PCB 4674 | PCB4674 Exam 2: Evolution - Florida
State University Updated and Latest Questions and
Correct Answers with Rationale
1. In a population at Hardy-Weinberg equilibrium, if the frequency of a recessive allele (q) is
0.3, what is the frequency of the heterozygous genotype?
A. 0.09
B. 0.21
C. 0.49
D. 0.42
Correct Answer: D
Expert Explanation: To find the heterozygous frequency, we use the formula 2pq from the
Hardy-Weinberg equation. Since q is 0.3, p must be 0.7 because p plus q equals 1.
Multiplying 2 times 0.7 times 0.3 results in a frequency of 0.42. Option A represents q
squared, while option C represents p squared. This calculation demonstrates how allele
frequencies predict genotype frequencies in an ideal population.
2. Which evolutionary force is most likely to reduce genetic variation within a small, isolated
population over time?
A. Mutation
B. Genetic Drift
C. Gene Flow
D. Natural Selection
Correct Answer: B
Expert Explanation: Genetic drift refers to random fluctuations in allele frequencies that
are particularly potent in small populations. Over time, these random changes often lead to
the fixation of some alleles and the loss of others, thereby reducing overall genetic
variation. Mutation and gene flow generally increase or maintain variation within a
population. Natural selection can reduce variation but is not inherently driven by
population size like drift is. Therefore, drift is the primary cause of variation loss in small
isolated groups.
3. If the selection coefficient (s) against a homozygous recessive genotype is 1.0, what does
this indicate about the fitness (w) of that genotype?
A. The fitness is 1.0, meaning it is perfectly adapted.
B. The fitness is 0.0, meaning the genotype is lethal or sterile.
,C. The fitness is 0.5, meaning it has moderate reproductive success.
D. The fitness cannot be determined from the selection coefficient alone.
Correct Answer: B
Expert Explanation: Fitness and selection coefficients are mathematically related by the
formula w equals 1 minus s. A selection coefficient of 1.0 means that the phenotype is being
completely selected against by the environment. Substituting 1.0 into the formula results in
a fitness value of 0.0, indicating zero reproductive success. This occurs in cases of lethal
mutations or complete sterility where individuals do not pass on their genes.
Understanding this relationship is fundamental to modeling how selection changes allele
frequencies over generations.
4. Which of the following conditions is NOT an assumption of the Hardy-Weinberg
equilibrium?
A. No mutation occurs.
B. The population size is very small.
C. Random mating must take place.
D. No migration or gene flow exists.
Correct Answer: B
Expert Explanation: The Hardy-Weinberg equilibrium assumes an infinitely large
population size to minimize the effects of genetic drift. If a population is very small, allele
frequencies will change due to chance alone, violating the equilibrium. Mutation, migration,
and non-random mating are all factors that can shift frequencies and are thus excluded in
the HWE model. The model serves as a null hypothesis to identify when evolution is
actually occurring. Therefore, a small population size is the opposite of what HWE requires.
5. A sudden reduction in population size due to a natural disaster that leaves a random
sample of survivors is known as:
A. The founder effect
B. Directional selection
C. A population bottleneck
D. Disruptive selection
Correct Answer: C
Expert Explanation: A population bottleneck occurs when an environmental event
drastically reduces the number of individuals in a population. The surviving individuals
possess only a fraction of the original genetic diversity, leading to a shift in allele
frequencies. This is a form of genetic drift because the survival is largely random rather
than based on fitness. The founder effect is similar but specifically involves the colonization
, of a new area by a small group. Both processes result in reduced genetic variation
compared to the source population.
6. In the mutation-selection balance model, if a deleterious recessive allele is frequently
replenished by mutation but cleared by selection, its equilibrium frequency is determined by:
A. The square of (s / mu)
B. The product of mu and s
C. The square root of (mu / s)
D. The ratio of p to q
Correct Answer: C
Expert Explanation: The mutation-selection balance predicts the persistence of harmful
alleles in a population despite selective pressure. The equilibrium frequency q is calculated
as the square root of the mutation rate divided by the selection coefficient. This formula
shows that as the mutation rate increases, the frequency of the deleterious allele also
increases. Conversely, a higher selection coefficient against the allele will lower its
equilibrium frequency. This model explains why rare genetic diseases persist in human
populations.
7. Which type of selection maintains genetic variation by favoring the heterozygote over both
homozygotes?
A. Overdominance
B. Stabilizing selection
C. Disruptive selection
D. Directional selection
Correct Answer: A
Expert Explanation: Overdominance, or heterozygote advantage, occurs when the fitness
of the heterozygote is higher than that of either homozygote. This process prevents the loss
of either allele because the heterozygotes continue to pass both to the next generation. A
classic example of this is the sickle-cell trait, which provides resistance to malaria in its
heterozygous form. Unlike directional selection, which pushes an allele toward fixation,
overdominance maintains a stable polymorphism. Thus, it is a key mechanism for
preserving genetic diversity in populations.
8. Effective population size (Ne) is often smaller than the census population size (N) because:
A. All individuals in the population reproduce equally.
B. There is a skewed sex ratio or high variance in reproductive success.
C. The population is in a state of rapid growth.
State University Updated and Latest Questions and
Correct Answers with Rationale
1. In a population at Hardy-Weinberg equilibrium, if the frequency of a recessive allele (q) is
0.3, what is the frequency of the heterozygous genotype?
A. 0.09
B. 0.21
C. 0.49
D. 0.42
Correct Answer: D
Expert Explanation: To find the heterozygous frequency, we use the formula 2pq from the
Hardy-Weinberg equation. Since q is 0.3, p must be 0.7 because p plus q equals 1.
Multiplying 2 times 0.7 times 0.3 results in a frequency of 0.42. Option A represents q
squared, while option C represents p squared. This calculation demonstrates how allele
frequencies predict genotype frequencies in an ideal population.
2. Which evolutionary force is most likely to reduce genetic variation within a small, isolated
population over time?
A. Mutation
B. Genetic Drift
C. Gene Flow
D. Natural Selection
Correct Answer: B
Expert Explanation: Genetic drift refers to random fluctuations in allele frequencies that
are particularly potent in small populations. Over time, these random changes often lead to
the fixation of some alleles and the loss of others, thereby reducing overall genetic
variation. Mutation and gene flow generally increase or maintain variation within a
population. Natural selection can reduce variation but is not inherently driven by
population size like drift is. Therefore, drift is the primary cause of variation loss in small
isolated groups.
3. If the selection coefficient (s) against a homozygous recessive genotype is 1.0, what does
this indicate about the fitness (w) of that genotype?
A. The fitness is 1.0, meaning it is perfectly adapted.
B. The fitness is 0.0, meaning the genotype is lethal or sterile.
,C. The fitness is 0.5, meaning it has moderate reproductive success.
D. The fitness cannot be determined from the selection coefficient alone.
Correct Answer: B
Expert Explanation: Fitness and selection coefficients are mathematically related by the
formula w equals 1 minus s. A selection coefficient of 1.0 means that the phenotype is being
completely selected against by the environment. Substituting 1.0 into the formula results in
a fitness value of 0.0, indicating zero reproductive success. This occurs in cases of lethal
mutations or complete sterility where individuals do not pass on their genes.
Understanding this relationship is fundamental to modeling how selection changes allele
frequencies over generations.
4. Which of the following conditions is NOT an assumption of the Hardy-Weinberg
equilibrium?
A. No mutation occurs.
B. The population size is very small.
C. Random mating must take place.
D. No migration or gene flow exists.
Correct Answer: B
Expert Explanation: The Hardy-Weinberg equilibrium assumes an infinitely large
population size to minimize the effects of genetic drift. If a population is very small, allele
frequencies will change due to chance alone, violating the equilibrium. Mutation, migration,
and non-random mating are all factors that can shift frequencies and are thus excluded in
the HWE model. The model serves as a null hypothesis to identify when evolution is
actually occurring. Therefore, a small population size is the opposite of what HWE requires.
5. A sudden reduction in population size due to a natural disaster that leaves a random
sample of survivors is known as:
A. The founder effect
B. Directional selection
C. A population bottleneck
D. Disruptive selection
Correct Answer: C
Expert Explanation: A population bottleneck occurs when an environmental event
drastically reduces the number of individuals in a population. The surviving individuals
possess only a fraction of the original genetic diversity, leading to a shift in allele
frequencies. This is a form of genetic drift because the survival is largely random rather
than based on fitness. The founder effect is similar but specifically involves the colonization
, of a new area by a small group. Both processes result in reduced genetic variation
compared to the source population.
6. In the mutation-selection balance model, if a deleterious recessive allele is frequently
replenished by mutation but cleared by selection, its equilibrium frequency is determined by:
A. The square of (s / mu)
B. The product of mu and s
C. The square root of (mu / s)
D. The ratio of p to q
Correct Answer: C
Expert Explanation: The mutation-selection balance predicts the persistence of harmful
alleles in a population despite selective pressure. The equilibrium frequency q is calculated
as the square root of the mutation rate divided by the selection coefficient. This formula
shows that as the mutation rate increases, the frequency of the deleterious allele also
increases. Conversely, a higher selection coefficient against the allele will lower its
equilibrium frequency. This model explains why rare genetic diseases persist in human
populations.
7. Which type of selection maintains genetic variation by favoring the heterozygote over both
homozygotes?
A. Overdominance
B. Stabilizing selection
C. Disruptive selection
D. Directional selection
Correct Answer: A
Expert Explanation: Overdominance, or heterozygote advantage, occurs when the fitness
of the heterozygote is higher than that of either homozygote. This process prevents the loss
of either allele because the heterozygotes continue to pass both to the next generation. A
classic example of this is the sickle-cell trait, which provides resistance to malaria in its
heterozygous form. Unlike directional selection, which pushes an allele toward fixation,
overdominance maintains a stable polymorphism. Thus, it is a key mechanism for
preserving genetic diversity in populations.
8. Effective population size (Ne) is often smaller than the census population size (N) because:
A. All individuals in the population reproduce equally.
B. There is a skewed sex ratio or high variance in reproductive success.
C. The population is in a state of rapid growth.
