Compare patterns of natural selection
Describe directional, stabilising and disruptive selection as different effects on phenotype distributions.
Section 17.2 · Natural and artificial selection
A Level; Cambridge International Biology 9700, 2028–2030. Written answers and calculations are self-checked; this is not fieldwork or practical-skills certification.
Before you begin
- Populations contain inherited variation, and alleles can be transmitted from parents to offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which selection pattern favours intermediate phenotypes?
What you will learn
- Explain directional, stabilising and disruptive selection
- Relate each pattern to which phenotypes have higher reproductive success
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Generation 1 | 20% stated phenotype |
| Generation 2 | 35% |
| Generation 3 | 52% |
| Interpretation | Trend only; inheritance and reproductive evidence needed |
Build the evolutionary explanation
Directional selection favours one extreme phenotype and shifts the population distribution. Stabilising selection favours intermediate phenotypes and selects against extremes, reducing variation around the mean. Disruptive selection favours both extremes over intermediate phenotypes and can split a distribution. In each case, environmental conditions affect relative reproductive success; allele frequencies may change if phenotype differences are inherited.
Interpret data with care
A graph shows phenotype frequencies, not necessarily allele frequencies directly. Describe which phenotypes are favoured before predicting how the distribution changes.
Pause and explain
A graph shows the mean trait value moving toward one extreme after a new predator appears. Name the pattern and explain the graph.
Worked example
After an environmental shift, one extreme body size has highest survival. Name the selection pattern.
Show the worked solution
- One extreme is favoured.
- The distribution is expected to shift toward that extreme.
Answer Directional selection.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A stable environment favours average clutch size over very small or large clutches. Identify the pattern.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- Intermediate phenotypes have higher reproductive success.
- Extremes are selected against.
Stabilising selection.
Look for these in your work
- Population process identified
- Cause and consequence linked
A food source is available at either end of a size range but not in the middle. Which phenotype pattern may be favoured?
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- Both extremes have an advantage.
- Intermediate forms are disadvantaged.
Disruptive selection may favour both extremes over intermediate phenotypes.
Look for these in your work
- Evidence interpreted
- Assumptions stated
A graph shows the mean trait value moving toward one extreme after a new predator appears. Name the pattern and explain the graph.
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- One extreme is favoured by the new condition.
- The distribution shifts in that direction.
Directional selection: the predator favours one extreme phenotype, so the population distribution shifts toward that extreme if the trait is heritable.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Calling any reduced range directional selection
- Confusing stabilising selection with no selection
What can you explain now?
Which selection pattern favours intermediate phenotypes?
Compare with the explanation
Stabilising selection
It tends to reduce the frequency of extreme phenotypes.
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Make it stick
Which phenotype classes are favoured by the three selection patterns?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.