biology : class12 Adaptation and fitness : Directional Selection, Stabilising Selection, Disruptive Selection : Types of Natural Selection MCQ Questions and Answers
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Adaptation and fitness
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MCQ Questions
1. In a population of beetles, a drought causes dark, heat-absorbing soil to become common. Over several generations, beetles with darker coloration leave more surviving offspring than pale beetles. Which conclusion best describes this change in fitness?
A.
Dark coloration is an adaptation because it increases reproductive contribution under the altered environment
B.
Dark coloration is automatically advantageous in every environment
C.
All beetles have individually adapted because each beetle intentionally changed its color
D.
Fitness has increased only if the total population size has increased
Correct Answer:
Dark coloration is an adaptation because it increases reproductive contribution under the altered environment
Explanation:
An adaptation is a heritable trait that improves survival or reproductive success in a particular environment. The darker coloration increases fitness under the new soil conditions, even though it may not be advantageous everywhere.
2. A bird population has eggshell thickness ranging from thin to thick. Extremely thin shells break easily, whereas extremely thick shells reduce hatching success because oxygen exchange is limited. Birds producing intermediate-thickness shells leave the most viable offspring. Which interpretation is most accurate?
A.
Intermediate shell thickness has the highest fitness in this environment because it maximizes successful offspring production
B.
Both extreme shell thicknesses are equally adaptive because they are phenotypically distinct
C.
The population is undergoing directional selection toward the thinnest shells
D.
Fitness is determined only by the survival of adult birds, not by hatching success
Correct Answer:
Intermediate shell thickness has the highest fitness in this environment because it maximizes successful offspring production
Explanation:
Fitness reflects relative contribution to the next generation. Since intermediate shells produce the greatest number of viable offspring, that phenotype has the highest fitness, consistent with stabilising selection.
3. A fish species occupies a lake where small fish can enter narrow refuges and large fish can defend territories, but intermediate-sized fish are vulnerable to both predators and competitors. If body size is heritable, what is the most likely fitness consequence?
A.
Both small and large fish may have greater fitness than intermediate fish, maintaining two advantageous phenotypic extremes
B.
Intermediate fish will necessarily have the greatest fitness because they are closest to the population mean
C.
Only large fish can be considered adapted because body size always increases competitive ability
D.
Small fish and large fish cannot both be favoured by natural selection in the same population
Correct Answer:
Both small and large fish may have greater fitness than intermediate fish, maintaining two advantageous phenotypic extremes
Explanation:
When both extremes have higher survival or reproductive success than the intermediate phenotype, disruptive selection can favour alternative adaptations. Fitness is environment-dependent rather than determined by closeness to the mean.
4. A population of insects is exposed to a pesticide. Before exposure, susceptibility and resistance are heritable, and resistant insects reproduce more successfully after treatment. Which statement correctly distinguishes adaptation from acclimation?
A.
An increase in the frequency of resistance alleles across generations represents adaptation, whereas a reversible physiological response in one insect would be acclimation
B.
Any physiological response of an individual insect is an adaptation because it improves survival
C.
Resistance is not an adaptation unless every insect in the population becomes resistant
D.
Acclimation necessarily changes allele frequencies more rapidly than natural selection
Correct Answer:
An increase in the frequency of resistance alleles across generations represents adaptation, whereas a reversible physiological response in one insect would be acclimation
Explanation:
Adaptation is a heritable, population-level change expressed across generations. Acclimation is generally a reversible adjustment within an individual's lifetime and does not by itself change allele frequencies.
5. Two populations of the same moth species contain identical color variants. Population X lives on pale tree bark, while Population Y lives on dark, soot-covered bark. If birds are the major predators, why can the same color phenotype have different fitness in the two populations?
A.
Fitness is relative to environmental conditions, so camouflage may improve survival in one population but reduce it in the other
B.
Fitness is an intrinsic property of a phenotype and cannot vary between environments
C.
Natural selection acts on color only when the phenotype is dominant
D.
The moths in Population Y must have acquired different color genes during their lifetimes
Correct Answer:
Fitness is relative to environmental conditions, so camouflage may improve survival in one population but reduce it in the other
Explanation:
A trait contributes to adaptation only in relation to a specific environment. A color that reduces detection on pale bark can increase predation risk on dark bark, demonstrating that fitness is context-dependent.
6. In a lizard population, individuals with longer legs initially run faster and escape predators more effectively. After prey becomes concentrated in dense vegetation, shorter-legged individuals capture more prey and produce more offspring. What does this example demonstrate?
A.
The fitness advantage of a trait can reverse when environmental conditions change
B.
A trait that was once adaptive remains adaptive because adaptations cannot lose value
C.
Natural selection always favours directional change toward larger structures
D.
The lizards have become acclimated only if their allele frequencies changed within one generation
Correct Answer:
The fitness advantage of a trait can reverse when environmental conditions change
Explanation:
Adaptation and fitness are environment-dependent. Longer legs may have increased fitness in open habitats, whereas shorter legs become advantageous in dense vegetation, potentially reversing the direction of selection.
7. A population contains three heritable beak-size classes. In a particular season, birds with large beaks and birds with small beaks obtain abundant but different seed types, while birds with intermediate beaks obtain little food. Which measurement would most directly establish that the two extreme classes have higher fitness?
A.
The proportion of surviving, reproductively successful offspring produced by each beak-size class
B.
The average beak size of the adult population before the breeding season
C.
The total number of seeds present in the habitat
D.
The number of mutations detected in the beak-development genes
Correct Answer:
The proportion of surviving, reproductively successful offspring produced by each beak-size class
Explanation:
Relative fitness is measured by reproductive contribution to the next generation. Comparing surviving offspring production directly tests whether the two extreme phenotypes are favoured over the intermediate phenotype.
8. A heritable trait increases an animal's survival but also greatly reduces its mating success. Under which condition would the trait most likely increase in frequency through natural selection?
A.
The survival benefit produces a greater overall contribution to the next generation than the mating disadvantage reduces
B.
The trait increases survival regardless of whether the animal produces offspring
C.
The trait is common in the population, even if its carriers leave fewer offspring
D.
The trait is beneficial to the individual but has no effect on lifetime reproductive success
Correct Answer:
The survival benefit produces a greater overall contribution to the next generation than the mating disadvantage reduces
Explanation:
Fitness integrates effects on survival and reproduction. A trait can spread when its total effect increases lifetime reproductive success, even if it imposes a cost in one component such as mating success.
9. In a population of plants, individuals with either very shallow or very deep roots survive a fluctuating water regime better than plants with intermediate roots. Shallow roots exploit brief surface moisture, whereas deep roots reach persistent groundwater. What is the likely long-term outcome if the trait is heritable?
A.
Both root-depth extremes may be maintained because each provides an adaptive advantage under different water conditions
B.
Intermediate roots must become fixed because they represent the population average
C.
Only shallow roots will increase because surface water is always more accessible
D.
Root depth cannot affect fitness unless it changes the plant's visible appearance
Correct Answer:
Both root-depth extremes may be maintained because each provides an adaptive advantage under different water conditions
Explanation:
Different environmental conditions can favour different phenotypes. If both shallow and deep roots increase reproductive success in their respective conditions, disruptive selection can maintain alternative adaptive forms.
10. A mutation gives some bacteria resistance to an antibiotic but slows their growth when the antibiotic is absent. Which statement best predicts the fitness of resistant bacteria across alternating environments?
A.
Their relative fitness will be higher during antibiotic exposure but may be lower without exposure because fitness depends on environmental context
B.
Their fitness will be permanently higher because resistance is always beneficial once acquired
C.
Their fitness cannot change because the mutation is inherited
D.
Their resistance will be an adaptation only if it causes faster growth in every environment
Correct Answer:
Their relative fitness will be higher during antibiotic exposure but may be lower without exposure because fitness depends on environmental context
Explanation:
A heritable trait can be adaptive in one environment and costly in another. Resistance increases fitness when antibiotics are present, but its growth cost can reduce fitness when the antibiotic is absent.
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