biology : class12 Phenotypic distribution shifts : Directional Selection, Stabilising Selection, Disruptive Selection : Types of Natural Selection MCQ Questions and Answers
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Phenotypic distribution shifts
MCQ questions and answers.
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MCQ Questions
1. In a population, the original phenotype distribution for body size is approximately normal with a mean of 50 units. After several generations, the distribution remains approximately normal, but its mean shifts to 58 units while its variance changes very little. Which pattern best explains this phenotypic distribution shift?
A.
Directional selection favouring larger body size
B.
Stabilising selection favouring intermediate body size
C.
Disruptive selection favouring both extreme body sizes
D.
Random sampling with no consistent selection pressure
Correct Answer:
Directional selection favouring larger body size
Explanation:
A persistent shift of the population mean toward one extreme, with little change in overall spread, is characteristic of directional selection.
2. A trait initially shows a broad bell-shaped distribution centred at an intermediate value. Following selection, individuals near the centre become less common, whereas individuals at both low and high extremes increase in frequency. Which change in distribution has occurred?
A.
A narrowing around the original mean
B.
A shift of the entire distribution toward the lower extreme
C.
A bimodal distribution produced by disruptive selection
D.
A uniform distribution produced by stabilising selection
Correct Answer:
A bimodal distribution produced by disruptive selection
Explanation:
Disruptive selection reduces the frequency of intermediate phenotypes and increases both extremes, often changing a single-peaked distribution into a bimodal one.
3. Two populations begin with the same phenotypic distribution. In population X, the mean remains nearly constant but the range and variance decrease over generations. In population Y, the mean increases substantially while the variance remains similar. Which interpretation is correct?
A.
X shows directional selection and Y shows stabilising selection
B.
X shows stabilising selection and Y shows directional selection
C.
Both X and Y show disruptive selection
D.
X shows disruptive selection and Y shows stabilising selection
Correct Answer:
X shows stabilising selection and Y shows directional selection
Explanation:
Stabilising selection concentrates phenotypes around the intermediate value, reducing variance without greatly changing the mean. Directional selection shifts the mean toward one extreme.
4. A quantitative trait has an initial mean of 20 and a standard deviation of 4. After selection, the mean is 24 and the standard deviation is 3. Which conclusion is most justified from the distributional change alone?
A.
The population has certainly undergone disruptive selection
B.
The population has shifted directionally toward higher values, with some reduction in spread
C.
The population has undergone only stabilising selection around the original mean
D.
The phenotype has become independent of selection because the variance decreased
Correct Answer:
The population has shifted directionally toward higher values, with some reduction in spread
Explanation:
The increase in the mean indicates a shift toward higher phenotypic values, consistent with directional selection; the reduced standard deviation indicates that the distribution also became somewhat narrower.
5. A population's phenotype frequencies before and after selection are as follows: low phenotype, 20% to 35%; intermediate phenotype, 60% to 30%; high phenotype, 20% to 35%. Which selection pattern is most consistent with these changes?
A.
Directional selection toward the low phenotype
B.
Directional selection toward the high phenotype
C.
Stabilising selection favouring the intermediate phenotype
D.
Disruptive selection against the intermediate phenotype
Correct Answer:
Disruptive selection against the intermediate phenotype
Explanation:
Both extreme phenotypes increase while the intermediate phenotype decreases, producing the characteristic frequency shift of disruptive selection.
6. A population has a phenotype distribution with a mean of 100 and most individuals between 90 and 110. After selection, most surviving individuals lie between 96 and 104, and the mean remains close to 100. Which statistical change best represents this shift?
A.
A substantial increase in the mean with unchanged variance
B.
A decrease in variance with little change in the mean
C.
An increase in variance caused by greater representation of both extremes
D.
A conversion from a unimodal to a bimodal distribution
Correct Answer:
A decrease in variance with little change in the mean
Explanation:
The concentration of phenotypes around the same central value indicates stabilising selection, which lowers phenotypic variance while preserving the mean approximately.
7. In a population, the proportion of individuals with a large-value phenotype rises steadily over five generations, while medium and small phenotypes decline. The distribution does not split into two peaks. What does this pattern indicate?
A.
A directional shift toward the large-value phenotype
B.
Stabilisation around the medium phenotype
C.
Disruption producing two equally favoured extremes
D.
A shift toward the original population mean
Correct Answer:
A directional shift toward the large-value phenotype
Explanation:
A gradual increase in one extreme and decline of the other phenotypes, without formation of two peaks, indicates directional selection toward the large-value phenotype.
8. A trait distribution changes from one broad peak to two peaks after selection, but the overall population mean remains almost unchanged. Which inference is most appropriate?
A.
The population experienced stabilising selection centred on the original mean
B.
The population experienced directional selection toward an unobserved phenotype
C.
The population experienced disruptive selection favouring both extremes
D.
The population experienced no selection because the mean did not change
Correct Answer:
The population experienced disruptive selection favouring both extremes
Explanation:
Disruptive selection can increase both extremes and reduce intermediates, creating two peaks even when the overall mean changes little because the extremes may be balanced.
9. A phenotype has an initial distribution with mean 40 and variance 16. After several generations, the mean becomes 40.5 and the variance becomes 4. Which selection pattern is most strongly indicated?
A.
Directional selection, because any mean change proves directional selection alone
B.
Stabilising selection, because the distribution narrowed around nearly the same mean
C.
Disruptive selection, because the variance decreased
D.
Directional selection toward both extremes simultaneously
Correct Answer:
Stabilising selection, because the distribution narrowed around nearly the same mean
Explanation:
The large reduction in variance with only a negligible change in the mean shows that intermediate phenotypes were favoured and extremes were removed, which is stabilising selection.
10. Which observed sequence across generations most clearly distinguishes disruptive selection from directional selection using phenotypic distributions?
A.
The mean moves toward one extreme and the distribution remains mainly single-peaked
B.
The mean remains constant and the distribution becomes narrower around the centre
C.
Intermediate phenotypes decline while both extreme classes become increasingly prominent
D.
All phenotypic classes decrease proportionally while the mean remains unchanged
Correct Answer:
Intermediate phenotypes decline while both extreme classes become increasingly prominent
Explanation:
Disruptive selection is identified by the loss of intermediate phenotypes and the increased representation of both extremes, unlike directional selection, which shifts the distribution toward one extreme.
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