Biology • Year 11 • Module 3 • Lesson 6
Types of Natural Selection
Lock in the three selection types, what each does to a bell-curve distribution, the key graph clues, and the named examples before moving to application tasks.
1. Complete the paragraph
Fill each blank with the correct term from the word bank. Use each term once only. 8 marks
Three main types of natural selection describe different patterns of change in phenotype distributions. _______________ selection favours one _______________ phenotype, causing the distribution to shift sideways and the population _______________ to move in one direction. _______________ selection favours the _______________ phenotype and selects against both extremes, causing the distribution to become narrower and _______________ in the population to decrease. _______________ selection favours both extreme phenotypes and acts against the middle, which can split the distribution and increase variation, potentially contributing to _______________ if the two groups become reproductively isolated.
2. Term–definition match
Write the matching term from this list in the right-hand column: stabilising selection • directional selection • disruptive selection • phenotypic variation • selection pressure • fitness landscape • intermediate phenotype • bimodal distribution. 8 marks
| # | Definition | Matching term |
|---|---|---|
| 2.1 | Selection that favours the intermediate phenotype and reduces the frequency of both extremes. | |
| 2.2 | Selection that favours one extreme phenotype, causing the distribution to shift in one direction. | |
| 2.3 | Selection that favours both extreme phenotypes and acts against the intermediate phenotype. | |
| 2.4 | Observable differences in traits within a population. | |
| 2.5 | An environmental factor that affects survival or reproduction, favouring some phenotypes over others. | |
| 2.6 | A model showing the relationship between genotype and fitness. | |
| 2.7 | A phenotype lying in the middle of the distribution range, neither extreme. | |
| 2.8 | A distribution with two peaks, produced when both extreme phenotypes are favoured. |
3. Match the graph pattern to the selection type
For each bell-curve change described, write the selection type and a matching real-world example from the lesson. 9 marks — 1 per cell
| Bell-curve change | Selection type | Named example from lesson |
|---|---|---|
| The whole distribution shifts sideways toward one extreme phenotype | ||
| The distribution becomes narrower around the middle; variation decreases | ||
| The middle declines; two peaks form at the extremes (bimodal pattern) |
4. True or false — with correction
Circle T or F. If the statement is false, write the corrected version on the line below. 8 marks — 1 for T/F, 1 for correction where needed
4.1 Stabilising selection narrows the phenotype distribution by selecting against the extremes. T / F
4.2 Industrial melanism in peppered moths is a classic example of stabilising selection. T / F
4.3 Disruptive selection favours both extreme phenotypes and acts against the intermediate phenotype, increasing variation. T / F
4.4 Disruptive selection always leads directly to the formation of two new species. T / F
Q1 — Cloze paragraph
In order: Directional • extreme • mean • Stabilising • intermediate • variation • Disruptive • speciation.
Q2 — Term–definition matches
2.1 stabilising selection • 2.2 directional selection • 2.3 disruptive selection • 2.4 phenotypic variation • 2.5 selection pressure • 2.6 fitness landscape • 2.7 intermediate phenotype • 2.8 bimodal distribution.
Q3 — Graph patterns table
- Shift sideways → directional selection → Peppered moths (industrial melanism) or antibiotic resistance.
- Narrower middle → stabilising selection → Human birth weight or egg size in birds.
- Split peaks (bimodal) → disruptive selection → Oyster size (very small and very large survive better than mid-sized).
Award 1 mark per cell. Accept other valid examples where explicitly stated in the lesson.
Q4 — True/False with correction
4.1 True.
4.2 False. Correction: Industrial melanism in peppered moths is a classic example of directional selection — darker moths were favoured in polluted environments, shifting the distribution toward one extreme.
4.3 True.
4.4 False. Correction: Disruptive selection can be a precursor to speciation by creating divergence between favoured extremes, but speciation only occurs if the two groups also become reproductively isolated — disruptive selection alone does not automatically guarantee that new species form.