Darwin's Observations and the Galapagos

Q1, Sample Band 6 response (7 marks), annotated

All 13 to 15 Galapagos finch species descended from a single South American ancestor that colonised the islands; this diversification of one species into many is called adaptive radiation. [1, one ancestor / adaptive radiation]

Each island had a different dominant food source (large hard seeds, cactus flowers, bark insects, buds and fruit), so a different selection pressure operated on the finch population on each island. [1, different food = different selection pressure]

Within each population there was heritable variation in beak shape. Finches whose beak shape was suited to the available food were more likely to feed successfully, survive and reproduce, while those with poorly suited beaks were less likely to, this is differential survival. [1, heritable variation in beak shape] [1, differential survival of suited beaks]

Because beak shape is heritable, the survivors passed their beak traits to offspring, so over many generations the frequency of beak shapes matched to each island's food increased, until distinct species with distinctive beaks were established. [1, beak shape becomes matched to food over generations]

Peter and Rosemary Grant's long-term study confirmed this mechanism operates in real time: after the 1977 drought, larger, harder seeds dominated, larger-beaked finches survived better, and average beak size measurably increased within a single generation. [1, Grant study as supporting evidence] This directly observable change strongly supports Darwin's theory that selection pressures acting on heritable variation produce adaptation over time. [1, links the evidence back to the theory]

Marking criteria.

• 1 mark States all finch species descended from one ancestor (adaptive radiation).
• 1 mark Explains different island food sources create different selection pressures.
• 1 mark Identifies heritable variation in beak shape.
• 1 mark Describes differential survival of finches with suited beaks.
• 1 mark States beak shape becomes matched to food over generations.
• 1 mark Uses the Grant study (1977 drought, measurable beak-size shift) as supporting evidence.
• 1 mark Explicitly links the evidence back to natural selection.

Q2, Sample Band 6 response (8 marks), annotated

The Galapagos finch data represents adaptive radiation, that is, divergent evolution from a common ancestor under different selection pressures. [1, finches = adaptive radiation] The Australian marsupial fauna represents convergent evolution, that is, unrelated lineages independently producing similar body forms under similar pressures. [1, marsupials = convergent evolution]

A strength of the finch data is that it is quantitative and directly observable: the Grant study measured real-time beak-size shifts, there is a clear ancestral species, and the selection pressures (island food sources) are identifiable. [1, strength of finch data] A limitation is that the historical selection pressures must be inferred after the fact; the long-term divergence was not observed directly. [1, limitation of finch data]

A strength of the marsupial data is that there are multiple independent pairs (thylacine/wolf, numbat/anteater, sugar glider/flying squirrel) all showing the same pattern, and Australia's geographic isolation is well documented, making independent evolution plausible. [1, strength of marsupial data] A limitation is that convergent morphology alone is not proof of natural selection; other mechanisms (e.g. developmental constraints) could in principle produce similar forms, so additional evidence is needed. [1, limitation of marsupial data]

The two lines complement each other: one shows divergence from a common ancestor under different pressures, the other shows convergence in unrelated lineages under similar pressures. Both point to the same underlying mechanism, selection acting on heritable variation to match phenotype to environment. [1, explains how the two lines complement each other]

In conclusion, multiple independent lines of evidence pointing to the same mechanism greatly strengthen the case for evolution by natural selection compared with relying on a single line, because each line guards against the limitations of the other. [1, justified conclusion]

Q3, Sample Band 6 response (6 marks)

The claim is flawed: the long delay before publication is a sign of careful evidence-building, not of weak evidence. [1, overall evaluative judgement]

What is flawed: the claim assumes that taking a long time means the scientist was unsure and the evidence was weak. In fact, Darwin used the time to collect about 5,000 specimens, keep detailed field notebooks, and compare specimens across many locations, building a large, varied body of evidence. [1, identifies the false assumption and Darwin's actual data collection]

Strong scientific evidence requires reliable collection methods, large sample sizes, and patterns that cannot easily be explained by other hypotheses. By waiting, Darwin assembled multiple independent lines of evidence (e.g. Galapagos finches, Australian fauna, comparative anatomy, biogeography) that all pointed to the same conclusion, which is far more convincing than a single rushed observation. [1, explains the criteria for strong evidence] [1, links the delay to multiple corroborating lines being stronger]

It is also wrong to assume a faster theory is automatically more convincing; speed says nothing about the quality or quantity of the supporting data. [1, rejects the “faster is more convincing” reasoning]

Defensible reformulation: “Darwin's 28-year delay before publishing reflects careful, thorough evidence-building. Strong scientific evidence comes from reliable collection, large samples, and multiple independent lines pointing to the same conclusion, not from how quickly a theory is published.” [1, accurate reformulation]

Marking criteria.

• 1 mark States an overall evaluative judgement (the claim is flawed).
• 1 mark Identifies the false assumption and notes Darwin's actual extensive data collection.
• 1 mark Explains the criteria for strong scientific evidence.
• 1 mark Links the delay to assembling multiple corroborating independent lines.
• 1 mark Rejects the idea that a faster theory is automatically more convincing.
• 1 mark Reformulates the claim into an accurate statement about how strong evidence is built.