biology : class12 Selection type comparison : Directional Selection, Stabilising Selection, Disruptive Selection : Types of Natural Selection MCQ Questions and Answers
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Selection type comparison
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MCQ Questions
1. A population of lizards shows a continuous range of body sizes. After several generations, predators preferentially remove the smallest lizards, and the population mean body size increases while variation around the new mean remains relatively similar. Which type of selection best explains this pattern?
A.
Directional selection, because one extreme phenotype is favoured
B.
Stabilising selection, because the mean phenotype is maintained
C.
Disruptive selection, because both extremes are favoured
D.
Stabilising selection, because phenotypic variation increases
Correct Answer:
Directional selection, because one extreme phenotype is favoured
Explanation:
Directional selection favours one extreme phenotype, causing the population mean to shift toward that extreme. Stabilising selection would reduce extremes without substantially shifting the mean, whereas disruptive selection would favour both extremes.
2. In a population of birds, individuals with intermediate beak depth obtain food most efficiently, while birds with very shallow or very deep beaks have lower survival. Over time, the average beak depth changes little, but phenotypic variation decreases. Which selection pattern is operating?
A.
Directional selection because one beak extreme is eliminated
B.
Stabilising selection because the intermediate phenotype is favoured
C.
Disruptive selection because two beak extremes are selected against
D.
Directional selection because the population becomes more uniform
Correct Answer:
Stabilising selection because the intermediate phenotype is favoured
Explanation:
Stabilising selection favours intermediate phenotypes and selects against both extremes. Consequently, the mean remains relatively constant while variation in the trait decreases.
3. A fish population feeds in an environment containing only very small and very large seeds. Fish with intermediate jaw sizes feed inefficiently and have lower reproductive success, whereas both small-jawed and large-jawed fish reproduce successfully. What is the most likely long-term result?
A.
A shift of the population mean toward small jaw size
B.
A reduction in variation around an intermediate jaw size
C.
Maintenance or increase of two distinct phenotypic extremes
D.
A complete loss of the large-jawed phenotype
Correct Answer:
Maintenance or increase of two distinct phenotypic extremes
Explanation:
Disruptive selection favours both extremes and selects against intermediate phenotypes. This can increase variation and may produce a population with two phenotypic peaks rather than one intermediate peak.
4. Which combination correctly compares the expected effects of directional, stabilising, and disruptive selection on a normally distributed quantitative trait?
A.
Directional: mean shifts; stabilising: variation decreases; disruptive: variation increases
B.
Directional: variation disappears; stabilising: mean shifts; disruptive: variation decreases
C.
Directional: two extremes increase; stabilising: mean shifts; disruptive: one extreme increases
D.
Directional: mean remains fixed; stabilising: both extremes increase; disruptive: variation disappears
Explanation:
Directional selection changes the population mean toward one extreme. Stabilising selection removes extreme phenotypes and reduces variation. Disruptive selection favours both extremes, increasing variation and potentially producing a bimodal distribution.
5. A drought causes plants with genetically determined deep roots to produce more seeds than plants with shallow roots. After several generations, the frequency of deep-root alleles rises, but there is no evidence that both deep- and shallow-root plants are favoured. This is best classified as:
A.
Stabilising selection because the population becomes genetically uniform
B.
Disruptive selection because root-depth variation exists
C.
Directional selection because one phenotypic extreme has higher fitness
D.
Stabilising selection because the original mean is preserved
Correct Answer:
Directional selection because one phenotypic extreme has higher fitness
Explanation:
The preferential reproductive success of one extreme, deep-rooted plants shifts the trait distribution toward that extreme. The mere presence of variation does not make selection disruptive.
6. In three populations, the following changes occur for the same quantitative trait: Population I shows a shift in its mean toward one extreme; Population II loses both extremes while retaining the same mean; Population III develops increased representation of both extremes and fewer intermediates. Which sequence identifies the selection types?
A.
Population I: stabilising; Population II: directional; Population III: disruptive
B.
Population I: directional; Population II: stabilising; Population III: disruptive
C.
Population I: disruptive; Population II: stabilising; Population III: directional
D.
Population I: directional; Population II: disruptive; Population III: stabilising
Correct Answer:
Population I: directional; Population II: stabilising; Population III: disruptive
Explanation:
A shift toward one extreme indicates directional selection. Removal of both extremes with little change in the mean indicates stabilising selection. Increased frequency of both extremes and reduced intermediates indicates disruptive selection.
7. A trait has three phenotypic classes: low, intermediate, and high. Their relative fitnesses under environmental condition X are 0.7, 1.0, and 0.6, respectively. Under condition Y, their relative fitnesses are 1.0, 0.5, and 1.0. Which comparison is correct?
A.
Condition X represents stabilising selection, whereas condition Y represents disruptive selection
B.
Condition X represents directional selection, whereas condition Y represents stabilising selection
C.
Both conditions represent directional selection because fitness differs among phenotypes
D.
Condition X represents disruptive selection, whereas condition Y represents stabilising selection
Correct Answer:
Condition X represents stabilising selection, whereas condition Y represents disruptive selection
Explanation:
Under condition X, the intermediate phenotype has the highest fitness and both extremes are selected against, which is stabilising selection. Under condition Y, both extremes have higher fitness than the intermediate phenotype, which is disruptive selection.
8. Which observation most reliably distinguishes directional selection from stabilising selection when only one generation of data is available?
A.
The trait is continuously distributed in the population
B.
The population contains individuals at both phenotypic extremes
C.
The mean phenotype shifts toward one extreme rather than remaining near its previous value
D.
Individuals with different phenotypes show different survival rates
Correct Answer:
The mean phenotype shifts toward one extreme rather than remaining near its previous value
Explanation:
Both directional and stabilising selection can produce unequal survival among phenotypes. A shift in the mean toward one extreme is characteristic of directional selection, whereas stabilising selection generally preserves the mean while reducing extreme phenotypes.
9. A population originally has a single peak for body size. After selection, the mean body size is almost unchanged, but the frequency of both very small and very large individuals declines sharply. Which conclusion is most justified?
A.
Selection is directional because both extremes have decreased
B.
Selection is stabilising because extremes are removed without a major mean shift
C.
Selection is disruptive because the population has become less variable
D.
Selection is directional because the intermediate phenotype has increased
Correct Answer:
Selection is stabilising because extremes are removed without a major mean shift
Explanation:
Stabilising selection favours intermediate phenotypes and eliminates both extremes. The resulting distribution is narrower, while the population mean remains approximately stable.
10. Two environments act on the same trait. In Environment A, individuals at one extreme have the greatest fitness. In Environment B, individuals at both extremes have greater fitness than intermediates. If the environments persist, which evolutionary outcomes are most likely?
A.
Environment A produces a mean shift; Environment B can produce a bimodal distribution
B.
Environment A reduces variation; Environment B preserves only the intermediate phenotype
C.
Environment A produces a bimodal distribution; Environment B shifts the mean toward one extreme
D.
Both environments produce stabilisation because selection acts on the same trait
Correct Answer:
Environment A produces a mean shift; Environment B can produce a bimodal distribution
Explanation:
Favouring one extreme in Environment A produces directional selection and shifts the mean. Favouring both extremes over the intermediate in Environment B produces disruptive selection, which can create a bimodal distribution.
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