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Characteristics and Classification of Living Organisms: Question 4

Syllabus 1.2

Structured Extended 6 marks

Scientists compare DNA samples from three species of wild dog: the grey wolf, the coyote and the black-backed jackal. For each pair of species, they calculate the percentage similarity between the sequences of bases in their DNA. Their results are:

  • Grey wolf and coyote: 96% similarity
  • Grey wolf and black-backed jackal: 89% similarity
  • Coyote and black-backed jackal: 88% similarity

(a) Using these results, identify which two of the three species are the most closely related. [1]

(b) Explain how the concept of DNA base sequence similarity supports your answer to (a). [2]

(c) State one aim of classification systems that this DNA evidence supports. [1]

(d) The grey wolf has the binomial name Canis lupus, and the coyote has the binomial name Canis latrans. State what this shared genus name suggests about the relationship between the two species, and link your answer to the DNA evidence given above. [2]

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Worked solution

The three percentage similarities given are 96% (grey wolf and coyote), 89% (grey wolf and black-backed jackal) and 88% (coyote and black-backed jackal). The highest of these is 96%, between the grey wolf and the coyote.

Part (b): Linking similarity to relatedness

Groups of organisms that share a more recent common ancestor have DNA base sequences that are more similar to one another than the sequences of organisms that share only a distant common ancestor. Since the grey wolf and coyote have the highest percentage DNA base sequence similarity (96%) of the three pairs compared, this evidence indicates that they share a more recent common ancestor than either species shares with the black-backed jackal, making them the most closely related pair.

Part (c): Aim of classification systems

This DNA evidence supports the aim that classification systems try to group organisms so that the groupings reflect their evolutionary relationships, placing more closely related organisms (those with a more recent common ancestor) closer together in the classification.

Part (d): Connecting the binomial names to the DNA evidence

In “Canis lupus” and “Canis latrans”, the shared first word, Canis, is the genus. Both the grey wolf and the coyote being placed in the same genus suggests that they are closely related species. This matches the DNA evidence from parts (a) and (b): their DNA base sequences are the most similar of the three species compared, which is consistent with them sharing a more recent common ancestor than either does with the black-backed jackal.

Final answers

  • (a) The grey wolf and the coyote are the most closely related pair.
  • (b) A more recent common ancestor gives more similar DNA base sequences; the grey wolf and coyote have the highest similarity (96%), so they are the most closely related.
  • (c) Classification systems aim to reflect the evolutionary relationships between organisms.
  • (d) The shared genus, Canis, suggests a close relationship, consistent with the high (96%) DNA base sequence similarity between the grey wolf and the coyote.