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A-Level Biology May/June 2025 Q2: When organisms reproduce, they pass on their alleles to the next generation. There are…
A-Level Biology · Paper 9700/42 · May/June 2025 · Question 2 · [6 marks]
When organisms reproduce, they pass on their alleles to the next generation. There are many factors that can affect how allele frequencies change over time in a population. Explain how genetic drift and the founder effect may affect allele frequencies in populations.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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Genetic drift refers to the change in allele frequencies within a population over generations due to random chance events, and it is not caused by selection pressure. The effects of genetic drift are much more significant in a small population.
The founder effect is a specific example of genetic drift. This occurs when a small number of individuals become isolated or migrate to start a new population. The gene pool of this new founding population is often not representative of the original population's gene pool, as the founders may, by chance, have a different set of allele frequencies. This process often leads to a reduction in genetic variation (a smaller gene pool) in the newly founded population compared to the original source population.
How the marks are awarded
- MP 1 — Awarded for stating that the change in allele frequencies is due to 'random chance'.
- MP 2 — Awarded for correctly stating that genetic drift is 'not caused by selection pressure', distinguishing it from natural selection.
- MP 7 — Awarded for explaining that the effect is more significant in a 'small population'.
- MP 8 — Awarded for describing the founder effect scenario: a 'small number of individuals become isolated or migrate'.
- MP 9 — Awarded for explaining that the new population's gene pool is 'not representative of the original population'.
- MP 5 — Awarded for stating that the outcome is a 'reduction in genetic variation' or a 'smaller gene pool'.
Common mistakes
- Confusing genetic drift with natural selection by incorrectly stating that alleles are lost because they are 'less fit' or disadvantageous.
- Failing to mention the crucial role of population size, i.e., that drift has a much greater impact in small populations.
- Mixing up the founder effect (migration of a small group) with the bottleneck effect (a drastic reduction in the size of a single population).
- Giving a vague answer, such as 'alleles are lost', without explaining that this happens by chance and results in a non-representative gene pool.
Examiner tip: When explaining related concepts like genetic drift and the founder effect, first define the overarching principle, then describe the specific example, explicitly stating how it illustrates that principle.
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