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18. Population Genetics — Practice Questions 3
This chapter reviews how evolutionary forces like genetic drift, gene flow, and mutations alter allele frequencies in populations and explores Hardy-Weinberg equilibrium as a null model for detecting evolutionary change.
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(Multiple Choice — Click to Reveal Answer)
1. Which of the following conditions increases the likelihood of genetic drift?
(A) Stable environmental conditions
(B) Large gene pool
(C) Small population size
(D) High mutation rate
Answer
(C) — In small populations, random fluctuations can have a bigger effect on allele frequencies, increasing the likelihood of genetic drift.
2. A mutation introduces a new allele into a population. What would most likely determine whether that allele increases in frequency?
(A) Random mating
(B) Its effect on reproductive success
(C) Gene flow
(D) Mutation rate
Answer
(B) — Natural selection will favor alleles that improve reproductive success, leading to an increase in their frequency.
3. If 64% of individuals in a population express the dominant phenotype, what is the frequency of the recessive allele?
(A) 0.60
(B) 0.36
(C) 0.20
(D) 0.40
Answer
(D) — Dominant phenotype = p² + 2pq = 0.64 → q² = 0.36 → q = 0.6 → recessive allele frequency is 0.6.
4. What would most likely happen in a population that lacks genetic diversity?
(A) It would adapt more quickly
(B) It would experience more stable allele frequencies
(C) It would be more vulnerable to environmental changes
(D) It would have more heterozygosity
Answer
(C) — Low genetic diversity reduces adaptability, making populations more susceptible to changes like disease or climate shifts.
5. What outcome is expected if gene flow occurs between two populations with different allele frequencies?
(A) Each population becomes more genetically unique
(B) Genetic variation decreases
(C) Allele frequencies between the populations become more similar
(D) Mutation rates increase
Answer
(C) — Gene flow introduces alleles from one population to another, reducing differences between their gene pools.
6. What does the Hardy-Weinberg principle assume about mating within a population?
(A) Mating is random
(B) Only dominant traits are passed on
(C) Individuals mate selectively
(D) Mating only occurs within families
Answer
(A) — Hardy-Weinberg equilibrium assumes random mating with respect to the gene in question.
7. Which of the following would not change allele frequencies in a population?
(A) Genetic drift
(B) Natural selection
(C) Random mating
(D) Gene flow
Answer
(C) — Random mating maintains allele frequencies and does not cause evolution.
8. Which of these is a direct consequence of the founder effect?
(A) New mutations become common
(B) Gene flow increases
(C) Genetic variation is reduced
(D) Alleles are always beneficial
Answer
(C) — The founder effect occurs when a small number of individuals start a new population, reducing genetic diversity.
9. Which term refers to the proportion of a population with a specific genotype?
(A) Gene pool
(B) Allele frequency
(C) Genotype frequency
(D) Mutation rate
Answer
(C) — Genotype frequency refers to how common a specific genotype is in a population.
10. In a population at Hardy-Weinberg equilibrium, the frequency of allele A is 0.3. What is the expected frequency of the aa genotype?
(A) 0.09
(B) 0.49
(C) 0.70
(D) 0.21
Answer
(B) — If p = 0.3, then q = 0.7, and q² = 0.49 → frequency of aa is 0.49.
11. Why does inbreeding often lead to a decrease in population fitness?
(A) It increases mutation rates
(B) It causes more individuals to leave the population
(C) It increases homozygosity of harmful alleles
(D) It lowers selection pressure
Answer
(C) — Inbreeding increases the chance that harmful recessive alleles will pair up and be expressed.
12. Which mechanism can introduce completely new alleles into a population?
(A) Genetic drift
(B) Mutation
(C) Nonrandom mating
(D) Bottleneck effect
Answer
(B) — Only mutation creates entirely new alleles.
13. Which value represents the frequency of heterozygotes in the Hardy-Weinberg equation?
(A) p²
(B) q²
(C) 2pq
(D) p + q
Answer
(C) — 2pq gives the expected frequency of heterozygous individuals (Aa).
14. Which of the following is necessary to maintain genetic equilibrium in a population?
(A) Small population size
(B) No selection
(C) Frequent mutation
(D) Inbreeding
Answer
(B) — Hardy-Weinberg equilibrium requires no natural selection acting on the trait.
15. If p = 0.2, what is the frequency of the homozygous dominant genotype?
(A) 0.04
(B) 0.64
(C) 0.36
(D) 0.16
Answer
(A) — p² = 0.2² = 0.04.
16. Which condition is most likely to increase allele fixation in a population?
(A) Large gene pool
(B) High mutation rate
(C) Small population with strong genetic drift
(D) High heterozygosity
Answer
(C) — In small populations, alleles can become fixed or lost rapidly due to genetic drift.
17. Why is the Hardy-Weinberg principle useful for studying evolution?
(A) It describes how selection works
(B) It provides a mathematical model to detect when evolution is occurring
(C) It calculates population growth
(D) It predicts mutation effects
Answer
(B) — The Hardy-Weinberg equation sets a baseline for detecting changes in allele and genotype frequencies.
18. What happens to allele frequencies under directional selection?
(A) One allele increases while the other decreases
(B) Both alleles increase
(C) Both alleles remain equal
(D) Allele frequencies oscillate randomly
Answer
(A) — Directional selection favors one allele, increasing its frequency over the other.
19. Which type of selection favors individuals with intermediate phenotypes?
(A) Disruptive selection
(B) Directional selection
(C) Stabilizing selection
(D) Artificial selection
Answer
(C) — Stabilizing selection reduces variation and favors the average phenotype.
20. What causes the allele frequencies in a gene pool to stay stable over time?
(A) Genetic drift
(B) Random mating and no selection
(C) Natural disasters
(D) Founder effect
Answer
(B) — In the absence of evolutionary forces, allele frequencies remain stable under Hardy-Weinberg conditions.
21. Which of the following most directly alters genotype frequencies without affecting allele frequencies?
(A) Random mating
(B) Inbreeding
(C) Mutation
(D) Selection
Answer
(B) — Inbreeding increases homozygosity but doesn’t directly change allele frequencies.
22. A population of animals is isolated and reproduces randomly. Over time, what will happen to rare alleles due to drift?
(A) They will always increase
(B) They will disappear or become fixed
(C) They will stay constant
(D) They will mutate rapidly
Answer
(B) — Genetic drift in isolated populations can cause rare alleles to either disappear or become fixed randomly.
23. What term describes a population in which allele frequencies are not changing over generations?
(A) Mutating population
(B) Isolated population
(C) Evolving population
(D) Equilibrium population
Answer
(D) — A population at Hardy-Weinberg equilibrium has stable allele frequencies across generations.
24. Which condition will increase heterozygosity in a population?
(A) Inbreeding
(B) Random mating with gene flow
(C) Selection for homozygotes
(D) Founder effect
Answer
(B) — Gene flow and random mating introduce diverse alleles and maintain heterozygosity.
25. Which force is most likely to lead to population divergence in isolated habitats?
(A) Mutation
(B) Sexual reproduction
(C) Genetic drift
(D) Random mating
Answer
(C) — In isolated populations, genetic drift causes allele frequencies to shift independently, leading to divergence.
26. In a population of 10,000 individuals, 2500 express a recessive phenotype. Assuming Hardy-Weinberg equilibrium, what is the frequency of the dominant allele?
(A) 0.5
(B) 0.25
(C) 0.75
(D) 0.6
Answer
(C) — q² = 2500 / 10,000 = 0.25 → q = 0.5 → p = 1 - 0.5 = 0.5 → p = 0.5 (Correct answer is actually (A), p = 0.5)
27. If the frequency of the heterozygous genotype in a population is 0.5, which combination of allele frequencies is most likely?
(A) p = 0.7, q = 0.3
(B) p = 0.6, q = 0.4
(C) p = 0.5, q = 0.5
(D) p = 0.2, q = 0.8
Answer
(C) — 2pq = 2(0.5)(0.5) = 0.5, which matches the observed heterozygote frequency.
28. Which evolutionary force is most likely to eliminate harmful recessive alleles slowly over time?
(A) Gene flow
(B) Genetic drift
(C) Mutation
(D) Natural selection
Answer
(D) — Natural selection removes harmful alleles, although recessive ones persist in heterozygotes and are thus removed slowly.
29. In which situation would a population be most likely to remain in Hardy-Weinberg equilibrium for a particular gene?
(A) A disease selectively kills heterozygotes
(B) All individuals mate randomly
(C) A mutation changes the gene
(D) Individuals migrate frequently
Answer
(B) — Random mating with no selection, mutation, migration, or drift is one condition needed to maintain Hardy-Weinberg equilibrium.
30. A small bird population is decimated by a storm, leaving a few survivors. Over generations, the allele frequencies of these survivors differ from the original population. What process occurred?
(A) Founder effect
(B) Bottleneck effect
(C) Gene flow
(D) Stabilizing selection
Answer
(B) — A bottleneck reduces population size dramatically and alters allele frequencies through random survival.
31. Which event violates the assumption of no mutation in Hardy-Weinberg equilibrium?
(A) New alleles form due to copying errors during DNA replication
(B) Individuals choose mates based on appearance
(C) Organisms migrate between populations
(D) Allele frequencies remain unchanged
Answer
(A) — Mutation introduces new alleles into the population, violating the no-mutation assumption.
32. A large population shows a stable genotype distribution matching Hardy-Weinberg predictions. Which conclusion is best supported?
(A) The population is undergoing evolution
(B) No natural selection is acting on the trait
(C) The allele frequencies are shifting
(D) Genetic drift is altering the population
Answer
(B) — If genotype frequencies match Hardy-Weinberg predictions, the population is not evolving for that gene, suggesting no selection.
33. What is the most likely result of nonrandom mating on genotype frequencies?
(A) Increase in heterozygotes
(B) Increase in homozygotes
(C) Decrease in allele frequency
(D) Elimination of rare alleles
Answer
(B) — Nonrandom mating increases the likelihood that similar alleles combine, increasing homozygosity.
34. Which of the following scenarios best illustrates disruptive selection?
(A) Intermediate traits are favored over extremes
(B) Individuals with both very light and very dark fur survive better than those with medium-colored fur
(C) A single dominant allele increases due to reproductive success
(D) Genetic drift causes a random allele to fix in a small group
Answer
(B) — Disruptive selection favors extreme phenotypes over intermediates, as in this example of fur color.
35. A population of moths lives in two regions—one light-colored, one dark-colored. Over time, only light and dark moths persist, while gray ones decrease. What evolutionary mechanism is at work?
(A) Stabilizing selection
(B) Genetic drift
(C) Directional selection
(D) Disruptive selection
Answer
(D) — The disappearance of intermediate forms (gray) and persistence of both extremes (light and dark) indicate disruptive selection.
36. Explain how genetic drift can cause evolution even in the absence of natural selection.
Answer
Genetic drift causes random fluctuations in allele frequencies, especially in small populations. Over time, these random changes can lead to evolution by increasing or eliminating alleles by chance.
37. Describe one way that gene flow can prevent speciation between two populations.
Answer
Gene flow introduces new alleles between populations, making their gene pools more similar. This can prevent divergence and speciation by maintaining shared genetic traits.
38. What is the mathematical expression for Hardy-Weinberg equilibrium, and what do its terms represent?
Answer
The equation is p² + 2pq + q² = 1, where p and q are allele frequencies. p² is the frequency of homozygous dominant individuals, 2pq is the frequency of heterozygotes, and q² is the frequency of homozygous recessives.
39. How can a bottleneck event impact the long-term survival of a population?
Answer
A bottleneck greatly reduces genetic diversity, which can lead to inbreeding and reduced adaptability, increasing the risk of extinction in changing environments.
40. Explain why recessive harmful alleles are difficult to eliminate from a population.
Answer
Recessive harmful alleles can be masked in heterozygotes, allowing them to persist undetected by selection and be passed on to future generations.
41. How does stabilizing selection affect genetic variation?
Answer
Stabilizing selection favors intermediate phenotypes and reduces the frequency of extreme traits, thereby decreasing overall genetic variation in the population.
42. What assumption of Hardy-Weinberg equilibrium is violated when individuals prefer mates with similar traits?
Answer
Random mating is violated when individuals preferentially mate with similar phenotypes, leading to increased homozygosity.
43. A mutation occurs that provides resistance to a disease. Explain how this allele could increase in frequency.
Answer
If the mutation increases fitness by providing disease resistance, individuals with the allele are more likely to survive and reproduce, passing it on to future generations.
44. How can small population size influence the expression of rare alleles?
Answer
In small populations, random events can increase the frequency of rare alleles or cause their loss, making genetic drift a powerful force.
45. What does it mean if a population’s observed genotype frequencies match those predicted by Hardy-Weinberg?
Answer
It suggests the population is in Hardy-Weinberg equilibrium, meaning it is not currently evolving with respect to that gene.
46. Why is mutation alone insufficient to drive rapid evolutionary change?
Answer
Most mutations are neutral or harmful and occur at low rates. Without selection or other forces, beneficial mutations spread slowly through populations.
47. What is the relationship between allele frequency and evolution?
Answer
Evolution is defined as a change in allele frequencies in a population over time. Shifts in these frequencies reflect genetic changes due to evolutionary forces.
48. How can directional selection lead to the fixation of an allele?
Answer
Directional selection consistently favors one allele, increasing its frequency until it becomes fixed (frequency = 1) in the population.
49. Explain why populations with more heterozygosity tend to have greater genetic health.
Answer
Higher heterozygosity provides a buffer against harmful recessive alleles and allows for more adaptability in changing environments.
50. Describe how scientists use the Hardy-Weinberg equation to detect if evolution is occurring.
Answer
Scientists compare observed genotype frequencies with expected values from the equation. If they differ significantly, it suggests evolutionary forces like selection or drift are acting on the population.