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Evolutionary consequences
MCQ questions and answers.
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MCQ Questions
1. A population experiences directional selection for increasingly larger body size over many generations. If the selected environmental condition remains stable, which evolutionary consequence is most likely?
A.
The population mean body size shifts toward larger values, reducing the frequency of alleles associated with smaller size
B.
The population mean remains unchanged while phenotypic variance becomes zero
C.
Both extremely small and extremely large individuals increase in frequency
D.
The population becomes genetically identical immediately after one generation
Correct Answer:
The population mean body size shifts toward larger values, reducing the frequency of alleles associated with smaller size
Explanation:
Directional selection consistently favours one extreme phenotype, causing the population mean and often the associated allele frequencies to shift toward that extreme over generations.
2. In a population, stabilising selection continually removes individuals at both extremes of birth weight. What is the most likely long-term evolutionary consequence?
A.
The average birth weight shifts steadily toward the heavier extreme
B.
Intermediate birth weight becomes more common, while phenotypic variation decreases
C.
Both extreme birth-weight phenotypes become more common than the intermediate phenotype
D.
Two reproductively isolated populations necessarily form
Correct Answer:
Intermediate birth weight becomes more common, while phenotypic variation decreases
Explanation:
Stabilising selection favours the intermediate phenotype and eliminates both extremes, maintaining a similar population mean while reducing variation around that mean.
3. A disruptive selection regime favours very small and very large individuals but disadvantages intermediate individuals. Which evolutionary outcome is most directly expected if this persists?
A.
The population becomes concentrated around a single intermediate phenotype
B.
Phenotypic variation decreases because both extremes are eliminated
C.
The frequency of both extreme phenotypes rises and the population distribution becomes bimodal
D.
The population mean must shift toward the larger extreme
Correct Answer:
The frequency of both extreme phenotypes rises and the population distribution becomes bimodal
Explanation:
Disruptive selection favours both extremes over the intermediate form, producing a bimodal distribution and increasing divergence within the population.
4. Which comparison correctly describes the effect of directional and stabilising selection on the population mean and phenotypic variation?
A.
Directional selection usually shifts the mean, whereas stabilising selection generally preserves the mean while reducing variation
B.
Directional selection reduces the mean, whereas stabilising selection always increases the mean
C.
Both forms necessarily produce two distinct phenotype clusters
D.
Both forms preserve variation without changing the distribution of phenotypes
Correct Answer:
Directional selection usually shifts the mean, whereas stabilising selection generally preserves the mean while reducing variation
Explanation:
Directional selection favours one extreme and changes the population mean, whereas stabilising selection favours intermediate phenotypes and narrows the distribution around the existing mean.
5. In a population subjected to persistent disruptive selection, why can the evolutionary divergence between the two favoured forms increase over time?
A.
Selection increases the reproductive success of intermediate individuals, causing convergence
B.
Selection removes both extreme phenotypes and preserves only the mean
C.
Selection favours contrasting phenotypes, allowing differences in allele frequencies and traits to accumulate between them
D.
Selection prevents any heritable variation from being transmitted
Correct Answer:
Selection favours contrasting phenotypes, allowing differences in allele frequencies and traits to accumulate between them
Explanation:
When two extreme phenotypes are favoured, traits and allele frequencies associated with each form can diverge, potentially creating conditions that contribute to population subdivision.
6. A pesticide initially kills most insects with low resistance, but resistant insects survive and reproduce. After many generations, which result best demonstrates the evolutionary consequence of directional selection?
A.
The population contains fewer resistance alleles because resistant insects reproduce less
B.
The frequency of resistance alleles increases and the population becomes more resistant on average
C.
Intermediate resistance disappears while both low and high resistance increase equally
D.
Resistance becomes present in every individual regardless of inheritance
Correct Answer:
The frequency of resistance alleles increases and the population becomes more resistant on average
Explanation:
Directional selection favours one extreme—in this case high resistance—so heritable resistance alleles become more frequent and the population mean resistance increases.
7. A stabilising selection pattern is observed for a quantitative trait, but the trait's mean remains almost constant for several generations. Which inference is most justified?
A.
Natural selection is absent because the mean has not changed
B.
Selection may be acting against both extremes while maintaining the intermediate phenotype
C.
Disruptive selection must be producing two equally common extremes
D.
Directional selection must be favouring the less common extreme
Correct Answer:
Selection may be acting against both extremes while maintaining the intermediate phenotype
Explanation:
Stabilising selection can produce strong evolutionary change in the distribution by reducing extremes and variation even when the population mean remains relatively unchanged.
8. If disruptive selection continues in a population and mating becomes increasingly associated with similar extreme phenotypes, which further consequence is most plausible?
A.
Reduced gene flow between the forms may promote genetic divergence and eventual reproductive isolation
B.
The two forms must immediately merge into one genetically uniform population
C.
The intermediate phenotype must become the only phenotype present
D.
Allele frequencies cannot differ between the two forms because they share the same habitat
Correct Answer:
Reduced gene flow between the forms may promote genetic divergence and eventual reproductive isolation
Explanation:
Disruptive selection can separate a population into contrasting forms. If mating or ecological differences reduce gene flow, genetic divergence and reproductive isolation may develop over time.
9. Two populations have the same initial mean for a heritable trait. Population X undergoes stabilising selection, whereas Population Y undergoes directional selection toward a higher value. After many generations, which pattern is expected?
A.
Both populations retain the same mean and the same phenotypic variance
B.
Population X tends to show reduced variance around a similar mean, while Population Y shows a higher mean
C.
Population X develops two extremes, while Population Y becomes concentrated at the original mean
D.
Population X shifts upward more rapidly than Population Y because intermediate phenotypes are favoured
Correct Answer:
Population X tends to show reduced variance around a similar mean, while Population Y shows a higher mean
Explanation:
Stabilising selection mainly narrows variation around the existing intermediate optimum, whereas directional selection changes the mean toward the favoured higher phenotype.
10. Which observation provides the strongest evidence that a population has undergone disruptive rather than directional selection?
A.
The average trait value has increased and the distribution remains unimodal
B.
The average trait value is nearly unchanged, but intermediate phenotypes have declined and two extreme classes have increased
C.
The average trait value is unchanged and variation has decreased around the intermediate phenotype
D.
One extreme phenotype has increased while the opposite extreme and intermediate forms have declined
Correct Answer:
The average trait value is nearly unchanged, but intermediate phenotypes have declined and two extreme classes have increased
Explanation:
Disruptive selection favours both extremes over intermediates, often producing a bimodal distribution. This distinguishes it from directional selection, which shifts the mean toward one extreme.
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