Biology • Year 11 • Module 3 • Lesson 4
Darwin's Observations and the Galapagos
Apply adaptive radiation and convergent evolution to data, match finch beaks to food sources, and reason about Darwin's evidence.
1. Complete the Galapagos finch table
Use the lesson content to complete the empty cells, matching each finch to its beak shape and food source. 8 marks, 1 per correct cell
| Finch species | Beak shape | Main food source |
|---|---|---|
| Large ground finch | ||
| Warbler finch | Long, thin beak | |
| Cactus finch | Cactus flowers | |
| Vegetarian finch | Parrot-like beak |
2. Analyse the Australian fauna data
Use the information below to answer the questions. 8 marks
Scenario. Australia became geographically isolated about 45 million years ago, leaving its marsupial ancestors to evolve independently. The result is a set of marsupials that closely resemble unrelated placental mammals on other continents: the thylacine resembles the wolf, the numbat resembles the anteater, and the sugar glider resembles the flying squirrel. The marsupial and placental lineages diverged more than 80 million years ago.
2.1 Name the evolutionary process that explains why unrelated marsupials and placental mammals look so similar. 1 mark
2.2 Explain why the similarity between the thylacine and the wolf is best explained by this process rather than by a recent common ancestor. 3 marks
2.3 Identify the kind of factor (a selection pressure) most likely responsible for producing similar body forms in unrelated lineages. 2 marks
2.4 State whether this is the same evolutionary process as the Galapagos finches, and briefly justify your answer. 2 marks
3. Cause-and-effect chain, how finch beaks diversified
Complete the cause-and-effect chain by filling in each empty box. The first box is done for you. 5 marks
Step 1: Each island has a different dominant food source, so each island population faces a different _________________________.
Step 2: Within each population there is _________________________ in beak shape that is heritable.
Step 3: On each island, finches with beaks suited to the local food are more likely to _________________________ and reproduce (differential survival).
Outcome: Over many _________________________, the islands develop distinct finch species, an example of _________________________.
4. Applied scenario, reading the Grant finch data
Peter and Rosemary Grant measured finches on Daphne Major. After the 1977 drought, most small soft seeds were gone and mainly large, hard seeds remained. The next generation of finches had a measurably larger average beak size. 4 marks
4.1 Identify the selection pressure created by the 1977 drought, and the trait it favoured. 2 marks
4.2 A student concludes: “The drought made individual finches grow bigger beaks.” Explain why this conclusion is wrong, and give the correct explanation. 2 marks
Q1, Galapagos finch table (marking criteria)
- Large ground finch: Deep, thick beak; food = hard seeds (crushes them).
- Warbler finch: long, thin beak; food = insects (probes for them in bark).
- Cactus finch: Curved, medium beak; food = cactus flowers.
- Vegetarian finch: parrot-like beak; food = buds and fruit (crushes them).
Award 1 mark per correctly completed cell.
Q2.1, Evolutionary process
Convergent evolution [1].
Q2.2, Why convergent rather than common ancestry
The thylacine is a marsupial and the wolf is a placental mammal, and these lineages diverged more than 80 million years ago, so they are not closely related [1]. Australia was isolated for about 45 million years, so the thylacine evolved independently [1]. Their similar body form arose because both faced similar selection pressures (a predatory hunting niche) and independently evolved similar adaptations, not because they inherited the form from a recent shared ancestor [1].
Q2.3, Selection pressure
Similar environmental selection pressures, that is, similar ecological niches or ways of life (e.g. hunting prey, eating ants and termites, gliding between trees) [1], which favour similar adaptations in unrelated lineages [1].
Q2.4, Same process as the finches?
No, it is not the same process [1]. The Galapagos finches show adaptive radiation (divergent evolution): one ancestor diversifying into many species. The Australian fauna shows convergent evolution: unrelated lineages independently evolving similar forms [1].
Q3, Cause-and-effect chain (marking criteria)
- Step 1: selection pressure [1]
- Step 2: variation [1]
- Step 3: survive [1]
- Outcome: generations [1]; adaptive radiation [1]
Q4.1, Selection pressure and favoured trait
The selection pressure is the drought, which removed small soft seeds and left mainly large, hard seeds (a change in food availability) [1]. The trait it favoured was a larger, stronger beak able to crack the remaining hard seeds [1].
Q4.2, Why the student is wrong
Individual finches do not grow bigger beaks in response to the drought [1]. Variation in beak size already existed in the population; larger-beaked birds survived better on the hard seeds (differential survival) and reproduced, so the average beak size of the next generation was larger. The population changed over a generation, the individuals did not change their own beaks [1].