A group of people whose ancestors immigrated to North America 200 years ago have certain allele frequencies that differ significantly from those in surrounding populations in the United States. Which of the following is the most likely explanation for the difference in allele frequencies?

Question

BlackTom AI · Accepted Answer

Reproductive isolation reduces or prevents gene flow between this immigrant group and surrounding populations, allowing allele frequencies to diverge over generations.

1. Relevant knowledge points and concepts
- Gene flow: movement of alleles between populations tends to homogenize allele frequencies.
- Reproductive isolation: behavioral, cultural, geographic, or social barriers that limit interbreeding and thus gene flow.
- Genetic drift and founder effect: when a small founding population establishes a new population, chance can lead to different allele frequencies; drift is strongest in small, isolated groups.
- Other concepts listed in the choices (sex linkage, gametic isolation, independent assortment, mutation) have more specific or different implications and do not best explain a broad, population-level difference after immigration.

2. Step-by-step reasoning
- Ancestors arrived 200 years ago (roughly 6–8 human generations). If the group remained largely endogamous (married within the group), gene flow with the surrounding population would be restricted.
- Restricted gene flow plus a likely small founding population allows genetic drift and founder effects to shift allele frequencies away from the surrounding population.
- Over successive generations, these effects produce statistically significant differences in allele frequencies.

3. Intermediate logical steps (no numerical calculations required)
- Initial small founder population → sampling of only some alleles from source population.
- Limited intermarriage → continued isolation → little or no influx of new alleles.
- Genetic drift amplifies frequency differences over generations → distinct allele profile now.

4. Why the correct answer is correct
Reproductive isolation directly explains how allele frequencies can remain different: it prevents or limits gene flow that would otherwise equalize allele frequencies. Founder effects and genetic drift operate within the context of isolation; thus reproductive isolation is the primary mechanism.

5. Why other options are incorrect
- Sex linkage: refers to genes on sex chromosomes; it cannot account for broad, population-wide differences across many loci.
- Gametic isolation: a prezygotic barrier between species that prevents fertilization; not relevant within a human population.
- Independent assortment: a principle of segregation at meiosis affecting individual inheritance patterns, not a cause of population divergence on its own.
- Mutation: mutations are rare and unlikely to produce pronounced allele-frequency differences across a whole group in only ~200 years.

6. Conclusion
Because limited interbreeding (reproductive isolation) best explains persistent, population-level allele frequency differences after immigration, the correct answer is Reproductive isolation. This matches the provided answer.

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