Biology • Year 11 • Module 3 • Lesson 9
Convergent, Divergent & Punctuated Equilibrium
Apply the convergent/divergent distinction to examples, reason about analogous and homologous structures, and interpret the two models of evolutionary pace.
1. Complete the evolution classification table
The table below lists four examples. Complete each empty cell using the lesson content. 8 marks, 1 per correct cell
| Example | Convergent or divergent? | Analogous or homologous structures? |
|---|---|---|
| Forelimbs of human, whale and bat (same bones, different functions) | ||
| Streamlined bodies of sharks and dolphins | ||
| Darwin's finches with different beak shapes from one ancestor | ||
| Wings of a butterfly and the wings of a sparrow |
2. Analyse a convergent evolution claim
Use the information below to answer the questions. 8 marks
Scenario. Marsupial moles live in arid Australia, while placental (true) moles live in Europe, Asia and North America. The two groups belong to completely different mammal lineages and do not share a recent common ancestor. Yet both have cylindrical bodies, reduced eyes, powerful digging forelimbs and a streamlined snout for burrowing. Both live underground and feed on soil invertebrates.
2.1 State whether the similarities between marsupial and placental moles are an example of convergent or divergent evolution, and justify your choice. 2 marks
2.2 Are the digging forelimbs of the two moles analogous or homologous structures? Explain your reasoning. 2 marks
2.3 Identify the shared selection pressure most likely responsible for driving both groups toward these similar features. 2 marks
2.4 Name one other example of convergent evolution from the lesson and state why the structures involved are described as analogous. 2 marks
3. Cause-and-effect chain, how adaptive radiation produces many species
Complete the cause-and-effect chain by filling in each empty box. The first box is done for you. 5 marks
Step 1: Different populations of the ancestor spread into different niches and face _________________________ selection pressures (different foods, predators, conditions).
Step 2: In each niche, natural selection favours _________________________ traits (for example, different mouth or jaw shapes for different foods).
Step 3: Over many generations the populations become increasingly _________________________ from one another, eventually becoming separate _________________________.
Outcome: One ancestor produces about 500 species; this rapid divergence into many niches is called _________________________.
4. Applied scenario, reading the pace of change
Read each scenario and decide which model of evolutionary pace it best supports. Explain your reasoning. 4 marks
4.1 A fossil lineage shows almost no change for 8 million years, then a new related form appears within about 100,000 years. 2 marks
4.2 A second fossil lineage shows small, steady changes accumulating continuously across the same time period, with no long periods of stasis. 2 marks
Q1, Classification table (marking criteria)
- Human/whale/bat forelimbs: Divergent; homologous structures (same origin, different functions).
- Shark and dolphin streamlined bodies: Convergent; analogous structures (different origins, similar function).
- Darwin's finch beaks: Divergent (adaptive radiation); homologous structures (one ancestor, modified beaks).
- Butterfly and sparrow wings: Convergent; analogous structures (flight, different origins).
Award 1 mark per correctly completed cell.
Q2.1, Convergent or divergent
This is convergent evolution [1]. The two moles are unrelated (they belong to different mammal lineages with no recent common ancestor) yet they have independently evolved similar features, which is the defining pattern of convergence [1].
Q2.2, Analogous or homologous
The digging forelimbs are analogous structures [1] because they have similar function (digging) but different evolutionary origins, they arose independently in two separate lineages rather than being inherited from a recent common ancestor [1].
Q2.3, Selection pressure
The shared selection pressure is the demands of a burrowing, underground (fossorial) lifestyle [1], which favours cylindrical bodies, reduced eyes and powerful digging forelimbs for moving efficiently through soil and feeding on soil invertebrates [1].
Q2.4, Other example
Accept: sharks (fish) and dolphins (mammals) with streamlined bodies; or the wings of birds, bats and insects [1]. These are analogous because the structures serve a similar function (fast swimming or flight) but evolved independently from different origins, so they reflect similar selection pressures rather than shared recent ancestry [1].
Q3, Cause-and-effect chain (marking criteria)
- Step 1: different [1]
- Step 2: different / advantageous / locally adaptive [1]
- Step 3: different (divergent); species [1]
- Outcome: adaptive radiation [1]
Q4.1, Long stasis then rapid change
This supports punctuated equilibrium [1] because the lineage shows a long period of little change (stasis) followed by a short, rapid burst of change in which a new form appears, which is the characteristic pattern of punctuated equilibrium [1].
Q4.2, Slow steady change
This supports gradualism [1] because the changes are small, steady and continuous over a long period, with no long periods of stasis, matching the gradualist model of slow, continuous change [1].