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Phenotypic variance increase
MCQ questions and answers.
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MCQ Questions
1. A population of birds shows heritable variation in beak depth. During a drought, both very shallow and very deep beaks provide higher feeding success than intermediate beaks. If this pattern persists across generations, which change is most directly expected?
A.
A decrease in phenotypic variance because intermediate beaks become more common
B.
An increase in phenotypic variance because selection favors both phenotypic extremes
C.
No change in phenotypic variance because selection affects only survival
D.
A directional shift in the mean with no change in the distribution's spread
Correct Answer:
An increase in phenotypic variance because selection favors both phenotypic extremes
Explanation:
This is disruptive selection: individuals at both extremes are favored while intermediates are selected against, causing the population distribution to become more spread out and increasing phenotypic variance.
2. In a population, the initial phenotype distribution has mean 50 units and a standard deviation of 6 units. Selection removes individuals between 47 and 53 units but favors individuals below 47 and above 53. Assuming the two extreme groups contribute equally to the next generation, which outcome is most likely?
A.
The mean remains approximately 50 units while phenotypic variance increases
B.
The mean shifts strongly below 47 while phenotypic variance decreases
C.
The mean shifts strongly above 53 while phenotypic variance remains unchanged
D.
Both the mean and phenotypic variance must decrease
Correct Answer:
The mean remains approximately 50 units while phenotypic variance increases
Explanation:
Equal survival of both extremes can preserve the central mean, but removal of intermediate phenotypes increases the spread of the surviving distribution and therefore raises phenotypic variance.
3. Which empirical result would provide the strongest evidence that disruptive selection, rather than directional selection, has increased phenotypic variance in a population?
A.
The mean shifts upward and the variance remains unchanged
B.
The mean shifts downward and the variance decreases
C.
Intermediate phenotypes decline while both tails become relatively more frequent
D.
All phenotypes become equally frequent without a change in the mean
Correct Answer:
Intermediate phenotypes decline while both tails become relatively more frequent
Explanation:
An increase in both tails accompanied by a reduction in intermediate phenotypes is the characteristic distributional signature of disruptive selection and produces increased phenotypic variance.
4. A trait has a broad-sense heritability of 0.80. Disruptive selection increases the observed phenotypic variance from 16 to 25 units squared. Which interpretation is most appropriate?
A.
The increase in phenotypic variance can never influence future generations
B.
A substantial part of the increased variance may be genetically transmitted, although environmental variance also contributes
C.
The entire increase must be environmental because selection cannot alter variance
D.
The population must have undergone directional selection toward a single extreme
Correct Answer:
A substantial part of the increased variance may be genetically transmitted, although environmental variance also contributes
Explanation:
High broad-sense heritability indicates that much phenotypic variation is associated with genetic differences, so selection favoring extreme phenotypes can increase genetic as well as observed phenotypic variance, while environmental effects may still contribute.
5. Two populations have the same mean body size after several generations. Population X has a variance of 4 cm², whereas Population Y has a variance of 25 cm² and a distinctly bimodal distribution. Which conclusion is best supported?
A.
Population X necessarily experienced disruptive selection
B.
Population Y may have experienced disruptive selection favoring two contrasting size classes
C.
Both populations must have identical selection histories because their means are equal
D.
Population Y must have experienced stabilizing selection because its mean is unchanged
Correct Answer:
Population Y may have experienced disruptive selection favoring two contrasting size classes
Explanation:
Equal means do not imply equal variation. The much larger variance and bimodal pattern in Population Y are consistent with selection favoring both phenotypic extremes and increasing phenotypic variance.
6. A researcher compares a trait before and after selection. The mean changes from 10.0 to 10.1 units, but the variance rises from 2.0 to 7.5 units squared, with fewer individuals near the original mean and more individuals at both tails. Which process best explains the result?
A.
Strong directional selection toward larger values
B.
Stabilizing selection around the original mean
C.
Disruptive selection causing increased phenotypic variance
D.
Random genetic drift necessarily producing reduced phenotypic variance
Explanation:
The minimal change in the mean combined with a large increase in variance and enrichment of both tails indicates that both extremes were favored, which is disruptive selection.
7. If intermediate phenotypes are selectively disadvantaged but the two extreme phenotypes differ in their environmental optima, what distributional change is most likely within the selected generation?
A.
A narrower unimodal distribution centered on the previous mean
B.
A broader distribution with increased representation at both extremes
C.
A complete loss of phenotypic variation
D.
A narrower distribution shifted toward only the higher extreme
Correct Answer:
A broader distribution with increased representation at both extremes
Explanation:
When both extremes have higher fitness than intermediates, selection removes central phenotypes and increases the spread of phenotypic values, producing greater phenotypic variance.
8. Which statistical pattern most clearly distinguishes an increase in phenotypic variance caused by disruptive selection from an increase caused merely by random environmental fluctuation?
A.
A larger variance with no consistent relationship between phenotype and fitness
B.
A larger variance accompanied by higher relative fitness at both phenotypic extremes than at intermediate values
C.
A larger mean accompanied by lower fitness at the larger extreme
D.
A smaller variance accompanied by equal fitness across all phenotypes
Correct Answer:
A larger variance accompanied by higher relative fitness at both phenotypic extremes than at intermediate values
Explanation:
Disruptive selection specifically links increased variance to a fitness advantage for both extremes and a disadvantage for intermediates, unlike random environmental fluctuation, which need not show that relationship.
9. A population initially has a continuous, unimodal distribution for a heritable trait. After repeated selection, two peaks appear, one near each extreme, while the central region declines. Why can this pattern increase phenotypic variance even if the population mean changes very little?
A.
The distance of individuals from the mean increases in both directions
B.
The mean becomes identical to every individual's phenotype
C.
The environmental component of variance becomes exactly zero
D.
Selection removes all genetic differences between the two peaks
Correct Answer:
The distance of individuals from the mean increases in both directions
Explanation:
Variance measures the average squared deviation from the mean. When individuals accumulate farther from the mean on both sides, variance increases even when the mean itself remains nearly constant.
10. In an experiment, intermediate-colored insects are preyed upon more often than either very light or very dark insects. After several generations, the frequency of both light and dark phenotypes rises, and the variance of coloration increases. Which statement is most accurate?
A.
Selection has reduced variation by favoring the phenotype closest to the mean
B.
Selection has increased phenotypic variance by favoring alternative extreme phenotypes
C.
Selection has produced a purely directional response toward dark coloration
D.
Selection has increased the mean but necessarily reduced the genetic component of variance
Correct Answer:
Selection has increased phenotypic variance by favoring alternative extreme phenotypes
Explanation:
Higher predation on intermediates gives both extremes a relative fitness advantage. Their increased frequency broadens the distribution and raises phenotypic variance, the defining effect tested here.
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