Selection and Evolution: Question 9
Syllabus 17.3
A research group is investigating the evolutionary relationships between a reference bird species, R, and four other bird species (A, B, C and D) thought to share a common ancestor with species R. The group sequences the same respiratory transport protein from all five species and counts how many amino acid positions differ from species R.
| Species | Amino acid differences from species R |
|---|---|
| A | 2 |
| B | 15 |
| C | 4 |
| D | 27 |
(a) Using the data in the table, state which of species A to D is most closely related to species R, and which is least closely related. Explain your reasoning. [3]
(b) Explain what is meant by the term "molecular clock", and outline how biologists could use amino acid or DNA sequence differences, together with a molecular clock, to estimate how long ago two species diverged from a common ancestor. [3]
(c) Explain one advantage of using molecular (DNA or amino acid sequence) evidence, rather than relying only on comparing the outward physical (morphological) features of species, when investigating evolutionary relationships. [2]
Show worked solution Hide worked solution
Worked solution
Part (a): Interpreting the amino acid difference data
The number of amino acid differences between a species and the reference species R reflects how much independent evolutionary change has accumulated since they last shared a common ancestor. Fewer differences mean less time (or fewer mutations) has passed since divergence, so the species are more closely related; more differences mean more time (or more mutations) has passed, so the species are more distantly related.
From the table:
- Species A has the fewest differences (2), so it is the most closely related to R.
- Species D has the most differences (27), so it is the least closely related to R.
Part (b): The molecular clock
The molecular clock is based on the assumption that, within a particular gene or protein, mutations accumulate at a roughly steady average rate over long periods of evolutionary time. This rate is first calibrated using pairs of species (or fossil evidence) whose divergence time is already known from another source, such as the fossil record.
Once calibrated, the clock allows the number of sequence differences observed between any two species (for the same gene/protein) to be converted into an estimated divergence time: since the rate of accumulation is assumed constant, a species showing roughly twice as many differences from R as another species is estimated to have diverged from R roughly twice as long ago. In this way, sequence data alone can be used to estimate how long ago different species shared a common ancestor.
Part (c): Advantage of molecular evidence over morphology alone
Comparing only outward physical features can be misleading, because convergent evolution can cause unrelated species to independently evolve similar-looking structures or features if they face similar environmental pressures or ways of life. This can make distantly related species appear deceptively similar, or closely related species that have adapted differently appear deceptively different.
DNA and amino acid sequences, by contrast, directly reflect inherited genetic material passed down from a common ancestor, so they are much less affected by this kind of environmental convergence. Molecular comparisons therefore give more objective, quantifiable evidence of true evolutionary relationships, and are especially useful when species look very similar (or very different) despite their actual degree of relatedness being the opposite.
Final answers
- (a) Species A is most closely related to R (fewest differences); species D is least closely related (most differences), fewer amino acid differences indicate a more recent common ancestor.
- (b) The molecular clock: sequence differences accumulate at a roughly constant, calibrated rate, so the number of differences between species can be used to estimate their divergence time.
- (c) Molecular evidence avoids being misled by convergent evolution, which can make morphology-based comparisons unreliable, and gives more direct, quantifiable evidence of shared ancestry.