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Quiz: Population Genetics and Hardy-Weinberg Equilibrium

Test your understanding of allele frequencies, the Hardy-Weinberg model, evolutionary forces, and macroevolution with these review questions.

1. Which of the following is one of the five conditions that must hold for a population to remain in Hardy-Weinberg equilibrium?

  1. Directional selection must favor heterozygotes
  2. The population must be very large, so that random fluctuations in allele frequency (genetic drift) do not occur
  3. Mutation must continually introduce new alleles at a high rate
  4. Individuals must migrate freely between populations
Show Answer

The correct answer is B. Hardy-Weinberg equilibrium requires a very large population size so that chance alone does not meaningfully alter allele frequencies from one generation to the next. The other four assumptions are no mutation, random mating, no natural selection, and no gene flow — the opposite of options A, C, and D, each of which describes a condition that would actually cause a population to evolve away from equilibrium.

Concept Tested: Hardy-Weinberg Assumptions

2. If a real population is found to violate the random mating assumption of Hardy-Weinberg equilibrium, what is the most direct expected consequence?

  1. Allele frequencies stay the same, but genotype frequencies deviate from the p²+2pq+q² proportions expected under random mating
  2. The population's total gene pool size changes immediately
  3. New mutations arise at an accelerated rate
  4. Genetic drift becomes impossible in that population
Show Answer

The correct answer is A. Non-random mating, such as assortative mating where similar phenotypes preferentially mate, changes how alleles are combined into genotypes without necessarily changing the underlying allele frequencies themselves. This shifts the observed genotype proportions away from the p², 2pq, and q² values predicted under random mating, even though p and q may remain unchanged in that generation.

Concept Tested: Hardy-Weinberg Equilibrium

3. Which statement correctly distinguishes genetic drift from natural selection?

  1. Genetic drift produces adaptive changes in allele frequency, while natural selection produces random changes
  2. Both genetic drift and natural selection act only in large populations
  3. Genetic drift and natural selection are two names for the same underlying mechanism
  4. Genetic drift is random and unrelated to fitness, while natural selection consistently favors alleles that increase survival and reproduction; drift's effects are strongest in small populations
Show Answer

The correct answer is D. Genetic drift changes allele frequencies purely by chance, regardless of whether an allele helps or hurts an organism's fitness, and its effects are magnified in small populations where random sampling has a larger proportional impact. Natural selection, in contrast, is a non-random process that consistently favors alleles associated with higher survival and reproductive success, and it can act in populations of any size.

Concept Tested: Genetic Drift

4. In a population of 2,000 diploid individuals, 180 individuals express a recessive phenotype caused by a single gene. Assuming the population is in Hardy-Weinberg equilibrium, approximately how many individuals are heterozygous carriers?

  1. 180
  2. 420
  3. 840
  4. 1,260
Show Answer

The correct answer is C. The recessive phenotype frequency is \(q^2 = 180/2000 = 0.09\), so \(q = \sqrt{0.09} = 0.30\) and \(p = 1 - 0.30 = 0.70\). The heterozygote frequency is \(2pq = 2(0.70)(0.30) = 0.42\). Multiplying by the population size gives \(0.42 \times 2000 = 840\) heterozygous carriers, far more than the 180 individuals who actually show the recessive phenotype.

Concept Tested: Hardy-Weinberg Equilibrium (Quantitative Application)

5. Researchers measure clutch size in a bird population before and after a drought. Birds with intermediate clutch sizes survive at the highest rate, while birds with very small or very large clutches have reduced survival. What mode of natural selection is illustrated?

  1. Directional selection
  2. Disruptive selection
  3. Sexual selection
  4. Stabilizing selection
Show Answer

The correct answer is D. Stabilizing selection favors intermediate phenotypes and selects against both extremes, narrowing the range of variation in the population over time. Because birds with intermediate clutch sizes have the highest survival here, while both very small and very large clutch sizes are disfavored, this scenario matches stabilizing selection rather than directional selection (favoring one extreme) or disruptive selection (favoring both extremes).

Concept Tested: Stabilizing Selection

6. A small group of 15 birds is blown by a storm to a remote island and establishes a new population isolated from the mainland. Which evolutionary phenomenon does this scenario best illustrate?

  1. Bottleneck effect, because the mainland population was drastically reduced by the storm
  2. Gene flow, because alleles are being exchanged between the mainland and island populations
  3. Directional selection, because the birds that survived the storm had superior flying ability
  4. Founder effect, because a small subset of the original population's genetic variation establishes a new, isolated population
Show Answer

The correct answer is D. The founder effect occurs when a small number of individuals becomes separated from a larger source population and establishes a new population, carrying only a limited, chance sample of the original population's genetic variation. Because the mainland population itself is not being reduced (only a subset leaves to found a new colony) and no ongoing allele exchange with the mainland is described, this is a founder event rather than a bottleneck or gene flow.

Concept Tested: Founder Effect

7. A conservation biologist compares two populations of the same endangered species: Population X has 5,000 individuals and Population Y has 50 individuals. Both experience the same rare, non-adaptive allele. Which prediction about the fate of that allele is best supported by population genetics theory?

  1. The allele frequency will change more due to chance in Population Y than in Population X, because genetic drift has a stronger effect on allele frequencies in smaller populations
  2. Both populations have identical evolutionary potential, since mutation rate does not influence genetic variation
  3. The allele frequency will change more due to chance in Population X because it contains more individuals in which mutations can occur
  4. Evolutionary potential depends solely on population size, not on reproductive mode or mutation rate
Show Answer

The correct answer is A. Genetic drift's magnitude is inversely proportional to population size, so random sampling effects on allele frequency are far more pronounced in the 50-individual Population Y than in the 5,000-individual Population X. A rare, non-adaptive allele in the small population is much more likely to drift toward fixation or loss purely by chance, whereas the large population's allele frequencies will remain comparatively stable across generations.

Concept Tested: Genetic Drift and Population Size

8. A population that reproduces only asexually and has an unusually low mutation rate is compared to a sexually reproducing population with a typical mutation rate and frequent recombination. Which conclusion about evolutionary potential is best supported?

  1. The asexually reproducing population has greater evolutionary potential because its genome remains stable across generations
  2. Both populations have identical evolutionary potential, since mutation rate does not influence genetic variation
  3. The sexually reproducing population likely has greater evolutionary potential because recombination during meiosis, combined with a typical mutation rate, generates more genetic variation for selection or drift to act upon
  4. Evolutionary potential depends solely on population size, not on reproductive mode or mutation rate
Show Answer

The correct answer is C. Evolutionary potential depends on the amount of heritable variation available for selection and drift to act upon. Sexual reproduction shuffles existing alleles into new combinations through crossing over and independent assortment, and a typical mutation rate continually supplies new alleles, together generating substantially more genetic variation than an asexual population with a low mutation rate and no recombination, which will change much more slowly.

Concept Tested: Genetic Variation and Evolutionary Potential

9. A species has been reduced to a small, genetically depleted population by a historical bottleneck event, similar to the northern elephant seal. Which conservation strategy would population genetics principles suggest is most important for the species' long-term evolutionary viability?

  1. Focus exclusively on increasing population size as quickly as possible, regardless of genetic diversity
  2. Maintain the population at its current small size to prevent further genetic change
  3. Establish habitat corridors or manage gene flow between fragmented subpopulations to help restore genetic diversity and counteract the effects of drift and inbreeding
  4. Eliminate all individuals carrying rare alleles to standardize the gene pool
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The correct answer is C. Bottlenecked populations suffer reduced genetic diversity, which limits their ability to adapt to future environmental change and increases vulnerability to inbreeding depression. Habitat corridors or managed gene flow between subpopulations reintroduce genetic variation and counteract the ongoing effects of drift, directly addressing the underlying population genetics problem rather than simply increasing raw numbers or further reducing variation.

Concept Tested: Conservation Applications of Population Genetics

10. A researcher wants to predict the likely fate of a neutral allele at frequency p = 0.5 in an isolated population of only 20 individuals over 50 generations. Which modeling approach would best capture the expected evolutionary outcome?

  1. A deterministic model assuming allele frequency remains exactly at 0.5 every generation, since no selection is acting
  2. A stochastic (probabilistic) simulation run across many replicate populations, since genetic drift is random and small population size makes fixation (p=1) or loss (p=0) likely within a modest number of generations, with considerable variation among replicate runs
  3. A model based solely on mutation rate, since mutation is the only force capable of changing allele frequency in this scenario
  4. A model that assumes gene flow will restore the allele to its original frequency each generation
Show Answer

The correct answer is B. Because the allele is neutral and the population is small, genetic drift — a random process — will be the dominant force shaping its frequency. A deterministic model is inappropriate because drift outcomes vary by chance between replicate populations; a stochastic simulation run many times captures this variability and correctly predicts that fixation or loss becomes likely well before 50 generations in a population of only 20 individuals.

Concept Tested: Genetic Drift (Modeling and Prediction)