Organisms and Populations — Class 12 MCQs with Answers

Organisms and Populations — important questions & MCQs with answers (Class 12 Biology)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Both species benefit:

   APredationBMutualismCParasitismDCompetition
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   Answer: Mutualism

   Explanation: Mutualism is the +/+ interaction — both partners gain, as bees do collecting nectar while pollinating flowers; confusing it with commensalism (+/0), where only one side benefits, is the common slip.

2. Q2 of 6Easy

   Exponential growth occurs when:

   AAlwaysBResources are scarceCPopulation is stableDResources are unlimited
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   Answer: Resources are unlimited

   Explanation: Exponential growth needs unlimited food, space and no predators, producing an unchecked J-shaped curve; real populations rarely sustain this once resources start becoming limiting.

3. Q3 of 6Medium

   Commensalism (+/0):

   AOne benefits, other unaffectedBBoth benefitCBoth harmedDNone affected
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   Answer: One benefits, other unaffected

   Explanation: Commensalism (+/0) benefits one partner while the other is neither helped nor harmed, e.g. barnacles riding on a whale for transport; mixing this up with mutualism, where both sides gain, is the classic trap.

4. Q4 of 6Medium

   Logistic growth has:

   AJ-shapedBS-shaped (sigmoid) curve with carrying capacityCLinearDRandom
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   Answer: S-shaped (sigmoid) curve with carrying capacity

   Explanation: Logistic growth follows dN/dt = rN(K−N)/K, giving an S-shaped (sigmoid) curve that flattens as N nears carrying capacity K — unlike unlimited exponential growth's J-shaped curve, which never levels off.

5. Q5 of 6Hard

   Gause's competitive exclusion principle:

   ANoneBAll species share resources equallyCPredators always winDTwo species with identical niches cannot coexist indefinitely
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   Answer: Two species with identical niches cannot coexist indefinitely

   Explanation: Gause's competitive exclusion principle holds that two species competing for the same limiting resource in an identical niche cannot coexist indefinitely — the more efficient competitor eventually eliminates the other.

6. Q6 of 6Hard

   Behavioural adaptation:

   ABody shape onlyBPermanent thick furCHibernation in winterDColour blue
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   Answer: Hibernation in winter

   Explanation: Hibernation is a behavioural adaptation — the animal actively lowers activity and metabolic rate to conserve energy through winter cold, unlike a permanent thick coat, which is a fixed structural adaptation, not a behaviour.

Organisms and Populations notes: key concepts (Class 12 Biology)

Organisms and populations: how abiotic factors shape distribution and the adaptations (regulate, conform, migrate, suspend) organisms show; population attributes — density, natality, mortality, age pyramids; exponential and logistic growth; and interactions — mutualism, competition, predation, parasitism and commensalism.

Responses to abiotic factors

Organisms regulate (constant internal state, e.g., mammals), conform (vary with surroundings), migrate, or suspend life (hibernation, aestivation, diapause).

Adaptations

Kangaroo rat conserves water; desert plants use CAM and thick cuticle; Allen's and Bergmann's rules link body shape/size to cold. Acclimatisation is short-term.

Population density

Number of individuals per unit area or volume at a given time; may be measured relatively (pug marks, % cover) when exact counting is impractical.

Natality and mortality

Natality = births adding to a population; mortality = deaths. Density rises with natality + immigration and falls with mortality + emigration.

Age pyramids

Graph of pre-reproductive, reproductive and post-reproductive groups; a broad base = expanding, bell = stable, narrow base = declining population.

Exponential growth

Under unlimited resources dN/dt = rN gives a J-shaped curve; r is the intrinsic rate of natural increase (birth rate minus death rate).

Logistic growth

Under limited resources dN/dt = rN(K−N)/K gives an S-shaped (sigmoid) curve levelling at K; growth is fastest when N = K/2.

Carrying capacity (K)

The maximum population size the environment can sustain; growth slows as N nears K, and dN/dt becomes zero when N = K.

Mutualism (+/+)

Both species benefit; e.g., lichens (fungus + alga), mycorrhiza (fungus + roots) and the obligate fig-wasp pollination partnership.

Competition (−/−)

Both suffer over shared resources; Gause's competitive exclusion says identical niches can't coexist, but resource partitioning allows coexistence.

Predation (+/−)

Predator kills and eats prey; it controls prey numbers and keeps diversity. Prey defend by camouflage, mimicry, thorns or toxins.

Parasitism (+/−)

Parasite benefits at the host's cost; often host-specific with special adaptations. Brood parasitism — the cuckoo lays eggs in a crow's nest.

Commensalism (+/0)

One benefits, the other is unaffected; e.g., an orchid on a mango branch, barnacles on a whale, or clownfish among sea-anemone tentacles.

Mimicry and camouflage

Camouflage — prey blends with its surroundings; mimicry — a harmless species resembles a harmful one (Batesian) to avoid being eaten.