Explain selection and genetic drift
Compare non-random selection with chance changes in allele frequencies, including founder and bottleneck effects.
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
Why can drift have a strong effect in small populations?
What you will learn
- Explain how selection and genetic drift affect allele frequencies
- Describe the founder effect and bottleneck effect as forms of genetic drift
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Recorded group | Olive 18; bronze 12 |
| Sample total | 30 beetles |
| Olive proportion | 18/30 = 60% |
| Evidence limit | Sample phenotype count; not a genotype census |
Build the evolutionary explanation
Natural selection changes allele frequencies because individuals with particular phenotypes leave different numbers of offspring. Genetic drift changes frequencies by chance, especially in small populations. A founder effect occurs when a small group establishes a new population with a non-representative sample of the source alleles. A bottleneck occurs when population size falls sharply, leaving a chance sample of prior variation. Both can reduce genetic variation and shift allele frequencies regardless of whether alleles are advantageous.
Interpret data with care
Drift is random with respect to fitness; selection is associated with differential reproductive success. A small population can experience both processes at once.
Pause and explain
A population falls sharply after habitat destruction. Explain how later allele frequencies may change even after population size recovers.
Worked example
A storm randomly leaves 12 survivors from a much larger population. What process may alter allele frequencies?
Show the worked solution
- Survival is a chance sample in a severe reduction.
- A bottleneck can shift frequencies by drift.
Answer A bottleneck effect: the small survivor group may have allele frequencies different from the original population by chance.
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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.
Distinguish selection from genetic drift in a small island population.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- Selection links a phenotype to reproductive success.
- Drift is chance change in allele frequency.
Selection occurs when phenotype differences cause unequal reproductive success. Drift is a chance change in allele frequencies, which can be strong in a small island population.
Look for these in your work
- Population process identified
- Cause and consequence linked
A few individuals colonise an island and carry an unusual allele frequency. Name and explain the effect.
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- The new population starts from a small sample.
- Its allele frequencies may not represent the source population.
Founder effect. The new population was established by a small, unrepresentative sample of the source alleles, so its allele frequencies can differ by chance.
Look for these in your work
- Evidence interpreted
- Assumptions stated
A population falls sharply after habitat destruction. Explain how later allele frequencies may change even after population size recovers.
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- The survivors carry a chance subset of previous variation.
- Their descendants inherit that reduced sample.
The bottleneck leaves a chance sample of the original alleles. Even if population size later increases, descendants may retain shifted frequencies and reduced genetic variation.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Treating drift as adaptation to the environment
- Assuming a bottleneck always removes only harmful alleles
What can you explain now?
Why can drift have a strong effect in small populations?
Compare with the explanation
Each random event affects a larger fraction of individuals
Chance sampling causes larger proportional frequency changes.
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Make it stick
How do founder and bottleneck effects differ in their causes?
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