biology : class12 Selection pressure direction : Directional Selection, Stabilising Selection, Disruptive Selection : Types of Natural Selection MCQ Questions and Answers
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Selection pressure direction
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MCQ Questions
1. In a beetle population, birds preferentially remove the lightest-coloured individuals because they are conspicuous on dark soil. If colour is heritable and other factors remain constant, what is the direction of selection pressure?
A.
Against the light-coloured extreme, favouring darker phenotypes
B.
Against both colour extremes, favouring intermediate phenotypes
C.
Against the dark-coloured extreme, favouring lighter phenotypes
D.
Equally against all colour phenotypes, producing no directional change
Correct Answer:
Against the light-coloured extreme, favouring darker phenotypes
Explanation:
Selection pressure is directed against one extreme phenotype, the light-coloured beetles. Therefore, darker phenotypes receive a relative survival advantage, producing directional selection toward darker colour.
2. A drought eliminates plants with very shallow roots, while plants with increasingly deep roots obtain more water and leave more offspring. Which description correctly identifies the pressure direction?
A.
Pressure is centred on the intermediate-root phenotype and opposes both extremes
B.
Pressure is directed against shallow roots and favours the deep-root extreme
C.
Pressure is directed against deep roots and favours shallow roots
D.
Pressure alternates equally between shallow and deep roots, maintaining both extremes
Correct Answer:
Pressure is directed against shallow roots and favours the deep-root extreme
Explanation:
The environmental pressure consistently disadvantages shallow roots while favouring progressively deeper roots. This one-sided pressure shifts the population toward the deep-root direction.
3. In a fish population, predators capture both the smallest and largest individuals more frequently, whereas medium-sized fish survive best. Relative to the phenotype distribution, how is selection pressure directed?
A.
Toward both size extremes simultaneously
B.
Only toward the largest-size extreme
C.
Against both extremes and toward the intermediate-size phenotype
D.
Only toward the smallest-size extreme
Correct Answer:
Against both extremes and toward the intermediate-size phenotype
Explanation:
Selection pressure acts from both directions by removing small and large fish. It therefore favours the intermediate phenotype rather than shifting the population toward one size extreme.
4. Suppose an environmental change makes very small and very large seeds easier for a bird to handle, while medium-sized seeds are difficult to process. What direction of selection pressure is expected on seed size?
A.
Pressure against both extremes, favouring medium-sized seeds
B.
Pressure toward only the smallest seeds
C.
Pressure toward only the largest seeds
D.
Pressure against the intermediate phenotype, favouring both extremes
Correct Answer:
Pressure against the intermediate phenotype, favouring both extremes
Explanation:
The intermediate seed phenotype is selectively disadvantaged, while both extremes gain a survival or reproductive advantage. The pressure therefore acts away from the centre toward both extremes.
5. A population initially contains short, intermediate, and tall plants. Grazing animals remove mostly tall plants, but after a change in grazing behaviour they remove mostly short plants. What determines the direction of selection in each period?
A.
Selection is always stabilising because two plant extremes are present
B.
The direction depends on which extreme is preferentially removed during that period
C.
Selection is always disruptive because grazing affects plant height
D.
Selection direction cannot be inferred unless the population size is known
Correct Answer:
The direction depends on which extreme is preferentially removed during that period
Explanation:
Selection pressure direction is identified by the phenotype being disadvantaged. Removing tall plants favours shorter plants, whereas removing short plants favours taller plants; the direction can therefore change with the environmental pressure.
6. In a bird population, a prolonged cold period causes individuals with low-fat reserves to die, while birds with moderately and very high reserves survive. Which conclusion about selection pressure is most accurate?
A.
The pressure is directed against the low-reserve extreme, not necessarily toward an intermediate optimum
B.
The pressure is directed equally against low and high reserves
C.
The pressure is directed against high reserves and favours low reserves
D.
The pressure favours only the intermediate-reserve phenotype
Correct Answer:
The pressure is directed against the low-reserve extreme, not necessarily toward an intermediate optimum
Explanation:
Only the low-reserve extreme is specifically disadvantaged. Since high-reserve birds also survive, the pressure is one-sided against low reserves rather than stabilising around an intermediate phenotype.
7. A pollutant becomes increasingly concentrated in a lake, and fish with greater detoxification capacity survive and reproduce more successfully across the measured range. Which pattern best describes the direction of selection pressure?
A.
Pressure shifts toward greater detoxification capacity, favouring one phenotypic extreme
B.
Pressure shifts toward intermediate detoxification capacity and removes both extremes
C.
Pressure removes the high-capacity extreme and favours low capacity
D.
Pressure acts only against intermediate-capacity fish and favours both extremes
Explanation:
As pollutant concentration increases, higher detoxification capacity provides increasing advantage. The pressure is therefore directional, acting toward the high-capacity extreme.
8. Two environments occur side by side: dry patches favour plants with short leaves, while moist patches favour plants with long leaves; plants with intermediate leaves perform poorly in both. What is the direction of selection pressure across the combined habitat?
A.
Pressure is exclusively toward short leaves
B.
Pressure is exclusively toward long leaves
C.
Pressure is away from the intermediate phenotype and toward both leaf-length extremes
D.
Pressure is against both short and long leaves and toward intermediate leaves
Correct Answer:
Pressure is away from the intermediate phenotype and toward both leaf-length extremes
Explanation:
Different environmental conditions favour opposite extremes, while the intermediate phenotype is disadvantaged in both settings. Selection pressure therefore acts in two directions away from the centre.
9. A change in soil colour causes pale moths to become more visible to predators, while dark moths remain camouflaged. If predation is the principal selective agent, which statement correctly describes the pressure?
A.
Predation is directed against the pale extreme and shifts selection toward dark coloration
B.
Predation is directed against both colour extremes and shifts selection toward intermediate coloration
C.
Predation is directed against the dark extreme and shifts selection toward pale coloration
D.
Predation has no directional effect because colour variation remains present
Correct Answer:
Predation is directed against the pale extreme and shifts selection toward dark coloration
Explanation:
The predator selectively removes the pale phenotype, so the pressure is directed against that extreme. Dark moths are relatively favoured, producing a shift toward darker coloration.
10. In a population with a heritable body-size trait, a new predator can capture only medium-sized individuals efficiently; very small individuals hide in narrow spaces and very large individuals deter attack. What does this reveal about selection-pressure direction?
A.
Pressure is directed toward the medium-sized phenotype
B.
Pressure is directed away from the medium-sized phenotype toward both extremes
C.
Pressure is directed only toward the large-sized phenotype
D.
Pressure is directed only toward the small-sized phenotype
Correct Answer:
Pressure is directed away from the medium-sized phenotype toward both extremes
Explanation:
Medium-sized individuals experience the greatest disadvantage, while both small and large individuals are protected. Thus, selection pressure acts away from the intermediate phenotype in two opposite directions.
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