count differences rather than judge by eye), and it works even when there are no fossils.
One big caution: The clock does not tick at the same rate for every gene or every lineage. Some genes mutate faster than others, and some lines speed up or slow down. So a molecular clock must be calibrated against the fossil record before it can give real dates.
So the clock gives estimates, never exact dates:
Why only an estimate: The rate at which differences build up is not fixed. It depends on the organism and its circumstances.
The points to remember
- Generation time: species that breed often copy their DNA more often, so they gain changes faster.
- Population size: chance changes spread differently in small and large populations.
- Selection: strong selection on a gene removes or spreads changes faster than neutral drift.
- So the clock is calibrated against dated fossils and gives a range, not one exact date.
Example: mice and elephants: Mice breed within weeks and elephants only after many years, so the same span of time leaves more sequence changes in a mouse lineage than in an elephant lineage.
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How this is tested: You may be asked to outline how molecular evidence is used to build cladograms, or to explain why more sequence differences mean an earlier divergence.
Data tasks and higher marks
- A common data task gives you a small table of sequence differences and asks you to deduce which species are most closely related (fewest differences) or to rank them.
- Higher-mark answers bring in the molecular clock: neutral mutations at a roughly constant rate → estimate divergence time.
- They also note that the clock must be calibrated against fossils.
Exam-style question — why more differences mean a longer time apart
Explain how the number of differences between the DNA base sequences of two species can be used to estimate how long ago they diverged from a common ancestor. [4]
How to score all four marks
- Same start. When the two species split from a common ancestor, their base sequences were identical.
- Independent mutation. After the split, mutations accumulate independently in each line, so the sequences gradually become different.
- More differences = more time. The more differences there are between the sequences, the longer ago they diverged (more time for mutations to build up); fewer differences means a more recent split.
- Molecular clock. Because (neutral) mutations accumulate at a roughly constant rate, the number of differences can estimate the divergence time — but the clock must be calibrated against fossils, as rates vary. (Award 1 mark per distinct point, up to 4.)
Final answer
The species started with identical sequences from a common ancestor; after the split each line accumulated mutations independently, so the number of differences grows with time. More differences therefore mean an earlier divergence, and because mutations accumulate at a roughly constant rate (the molecular clock, calibrated against fossils) the count can estimate the time since divergence.
Why this scores full marks: It gives the cause (identical start → independent mutations accumulate), states the rule (more differences = longer ago / fewer = more recent), and brings in the molecular clock with the calibration caution.
A common way to lose marks is to only state the rule ('more differences = less related') without explaining that mutations accumulate over time.