Biology · Year 11 · Module 3 · Lesson 6
HSC Exam Practice
Types of Natural Selection
Short answer
1.Short answer
Distinguish between directional, stabilising and disruptive selection. Use the effect on the phenotype distribution to help distinguish them.
Explain why industrial melanism in peppered moths is classified as directional selection rather than stabilising selection.
Explain why stabilising selection reduces phenotypic variation in a population.
Explain why disruptive selection can be a precursor to speciation, but does not automatically guarantee new species will form.
For each of the following scenarios, identify the selection type: (i) Human birth weight, where intermediate weights have the highest survival. (ii) Oyster shells, where very small and very large survive better than mid-sized. (iii) Dark-coloured beetles becoming more common on dark soil.
Data response
2.Data response — tail-length distribution in a bird population
A researcher measured tail lengths in a population of birds. The table below shows the survival rates for five tail-length categories.
| Tail-length category | % of initial population | Survival rate (%) |
|---|---|---|
| Very short | 8% | 35% |
| Short | 18% | 50% |
| Intermediate | 48% | 88% |
| Long | 18% | 52% |
| Very long | 8% | 32% |
(a) Identify the type of selection operating in this population and justify your answer using specific data from the table.
(b) Predict how the distribution of tail lengths in this population will change over many generations if selection pressures remain constant.
(c) A student argues that the data shows very long tails are “better” than very short tails because survival rates are slightly higher (32% vs 35%). Evaluate this student’s reasoning.
Extended response
3.Extended response
Assess the claim that “only directional selection is important for evolution because it is the only type that changes a population.” In your response, define all three types of selection, explain what each one does to allele frequencies and phenotypic variation, use at least one named example for each type, and reach a justified conclusion.
Biology · Year 11 · Module 3 · Lesson 6
Answer Key & Marking Guidelines
Section 1 · Short answer · 3 marks · Band 2–3
Sample response. Directional selection favours one extreme phenotype; the distribution shifts sideways and the mean moves in one direction. Stabilising selection favours the intermediate phenotype and selects against both extremes; the distribution narrows around the middle and variation decreases. Disruptive selection favours both extreme phenotypes and selects against the intermediate; the distribution can split into two peaks (bimodal) and variation increases.
Marking notes. 1 mark per correctly defined type including the effect on distribution. Accept brief descriptions; exact wording not required.
Section 1 · Short answer · 2 marks · Band 3
Sample response. Industrial melanism is directional selection because one extreme phenotype (dark colouration) was favoured on soot-darkened bark, causing the distribution of moth colouration to shift toward the dark end [1]. Stabilising selection would require the intermediate phenotype to have the highest fitness and both extremes to be selected against, but in the peppered moth case, one extreme (dark) consistently had higher fitness than both the intermediate and the other extreme (pale) [1].
Marking notes. 1 mark for identifying directional selection as one-extreme-favoured causing a shift. 1 mark for distinguishing it from stabilising selection (not intermediate-favoured; one extreme consistently best).
Section 1 · Short answer · 2 marks · Band 3
Sample response. Stabilising selection reduces variation because the intermediate phenotype has the highest fitness while both extremes are selected against [1]. As individuals with extreme phenotypes survive and reproduce less successfully, the alleles responsible for those extreme phenotypes decline in frequency over generations. The result is a narrower distribution around the mean, with fewer extreme phenotypes in the population [1].
Marking notes. 1 mark for identifying that intermediate is favoured and both extremes are selected against. 1 mark for explaining the mechanism (extreme alleles decline in frequency; distribution narrows).
Section 1 · Short answer · 2 marks · Band 3–4
Sample response. Disruptive selection can be a precursor to speciation because it creates divergence between two favoured extreme phenotypes, increasing the chance that the two groups will develop different adaptations and eventually become reproductively isolated [1]. However, reproductive isolation is also required for speciation to actually occur — disruptive selection alone creates the divergence but does not automatically stop gene flow between the two favoured extreme groups [1].
Marking notes. 1 mark for explaining how disruptive selection creates divergence between extremes. 1 mark for specifying that reproductive isolation is also required, and that disruptive selection alone does not guarantee speciation.
Section 1 · Short answer · 3 marks · Band 3
Sample response. (i) Human birth weight — stabilising selection (intermediate weight has highest survival; extremes selected against). (ii) Oyster shells — disruptive selection (both extreme sizes survive better than intermediate). (iii) Dark beetles — directional selection (one extreme, dark colouration, favoured; distribution shifts).
Marking notes. 1 mark each. All three must match the correct type for full marks.
Section 2 · Data response · 2 marks · Band 4
Sample response. Stabilising selection is operating [1]. The intermediate tail-length category has the highest survival rate (88%), while both extremes have the lowest survival (very short 35%; very long 32%). Both extremes are selected against and the intermediate phenotype is favoured — the defining pattern of stabilising selection [1].
Marking notes. 1 mark for correctly identifying stabilising selection. 1 mark for using specific survival data (intermediate = highest; both extremes = lowest) as justification.
Section 2 · Data response · 3 marks · Band 4–5
Sample response. Over many generations, the frequency of intermediate tail lengths will increase and the frequency of very short and very long tail lengths will decrease [1]. Because both extreme phenotypes have lower survival, individuals carrying extreme-length alleles will reproduce less and those alleles will decline in frequency. The distribution will become narrower around the intermediate phenotype [1]. The mean tail length will remain approximately constant (intermediate), but variation will decrease as the extremes are progressively removed from the population [1].
Marking notes. 1 mark for predicting increase in intermediate + decrease in extremes. 1 mark for explaining why (lower survival → fewer offspring → extreme alleles decline). 1 mark for noting distribution will narrow around intermediate while mean stays approximately the same.
Section 2 · Data response · 2 marks · Band 4–5
Sample response. The student’s reasoning is flawed [1]. Both very short (35%) and very long (32%) tail lengths have very low survival compared to the intermediate (88%). The tiny difference between 32% and 35% is negligible compared to the overall pattern; both extremes are strongly selected against by this data. Describing one extreme as “better” focuses on a minor difference within the disfavoured extreme category while ignoring the key finding: the intermediate phenotype is overwhelmingly more fit than either extreme. Both extremes are losing out to the intermediate, which is the hallmark of stabilising selection [1].
Marking notes. 1 mark for identifying the student’s reasoning as flawed. 1 mark for a clear explanation focusing on both extremes being strongly disfavoured relative to the intermediate, making the small difference between extremes irrelevant to the pattern of selection.
Section 3 · Extended response · 9 marks · Band 5–6
Sample response. The claim that only directional selection is important for evolution is incorrect. All three selection types change allele frequencies and therefore all three drive evolution.
Directional selection occurs when one extreme phenotype has the highest fitness. The distribution shifts toward that extreme and the mean moves. Alleles associated with the favoured extreme increase in frequency while alleles associated with other phenotypes decrease. Industrial melanism in peppered moths is a classic example: pollution darkened tree bark, favouring dark moths, and dark colouration alleles increased in frequency while pale alleles declined.
Stabilising selection occurs when the intermediate phenotype has the highest fitness and both extremes are selected against. The distribution narrows around the mean. Alleles associated with extreme phenotypes decline in frequency over generations, reducing phenotypic variation. Human birth weight is the classic example: both very low and very high birth weight carry higher risks, while intermediate weights have the best survival. Stabilising selection actively eliminates alleles at both extremes — this is evolutionary change even though the mean stays approximately constant.
Disruptive selection occurs when both extreme phenotypes have higher fitness than the intermediate. The distribution can split into two peaks and variation increases. Oysters provide the example: very small and very large oysters are better protected from predation than intermediate-sized ones, so both extreme phenotypes increase in frequency while the intermediate declines.
All three types change allele frequencies — the standard definition of evolution. Directional selection changes the mean; stabilising selection changes the frequency of extreme alleles (reducing them) and narrows variation; disruptive selection changes the frequency of intermediate alleles (reducing them) and can increase divergence. Claiming only directional selection “changes” a population confuses the direction of change with the occurrence of change.
The claim is therefore false. Stabilising and disruptive selection drive evolutionary change just as directional selection does, through different mechanisms and toward different population outcomes. Depending on environmental conditions, any of the three types may be dominant at a given time.
Marking criteria (9 marks).
- 1 mark — Explicit evaluative judgement: the claim is incorrect.
- 1 mark — Directional selection correctly defined (one extreme favoured; mean shifts) + named example.
- 1 mark — Directional selection’s effect on allele frequencies explained.
- 1 mark — Stabilising selection correctly defined (intermediate favoured; extremes selected against; narrower distribution) + named example.
- 1 mark — Stabilising selection changes allele frequencies (extreme alleles decline) — is evolutionary change.
- 1 mark — Disruptive selection correctly defined (both extremes favoured; intermediate against; bimodal) + named example.
- 1 mark — Disruptive selection’s effect on allele frequencies and variation explained.
- 1 mark — Explains why the claim is flawed: all three types change allele frequencies; evolution is defined as change in allele frequency, not just directional mean shift.
- 1 mark — Synthesised conclusion using precise lesson vocabulary throughout.