Explain why scientists compare the base sequences of DNA from different species when classifying organisms, and describe what a high percentage of shared base sequences suggests about two species.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists can now analyse the base sequences of DNA from different organisms. This information is used alongside physical and behavioural characteristics when classifying species and investigating biodiversity.
Model answer (5 marks)
DNA base sequences carry the genetic information that determines an organism’s characteristics. By comparing the order of bases in the DNA of different species, scientists can see how similar the genetic material is. Species that are closely related share a larger proportion of identical base sequences, whereas species that are more distantly related share fewer identical bases. DNA comparison is more reliable and objective than using only physical traits, because convergent evolution can make unrelated species look alike. A high percentage of shared base sequences indicates that the two species have a recent common ancestor. Over time, mutations accumulate and the sequences diverge, so species that split earlier have fewer shared bases.
Examiner tips
- Use the word ‘DNA base sequences’ and explain they carry genetic information; link similarity to relatedness; mention reliability over morphology; state high similarity = recent common ancestor; note mutation causes divergence over time
Common mistakes
- Confusing DNA similarity with physical similarity; forgetting to mention convergent evolution; not linking high similarity to recent common ancestor; using vague terms like ‘similar’ without specifying genetic bases
Mark scheme (5 marks)
- DNA base sequences (or the order of bases) carry genetic information / determine characteristics
- Closely related species share more similar / more base sequences in common
- DNA evidence is more reliable / objective than using physical characteristics alone (because physical features can be misleading / convergent evolution can make unrelated species look similar)
- A high percentage of shared base sequences suggests the two species share a recent common ancestor
- Over time, mutations cause base sequences to change / diverge, so species that separated longer ago have fewer shared sequences
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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