Population Genetics (College Board AP® Biology): Exam Questions

A small population of fish (Lacustris variabilis) inhabits a series of connected freshwater lakes.

• Occasionally, storms allow fish to migrate between lakes.

• Random natural disasters, like droughts, periodically reduce fish numbers drastically.

• A new mutation arose in one lake that affects fin size.

Researchers tracked allele frequencies of two fin-shape alleles (B and b) over 20 generations. The graph below shows the frequency changes of the two alleles.

Identify the evolutionary mechanism responsible for the initial random changes in allele frequencies after a drought drastically reduced the fish population size.

Explain how migration between lakes could disrupt Hardy-Weinberg equilibrium and alter the trends shown in the graph.

Predict what would happen to the frequency of the B allele if a new beneficial mutation occurred in the population.

Explain why small populations like these lake fish are more likely to show significant allele frequency changes compared to large populations.

A species of finch (Fringilla insulae) lives on a chain of small, isolated islands in the Pacific Ocean.

• Each island hosts a small population of finches

• Finches from the mainland occasionally migrate to the islands

• On the islands, some random events (like hurricanes) drastically reduce finch populations, causing genetic drift

• A new mutation appeared in one island population, leading to a slightly different beak shape

• Researchers collected data on the frequency of the B allele (a beak shape gene), where:

  • B = broad beak (advantageous for cracking seeds)

  • b = narrow beak (neutral trait)

Scientists want to determine whether the island populations are in Hardy-Weinberg equilibrium over time, based on observed allele frequencies. Table 1 shows some data collected about the allele frequencies of the B and b alleles in the population of finches.

Table 1. Allele frequencies for beak shape in Island finch population

(i) Describe how genetic drift and migration can alter allele frequencies in these island populations.

(ii) Explain how a new mutation in the beak-shape gene could affect genetic variation.

(i) Identify two conditions that must be met for a population to maintain Hardy-Weinberg equilibrium.

(ii) Justify why small island populations are unlikely to meet these conditions.

(i) Use data from Table 1 to calculate the frequency of the BB genotype at Generation 10, assuming Hardy-Weinberg conditions.

(ii) Determine whether the population is likely evolving, based on the change in allele frequencies over 10 generations.

(i) Predict how ongoing migration from the mainland would affect the genetic diversity of the island populations over time.

(ii) Justify your prediction in part (i).

A researcher was investigating the genetic diversity in four different breeds of cat. 

They calculated the mean genetic diversity for each breed. The value of this mean was somewhere between 0 and 1. A mean value of 1 indicated maximum genetic diversity, while a mean value of 0 indicated no genetic diversity.

The results are shown in Table 1 below.

Describe what the data in Table 1 shows about the differences in genetic diversity between the British shorthair and Oriental shorthair breeds.

Describe what the data in Table 1 shows about the differences in genetic diversity between the American shorthair and Exotic shorthair breeds.

Predict how the introduction of a new desirable mutation would affect the mean genetic diversity of the British shorthair population over time.

Explain how artificial selection might account for the observed differences in genetic diversity in cats.