Biology • Year 11 • Module 3 • Lesson 5
Darwin, Wallace and Natural Selection
Apply the four conditions to new scenarios, correct teleological language, and reason about MRSA and the Lamarck–Darwin distinction.
1. Apply the four conditions — beetle population
Read the scenario and answer the questions below. 8 marks
Scenario. A population of beetles lives on tree bark. Some beetles have green colouration and some have brown colouration. The tree bark is dark. Birds are the main predator. Brown beetles blend in with the dark bark and are harder for birds to spot; green beetles are more visible. The colour difference is heritable.
1.1 Identify each of the four conditions of natural selection as it applies to this beetle scenario. 4 marks
1.2 Predict what will happen to the frequency of brown beetles and green beetles over many generations. Explain your reasoning using natural selection. 2 marks
1.3 A student says: “The brown beetles decided to be darker because it helped them survive.” Explain why this description is incorrect, and rewrite it using precise natural selection language. 2 marks
2. Lamarck vs Darwin — comparison table
Complete the empty cells in the table comparing Lamarckian and Darwinian views of evolution. 6 marks — 1 per cell
| Aspect | Lamarckian view | Darwinian / natural selection view |
|---|---|---|
| Source of change | Organism changes during lifetime because of need or use/disuse | |
| Inheritance | Heritable genetic variation passed from parent to offspring | |
| Population effect | ||
| Giraffe neck example | A giraffe stretches its neck during life and passes the longer neck to offspring |
3. Cause-and-effect chain — MRSA antibiotic resistance
Complete the cause-and-effect chain by filling in each empty box. 5 marks
Step 1: Antibiotics are applied. The antibiotic acts as a _________________________ that kills more susceptible bacteria than resistant ones.
Step 2: Resistant bacteria survive and _________________________ in greater numbers.
Step 3: The resistance allele is passed to offspring, so the frequency of _________________________ increases in the population.
Outcome: Over generations, resistance becomes more common — the _________________________ has evolved. This is natural selection acting on _________________________ that already existed in the population.
4. Fix the wrong explanation
Each student explanation below contains an error in natural selection language. For each, identify the error and rewrite the explanation correctly. 6 marks
4.1 “Bacteria became resistant to penicillin because repeated exposure forced them to adapt and develop resistance genes.” 2 marks
4.2 “Giraffes evolved longer necks because they needed to reach higher leaves and kept stretching.” 2 marks
4.3 “The population of cheetahs got faster because the slowest ones wanted to improve.” 2 marks
Q1.1 — Four conditions in the beetle scenario
Variation: individuals differ in body colour (green vs brown). Heritability: the colour difference is heritable (stated in scenario). Differential survival/reproduction: brown beetles are harder to spot and caught less often; green beetles are more visible and caught more often. Selection pressure: bird predation on dark bark favours brown beetles over green ones. [1 mark each]
Q1.2 — Frequency prediction
Brown beetles will increase in frequency; green beetles will decrease in frequency [1]. Brown beetles survive and reproduce more successfully due to lower predation, passing the brown-colour allele to more offspring. Over generations, natural selection shifts allele frequencies so brown becomes more common [1].
Q1.3 — Correcting teleological language
Error: the statement implies beetles made a conscious choice to become darker (teleological/intentional language) [1]. Correction: Some beetles already had brown colouration due to heritable variation. Bird predation acted as a selection pressure; brown beetles survived more often and left more offspring. Over generations, the frequency of brown colouration increased in the population through natural selection [1].
Q2 — Lamarck vs Darwin table
- Source of change (Darwin): Variation already exists within the population; natural selection acts on that existing variation.
- Inheritance (Lamarck): Acquired traits (gained during the organism’s lifetime) are passed to offspring.
- Population effect (Lamarck): Individuals direct the change themselves.
- Population effect (Darwin): Selection shifts allele frequencies across generations; populations evolve, not individuals.
- Giraffe neck (Darwin): Giraffes already varied in neck length due to heritable differences; those with slightly longer necks fed more successfully, survived better and left more offspring; average neck length increased over many generations.
Award 1 mark per correctly completed cell (6 cells total).
Q3 — Cause-and-effect chain
- Step 1: selection pressure [1]
- Step 2: reproduce / leave more offspring [1]
- Step 3: the resistance allele [1]
- Outcome: population (bacterial population / the population); heritable variation / existing variation [1 each]
Q4.1 — Bacteria correction
Error: implies antibiotic exposure forces bacteria to develop resistance (teleological/Lamarckian) [1]. Correction: Some bacteria already carried resistance alleles due to random mutation before antibiotic treatment. Penicillin acted as a selection pressure, killing susceptible bacteria more often while resistant bacteria survived and reproduced. The resistance allele increased in frequency over generations [1].
Q4.2 — Giraffe correction
Error: implies giraffes stretched their necks because they needed to and passed that acquired length on to offspring (Lamarckian) [1]. Correction: Giraffes already varied in neck length due to heritable differences. Those with longer necks could reach more food, survived better and left more offspring. Over many generations, alleles associated with longer necks became more common through natural selection [1].
Q4.3 — Cheetah correction
Error: implies slow cheetahs intentionally decided to improve (teleological) [1]. Correction: Cheetahs varied in speed due to heritable differences. Faster individuals caught more prey and survived to reproduce more often. Over generations, alleles associated with greater speed became more frequent in the population through natural selection [1].