HSC Exam Practice

Genetic divergence is the accumulation of genetic differences between two populations or lineages after they stop or reduce interbreeding [1]. Over time, mutations arise independently in each lineage — because these mutations are not shared between the separated groups, sequence differences build up — the longer the separation, the greater the number of differences [1].

mtDNA is maternally inherited (not recombined from two parents), while nuclear DNA is inherited from both parents through meiosis and recombination [1]. mtDNA also tends to mutate faster than much nuclear DNA [1]. These two characteristics make it useful for tracing lineages: the maternal inheritance means it passes largely unchanged down the female line (making lineage tracing clearer), and the faster mutation rate means differences accumulate more quickly, making it useful for distinguishing populations that diverged relatively recently [1].

DNA barcoding is a technique that identifies species using a short, standardised DNA sequence (commonly the CO1 gene from mitochondrial DNA) matched against a reference library [1]. Two situations where preferred: (1) when the specimen is damaged, processed (e.g. filleted fish) or incomplete so that morphological features are absent [1]; (2) when organisms are at an immature or larval stage that looks very different from the adult, or when two species are so morphologically similar that visual identification is unreliable [1].

Cytochrome c is found in all aerobic organisms and performs the same essential role in cellular respiration in all of them [1]. Because it is universal, its amino acid sequence can be compared across a vast range of species — from bacteria to primates — making comparisons possible even between extremely distantly related lineages [1]. Additionally, because its function is so fundamental, changes in its sequence that disrupt function are selected against, meaning the sequence changes relatively slowly — making it suitable for revealing deep ancestry that shorter-timescale markers might miss [1].

Two species can look similar but have large DNA differences if their similarity resulted from convergent evolution — independent adaptation to similar selective pressures producing similar physical features without shared recent ancestry [1]. In this case the physical appearance reflects the environment and selection pressures rather than genetic relatedness; the DNA sequences accumulated mutations independently over a long period since the two lineages diverged from a common ancestor [1].

(i) 2 marks. As evolutionary distance increases, the number of amino acid differences in cytochrome c also increases [1]. For example, chimpanzee shows 0 differences (most similar) while baker’s yeast shows 44 differences (most distant); horse shows 12 differences and tuna shows 21, reflecting intermediate evolutionary distances [1].

(ii) 3 marks. Chimpanzees and humans share a very recent common ancestor (approximately 6–7 million years ago) so very few mutations have had time to accumulate in the shared cytochrome c sequence; the sequences remain identical [1]. Baker’s yeast and humans share a common ancestor far deeper in evolutionary history — billions of years ago — and since that divergence, vast numbers of mutations have accumulated independently in both lineages, resulting in 44 amino acid differences [1]. The greater the time since common ancestry, the more opportunities for independent mutations to accumulate [1].

(iii) 2 marks. Conclusion: humans and chimpanzees share a very recent common ancestor, reflected in their identical cytochrome c sequences [1]. Limitation: cytochrome c is only one protein and the molecular clock can be unreliable; conclusions drawn from a single protein may not fully represent the overall genetic relationship, and should ideally be corroborated by comparing additional genes or other molecular tools such as BLAST or full-genome comparison [1].

Marking criteria (1 mark each):

• Identifies the claim as an overstatement and states an evaluative position (not always more reliable).
• Names at least one molecular tool and explains its strength (e.g. cytochrome c — universal, allows broad comparison; DNA barcoding — works when morphology is absent; BLAST — database comparison).
• Explains a strength of morphological evidence (e.g. accessible without specialised equipment, captures structural and functional information).
• Explains a limitation of molecular evidence (e.g. unreliable molecular clock, incomplete barcode databases, requires specialised equipment and interpretation).
• Correctly applies convergent evolution to explain a situation where molecular evidence is more reliable (similar-looking but unrelated organisms).
• Reaches an explicit evaluative conclusion: neither is universally superior; the most reliable conclusions use both, with molecular evidence especially valuable when morphology is misleading.
• Quality mark: coherent, precise terminology, logical argument.