Biology • Year 11 • Module 3 • Lesson 6
Types of Natural Selection
Apply the three selection types to data and scenarios, interpret before-and-after distributions, and reason about the connection between disruptive selection and speciation.
1. Classify the selection scenarios
For each scenario, identify the selection type (directional, stabilising or disruptive) and justify your answer. 9 marks — 1 for type, 2 for justification
Scenario 1. In a population of marine snails, very small snails are eaten by small fish that can swallow them whole. Very large snails are too heavy to be attacked by most predators. Intermediate snails are the most frequently eaten. Over time, the population develops more very small and very large snails, and fewer intermediate ones.
Selection type: ___________________________
Justification:
Scenario 2. In a population of Arctic foxes, foxes with extremely thick or extremely thin fur coats both have lower survival than those with intermediate coat thickness. Foxes with intermediate thickness are best insulated for the typical winter temperature range.
Selection type: ___________________________
Justification:
Scenario 3. Industrial pollution darkens tree bark in a forest. Pale moths become easier for birds to spot against the dark bark, while dark moths blend in better. Dark moths survive and reproduce at higher rates, and the moth population gradually shifts toward darker colouration.
Selection type: ___________________________
Justification:
2. Interpret a human birth weight data set
The table shows the birth weight distribution and infant mortality rate for a large population sample. Use the data to answer the questions. 6 marks
| Birth weight category | % of births in this category | Relative infant mortality (%) |
|---|---|---|
| Very low (<2 kg) | 5% | High (22%) |
| Low (2–2.5 kg) | 9% | Moderate-high (8%) |
| Intermediate (2.5–4 kg) | 72% | Lowest (1.5%) |
| High (4–5 kg) | 12% | Moderate (5%) |
| Very high (>5 kg) | 2% | High (18%) |
2.1 Identify the selection type shown by this data and explain your reasoning using specific values from the table. 2 marks
2.2 What would you predict would happen to the distribution of birth weights in this population over many generations? 2 marks
2.3 A student argues that this selection type will eventually eliminate all variation in birth weight until every baby is born at exactly the same weight. Evaluate this claim. 2 marks
3. Cause-and-effect: disruptive selection and speciation
Complete the cause-and-effect chain. 5 marks
Step 1: Both extreme phenotypes (very small and very large) have higher _________________________ than intermediate phenotypes.
Step 2: This is _________________________ selection. The distribution of shell sizes becomes _________________________ (two peaks).
Step 3: If the two groups (very small and very large) also develop _________________________, gene flow between them would stop.
Outcome: _________________________ could potentially result, producing two distinct lineages from one ancestral population.
Q1 — Scenario classifications
Scenario 1: Disruptive selection. Both extreme phenotypes (very small and very large snails) have higher survival than intermediate phenotypes. The intermediate phenotype is selected against. This is the defining pattern of disruptive selection, and is consistent with the oyster-size example in the lesson [1 for type; 2 for justification citing both extremes favoured and intermediate selected against].
Scenario 2: Stabilising selection. The intermediate coat thickness has the highest survival. Both extremes (very thick and very thin) are selected against. This matches the stabilising-selection pattern — the distribution will narrow around the optimal intermediate, similar to the human birth weight example [1 for type; 2 for justification].
Scenario 3: Directional selection. One extreme phenotype (dark colouration) has higher fitness under the current selection pressure (pollution-darkened bark). The distribution shifts sideways toward the darker extreme over time. This is the industrial melanism example from the lesson [1 for type; 2 for justification].
Q2.1 — Selection type and data
Stabilising selection [1]. The intermediate birth weight category (2.5–4 kg) has the highest frequency (72% of births) and the lowest mortality (1.5%), while both extremes (<2 kg and >5 kg) have mortality rates of 18–22%. Both extremes are selected against, and the intermediate phenotype has the highest fitness — the defining pattern of stabilising selection [1].
Q2.2 — Prediction for birth weight distribution
Over many generations, the distribution of birth weights will become narrower around the intermediate range [1]. Very low and very high birth weights will decline in frequency because those phenotypes have higher mortality, while intermediate weights will make up an increasing proportion of births [1].
Q2.3 — Evaluating the student’s claim
The claim is incorrect [1]. Stabilising selection reduces variation by removing the extremes, but it does not eliminate all variation — it narrows the distribution around the optimal intermediate phenotype and maintains variation within the selected range. Complete uniformity is extremely unlikely because new variation is continuously introduced through mutation, recombination and environmental fluctuation. The lesson’s misconceptions box notes that stabilising selection maintains variation in the population rather than eliminating it entirely [1].
Q3 — Cause-and-effect chain
- Step 1: fitness [1]
- Step 2: disruptive; bimodal [1]
- Step 3: reproductive isolation [1]
- Outcome: speciation [1]
Note: Step 2 requires two correctly filled blanks for 1 mark.