In a small island population of birds, a rare genetic mutation causes some individuals to have unusually long tail feathers. Over several generations, this trait becomes much more common in the island population than in the mainland population. Which of the following best explains this change in allele frequency

Question

BlackTom AI · Accepted Answer

This question asks us to explain why a rare genetic mutation for long tail feathers becomes more common on the island over generations. Since the options provided are unavailable, I will first discuss the core idea and then outline why genetic drift fits best conceptually.

In small populations, allele frequencies can change from one generation to the next simply due to random sampling of alleles when individuals reproduce. This random fluctuation is known as genetic drift. On an isolated island, the bird population is relatively small, so chance events (e.g., which individuals happen to mate and reproduce) can disproportionately increase or decrease the frequency of the long-tail allele. Over many generations, such random changes can lead to the allele becoming more common than in the mainland population, even without any selective advantage.

Discounting other forces helps reinforce the drift explanation:
- Natural selection would require the long-tail allele to confer a reproductive or survival advantage, which is not stated here; if that were the case, the increase could be due to positive selection rather than drift.
- Gene flow (migration) from the mainland would typically introduce mainland alleles and slow the divergence between island and mainland populations, so migration is unlikely to explain the island-specific rise in allele frequency.
- Mutation alone would introduce new alleles at a very low rate and would not, by itself, explain a rapid increase in frequency of an already rare allele across generations.

Additionally, if the island experienced a bottleneck or founder effect, the allele frequency could shift dramatically due to the small subset of individuals founding the population, further amplifying drift's impact.

In sum, the scenario aligns with genetic drift: a small, relatively isolated population undergoing random, non-selective changes in allele frequencies over generations, leading to a higher prevalence of the long-tail mutation compared to the mainland.

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