HSC Exam Practice

Sample response. Adaptive radiation is the diversification of one ancestral species into many species, each adapted to a different ecological niche or food source.

Marking notes. 1 mark for linking one ancestral species diversifying into many adapted to different niches/food sources.

Sample response. Adaptive radiation is one ancestral species diversifying into many, each adapted to a different niche, for example the Galapagos finches. Convergent evolution is unrelated species independently evolving similar traits due to similar selection pressures, for example the thylacine (marsupial) and the wolf (placental). The key difference is that adaptive radiation starts from a recent common ancestor and diverges, while convergent evolution involves unrelated lineages becoming similar.

Marking notes. 1 mark for the distinction (one ancestor diverging vs unrelated lineages converging); 1 mark for a valid finch/radiation example; 1 mark for a valid convergent example.

Sample response. (1) Heritable variation exists within the population; (2) resources are limited, so there is a struggle for survival; (3) individuals with advantageous traits are more likely to survive and reproduce (differential survival).

Marking notes. 1 mark for any two conditions stated correctly; 2 marks for all three.

Sample response. During the voyage Darwin was simply a collector and did not recognise the birds as a single related group; the ornithologist John Gould later identified them as 13 distinct finch species all from one ancestor [1]. This shows the theory was built slowly from many lines of evidence analysed after the voyage, not from a single “eureka” moment at the Galapagos [1].

Marking notes. 1 mark for noting Gould identified the finches after the voyage; 1 mark for the point that the theory developed gradually from evidence, not instantly.

Sample response. After the 1977 drought, small soft seeds disappeared and mainly large, hard seeds remained [1]. Larger-beaked finches could crack these seeds, so they survived and reproduced better (differential survival), while smaller-beaked birds died [1]. Because beak size is heritable, the next generation had a measurably larger average beak size, a population change observed within one generation, which directly demonstrates natural selection in action [1].

Marking notes. 1 mark for the change in food/seed availability; 1 mark for differential survival of larger-beaked birds; 1 mark for the heritable, measurable shift in the next generation linked to natural selection.

Sample response. The thylacine is a marsupial and the wolf is a placental mammal; these lineages are not closely related and diverged long ago [1]. Their similar body forms arose independently because both occupied a similar predatory niche and faced similar selection pressures, so the similarity is convergent evolution, not the result of a recent common ancestor [1].

Marking notes. 1 mark for identifying the two as unrelated lineages (marsupial vs placental); 1 mark for explaining the similarity arose independently under similar selection pressures.

Sample response. They are secondary sources because we are studying observations collected and reported by others (Darwin, the Grants) rather than collecting the data ourselves [1]. A feature making them strong evidence is the large sample size and the multiple independent examples (many finch species; several marsupial/placental pairs) that point to the same conclusion [1].

Marking notes. 1 mark for a valid reason they are secondary sources; 1 mark for a valid strength (reliable collection, large sample, multiple corroborating examples, patterns not easily explained otherwise).

Sample response. The Galapagos finch data represents adaptive radiation (divergent evolution from a common ancestor), while the Australian marsupial fauna represents convergent evolution (unrelated lineages independently producing similar body forms).

The finch data is strong evidence because it includes quantitative, directly observable data, the Grant study measured beak-size shifts after the 1977 drought, there is a clear ancestral species, and the selection pressures (island food sources) are identifiable. A limitation is that the long-term historical selection pressures are inferred after the fact rather than observed directly.

The marsupial data is strong because there are multiple independent pairs (thylacine/wolf, numbat/anteater, sugar glider/flying squirrel) all showing the same pattern, and Australia's geographic isolation about 45 million years ago is well documented, making independent evolution plausible. A limitation is that convergent morphology alone is not proof of natural selection, so it needs to be combined with other evidence.

Together, these two independent lines, one showing divergence from a common ancestor under different pressures, the other showing convergence in unrelated lineages under similar pressures, both point to the same mechanism: selection acting on heritable variation to match phenotype to environment.

In conclusion, multiple independent lines of evidence pointing to the same conclusion greatly strengthen the case for evolution compared with a single line, because each line offsets the limitations of the other.

Marking criteria.

• 1 mark Identifies finches as adaptive radiation / divergent evolution.
• 1 mark Identifies Australian fauna as convergent evolution.
• 1 mark States a strength of the finch data (quantitative, Grant study, clear ancestor).
• 1 mark States a limitation of the finch data.
• 1 mark States a strength of the marsupial data (multiple pairs, documented isolation).
• 1 mark States a limitation of the marsupial data.
• 1 mark Explains how the two lines complement each other (same mechanism).
• 1 mark Reaches a justified conclusion that multiple independent lines strengthen the case, using precise vocabulary.