Biology • Year 11 • Module 3 • Lesson 14
Molecular Evidence
Apply sequence comparison logic, cytochrome c data, and the strengths and limits of molecular versus morphological evidence to real-world data and scenarios.
1. Interpret cytochrome c amino acid difference data
The table below shows the number of amino acid differences in cytochrome c between human and five other species. More amino acid differences indicate greater evolutionary distance from humans. 7 marks
| Species compared to human | Amino acid differences in cytochrome c |
|---|---|
| Chimpanzee (Pan troglodytes) | 0 |
| Rhesus monkey (Macaca mulatta) | 1 |
| Horse (Equus caballus) | 12 |
| Tuna (Thunnus thynnus) | 21 |
| Baker’s yeast (Saccharomyces cerevisiae) | 44 |
Data derived from: Margoliash, E. (1963). Proceedings of the National Academy of Sciences, 50(4), 672–679.
1.1 Which species is most closely related to humans according to this data? Justify your answer. 2 marks
1.2 Explain, using the concept of mutation accumulation, why tuna shows more differences from humans than a horse does. 2 marks
1.3 Yeast is a single-celled fungus yet still shares a working cytochrome c with humans. Explain why this is valid evidence for common ancestry of all aerobic life. 3 marks
2. Cause-and-effect chain — when molecular evidence outperforms morphology
Complete the chain below. 5 marks
Cause 1: Two unrelated species evolve in similar environments with similar selective pressures.
Effect 1 / Cause 2:
Cause 3: A scientist uses only physical appearance to classify these two species and places them in the same taxonomic group.
Effect 3 / Cause 4:
Cause 5: A second scientist compares the DNA sequences of the two species using BLAST.
Effect 5 (Overall outcome):
3. Case study — DNA barcoding exposes seafood fraud
Read the scenario, then answer the questions. 7 marks
Scenario. A consumer advocacy group purchased 60 samples of fish labelled as “red snapper” from restaurants and supermarkets across Australia. Each sample underwent DNA barcoding using the mitochondrial CO1 gene. Results showed that only 11 of the 60 samples (18%) were actually red snapper (Lutjanus campechanus). The remaining 49 samples contained 13 different species, several of which were cheaper fish that are visually similar once filleted. Morphological identification of a skinless fillet is extremely difficult even for trained seafood inspectors.
3.1 Explain why morphological identification failed in this situation, using what you know about the limits of that method. 2 marks
3.2 Explain why DNA barcoding was successful where morphology failed. Include in your answer which specific region of DNA was used and why that region is suitable. 3 marks
3.3 Predict one limitation that DNA barcoding might face in this kind of investigation. Justify using lesson content. 2 marks
Q1.1 — Most closely related species (2 marks)
The chimpanzee is most closely related to humans [1]. Its cytochrome c amino acid sequence is identical to that of humans (0 differences), indicating they share a very recent common ancestor and the lineages have had the least time to accumulate mutational differences [1].
Q1.2 — Why tuna shows more differences (2 marks)
The lineage leading to tuna diverged from the lineage leading to humans much earlier than the horse lineage did [1]. Over the longer time since the human–fish split, more mutations have accumulated independently in both lineages, resulting in more total amino acid differences in the shared cytochrome c protein by the time we compare them today [1].
Q1.3 — Yeast sharing cytochrome c as common ancestry evidence (3 marks)
Yeast and humans share a functional cytochrome c despite being separated by an enormous evolutionary distance [1]. This shared molecule could not have arisen independently in both lineages — a functional cytochrome c performing the same essential role in cellular respiration implies that this molecule was already present in a common ancestor from which both lineages descended [1]. The fact that both lineages retained a working version of the gene across billions of years of evolution demonstrates deep universal common ancestry for all aerobic life [1].
Q2 — Cause-and-effect chain (sample answers)
Effect 1 / Cause 2: Both species independently evolve similar structural features that suit that environment (convergent evolution produces superficially similar appearances in unrelated organisms).
Effect 3 / Cause 4: The classification is incorrect — the two species are placed in the same group even though they are not closely related. Their shared appearance reflects convergent adaptation to similar pressures, not common ancestry.
Effect 5 (Overall outcome): The DNA comparison reveals that the two species have very different sequences, showing they diverged from a common ancestor a long time ago. The molecular evidence overturns the morphological classification and gives a more accurate picture of the true evolutionary relationship.
Q3.1 — Why morphology failed (2 marks)
Once a fish has been filleted and the skin, scales and distinctive anatomical features removed, the visible characteristics used for morphological identification are no longer present [1]. Many fish species have very similar flesh colour, texture and fat content once processed, so even trained inspectors cannot distinguish them reliably from appearance alone — this is a recognised limitation of morphological identification when specimens are incomplete or processed [1].
Q3.2 — Why DNA barcoding succeeded (3 marks)
DNA barcoding works because a species-specific DNA sequence is preserved in the tissue regardless of how the specimen has been processed [1]. The CO1 (cytochrome c oxidase 1) gene from mitochondrial DNA was used — it is located in the mitochondria, which are present in abundance in muscle tissue [1]. CO1 is a short, standardised region that is consistent within species but different enough between species to act as a reliable identifier when matched against the barcode reference library [1].
Q3.3 — One limitation (2 marks)
A valid limitation is that DNA barcoding depends on a reliable, comprehensive reference database [1]. If a species is not yet catalogued in the barcode library — for example a rare or recently described species — the barcode cannot be matched and identification fails. Also, highly processed samples (e.g. heavily heat-treated) may yield degraded DNA that cannot produce a usable barcode sequence [1].