biology : class12 Mean trait value changes : Directional Selection, Stabilising Selection, Disruptive Selection : Types of Natural Selection MCQ Questions and Answers
Practice important
Mean trait value changes
MCQ questions and answers.
Page 1
of
1
|
10
MCQ Questions
1. In a population, the mean beak depth is 10.0 mm before selection. After several generations, birds with deeper beaks consistently leave more offspring, while the distribution remains approximately unimodal. Which outcome most strongly indicates directional selection?
A.
The mean beak depth gradually increases above 10.0 mm
B.
The mean beak depth remains 10.0 mm while the variance decreases
C.
The mean beak depth remains 10.0 mm while two extreme peaks develop
D.
The mean beak depth fluctuates randomly around 10.0 mm without a consistent trend
Correct Answer:
The mean beak depth gradually increases above 10.0 mm
Explanation:
Directional selection consistently favors one extreme phenotype, causing the population mean trait value to shift toward that extreme.
2. A population has a mean body size of 50 g. Selection favors both very small and very large individuals equally, whereas intermediate individuals have lower reproductive success. If the two extremes contribute equally to the next generation, what is the most likely change in the mean body size?
A.
It will increase substantially above 50 g
B.
It will decrease substantially below 50 g
C.
It will remain approximately 50 g despite changes in the distribution
D.
It will first increase and then permanently decrease
Correct Answer:
It will remain approximately 50 g despite changes in the distribution
Explanation:
Under symmetrical disruptive selection, both extremes are favored equally, so their opposing effects cancel and the mean remains near the original value, even though intermediate phenotypes decline.
3. A stabilizing-selection experiment begins with a mean offspring birth mass of 3.4 kg. Individuals near 3.4 kg have the highest survival, while both lighter and heavier offspring are selected against. Which result is most consistent with this selection regime after several generations?
A.
The mean shifts steadily toward a much higher birth mass
B.
The mean shifts steadily toward a much lower birth mass
C.
The mean remains near 3.4 kg while extreme values become less common
D.
The mean separates into two distinct values around 3.4 kg
Correct Answer:
The mean remains near 3.4 kg while extreme values become less common
Explanation:
Stabilizing selection favors intermediate phenotypes. Consequently, the mean generally remains close to the intermediate optimum while variation around the mean is reduced.
4. A trait has an initial mean value of 20 units. In population X, only individuals above 20 units reproduce disproportionately. In population Y, individuals below and above 20 units are favored equally, while intermediate individuals are disfavored. Which comparison is most accurate?
A.
The mean increases in X but remains approximately unchanged in Y
B.
The mean remains unchanged in X but increases in Y
C.
The mean decreases in both X and Y
D.
The mean increases equally in both X and Y
Correct Answer:
The mean increases in X but remains approximately unchanged in Y
Explanation:
Population X experiences directional selection toward higher values, shifting the mean upward. Symmetrical disruptive selection in population Y favors both extremes equally, so the mean can remain near its initial value.
5. In a population undergoing directional selection toward smaller body size, the mean body mass changes from 80 g to 72 g over five generations. Which interpretation is best supported by this change?
A.
Selection has shifted the population mean toward the favored smaller phenotype
B.
Selection has reduced variation but has not affected the population mean
C.
Selection has produced two equally frequent body-size classes around 80 g
D.
Selection has favored the intermediate body mass of 80 g
Correct Answer:
Selection has shifted the population mean toward the favored smaller phenotype
Explanation:
A consistent decrease in the mean indicates directional change toward the smaller extreme, which is the phenotype receiving greater reproductive representation.
6. Two populations begin with the same mean leaf size. In population P, intermediate leaves are favored over both small and large leaves. In population Q, small and large leaves are favored equally over intermediate leaves. Which statement about their means is most accurate?
A.
Both populations must show a large increase in mean leaf size
B.
The mean in P and Q can remain similar even though their distributions change in opposite ways
C.
The mean in P must decrease while the mean in Q must increase
D.
The mean in P must increase while the mean in Q must decrease
Correct Answer:
The mean in P and Q can remain similar even though their distributions change in opposite ways
Explanation:
Stabilizing selection in P and symmetrical disruptive selection in Q can both leave the mean near its original value, although P concentrates values around the mean and Q increases representation of both extremes.
7. A population has phenotypic trait values with a mean of 6.0. After selection, individuals with values near 6.0 are removed more often than individuals with values near 4.0 or 8.0. If the two favored extremes are equally represented, what should happen to the mean?
A.
It should move toward 4.0
B.
It should move toward 8.0
C.
It should remain near 6.0
D.
It should move outside the range from 4.0 to 8.0
Correct Answer:
It should remain near 6.0
Explanation:
Equal selection for values symmetrically positioned below and above the initial mean produces opposing shifts that cancel, so the mean remains near 6.0 despite reduced intermediate representation.
8. A trait distribution has a mean of 100 units. After selection, the mean is 100 units, but the frequency of values between 95 and 105 units has increased while values below 90 and above 110 have declined. Which selection pattern best explains this result?
A.
Directional selection favoring values above 110 units
B.
Directional selection favoring values below 90 units
C.
Stabilizing selection favoring intermediate values
D.
Disruptive selection favoring both values below 90 and above 110 units
Explanation:
The unchanged mean combined with a concentration of phenotypes near the existing mean is characteristic of stabilizing selection, which removes both extremes.
9. Disruptive selection favors two extreme phenotypes, but the higher-valued extreme has a greater reproductive contribution than the lower-valued extreme. What is the most likely long-term effect on the population mean?
A.
The mean must remain exactly unchanged because disruptive selection always preserves the mean
B.
The mean may shift toward the higher-valued extreme
C.
The mean must shift toward the lower-valued extreme
D.
The mean must become equal to the intermediate phenotype
Correct Answer:
The mean may shift toward the higher-valued extreme
Explanation:
Disruptive selection does not necessarily keep the mean constant. If the two favored extremes are not equally represented, the more successful extreme can pull the mean toward itself.
10. A population's mean toxin concentration rises from 2.0 to 3.1 mg per gram over successive generations. The frequency of high-toxin individuals increases, while low- and intermediate-toxin individuals contribute fewer offspring. Which conclusion is most justified?
A.
The population is experiencing directional selection toward higher toxin concentration
B.
The population is experiencing stabilizing selection around 2.0 mg per gram
C.
The population is experiencing symmetrical disruptive selection with no mean shift
D.
The population is experiencing selection that necessarily reduces the mean toxin concentration
Correct Answer:
The population is experiencing directional selection toward higher toxin concentration
Explanation:
A sustained increase in the mean, together with greater reproductive contribution from high-valued individuals, indicates directional selection toward higher toxin concentration.
Short Questions and Answers
Practice short questions and answers related to
Mean trait value changes
.
Short questions are currently unavailable for this page.