Chapter 6: Evolution Application Questions | biology Class 12 CBSE

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Chapter 6: Evolution Application Questions | biology Class 12 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A doctor notices that a common antibiotic is no longer effective against a bacterial strain that was previously susceptible. With reference to Darwin's theory of natural selection, explain how this resistance could have developed.

The bacterial population originally contained individuals with random pre-existing mutations, some of which happened to confer resistance to the antibiotic. These variants were present in small numbers before antibiotic exposure, as mutations are random and directionless.
When the antibiotic was introduced, sensitive bacteria were killed, but the resistant variants survived and reproduced. Over multiple generations — which in bacteria can mean millions of individuals within hours — resistant variants outgrew the sensitive population, changing population characteristics drastically.
This is a classic example of natural selection by anthropogenic action: the antibiotic acted as the selection pressure, and the heritable resistance trait was favoured. Evolution in bacteria occurs in months or years (not centuries) due to short generation times, explaining the rapid emergence of resistant strains.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 2

Applied Concept

A biologist studying two isolated islands finds that both islands have a predatory carnivore — one is a placental mammal and the other is a marsupial — but they look strikingly similar. Explain this observation using the concepts of adaptive radiation and convergent evolution.

This is an example of convergent evolution: two unrelated lineages (placental and marsupial mammals) independently evolved similar body plans and ecological roles (carnivore) in response to similar environmental pressures and niches available on each island.
Each island underwent its own adaptive radiation, where the ancestral stock on that island diversified into available ecological niches, including that of an apex carnivore. The marsupial lineage in one island and the placental lineage in the other independently filled the same ecological role.
A real-world parallel is the Tasmanian wolf (marsupial) and the placental wolf of North America, described in the NCERT chapter. These animals look similar not because of shared ancestry but because similar habitats exerted similar selection pressures, producing analogous structures — a hallmark of convergent evolution.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 3

Applied Concept

A population genetics study finds that in a small island population of birds, the frequency of a particular wing-colour allele changed dramatically after a cyclone drastically reduced the population. Identify the evolutionary force at work and explain its consequence.

The evolutionary force at work is genetic drift — a change in allele frequency due to chance events (the cyclone) rather than natural selection. In small populations, random sampling effects are powerful: the few survivors may carry a very different allele frequency profile than the original population.
Since the surviving population is small, its gene pool is a non-representative subset of the original. This violates Hardy-Weinberg equilibrium, as allele frequencies will differ from expected values, indicating evolutionary change has occurred by chance.
If the small surviving population becomes the founding group for subsequent generations and its allele frequency is very different from the original, this is termed the founder effect. Over time, if the population remains isolated, the changed allele frequencies may lead to such divergence that a new species appears — illustrating how genetic drift can drive speciation without any adaptive advantage.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 4

Applied Concept

Fossil evidence from different sedimentary rock layers of a given region shows that early rock layers contain only simple marine invertebrates, while upper layers contain fish, then amphibians, and finally reptiles. How does this pattern support the theory of evolution?

The sequential appearance of increasingly complex life forms in progressively younger rock layers is direct palaeontological evidence for evolution. It shows that life forms did not all appear simultaneously (as special creation claims) but arose at different geological periods in a pattern consistent with evolutionary descent.
Different-aged rock sediments contain fossils of different life forms that died during the formation of those sediments. Certain life forms are restricted to specific geological time spans, confirming that species arose, persisted, and were replaced by new forms over time.
The sequence observed — marine invertebrates, then fish (~350 mya), then amphibians (fish with lobefins invading land), then reptiles — precisely matches the evolutionary timeline described in the NCERT chapter. This congruence between geological strata and the predicted evolutionary sequence provides strong evidence for the gradual evolution of vertebrate life from marine origins to terrestrial forms.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 5

Applied Concept

A farmer uses the same pesticide every year and initially finds it effective. After a few years, the pesticide no longer controls the pest population. Explain this observation using the principles of evolution discussed in the chapter.

The pest population originally contained individuals with pre-existing random mutations that happened to confer pesticide resistance. These were rare before pesticide use, as mutations arise randomly and not in response to need.
Repeated pesticide application created a strong selection pressure: susceptible individuals were killed, while resistant ones survived and reproduced. Over a few years — a short time that represents many pest generations — resistant variants became the dominant form in the population, rendering the pesticide ineffective.
This mirrors antibiotic resistance in bacteria and is described in the chapter as 'evolution by anthropogenic action.' The excess use of herbicides, pesticides, and antibiotics selects for resistant varieties in much shorter time scales than classical evolution, because microbes and insects have short generation times and large population sizes, allowing rapid spread of advantageous mutations.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

With reference to Darwin's finches, explain how a single ancestral species colonising the Galapagos Islands could give rise to multiple species with different ecological roles.

When the ancestral seed-eating finch colonised the Galapagos Islands, different subpopulations began occupying different ecological niches — trees, ground, different food sources — on the various islands. Variation within the population provided the raw material for selection.
In each habitat, different beak shapes conferred feeding advantages: a strong, thick beak for crushing seeds, a slender beak for catching insects, a broader beak for eating fruit. Natural selection favoured birds whose beak shape matched their available food, leading to differential reproduction and divergence.
Over many generations, geographically and ecologically isolated subpopulations diverged sufficiently to become distinct species — a process called adaptive radiation. Darwin observed many varieties of finches on the same island, conjecturing they all evolved from a single ancestor. This example illustrates how adaptive radiation from one ancestral stock can produce multiple species, each occupying a distinct niche in the same geographical area.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 7

Applied Concept

In a population of 1000 individuals, allele A has a frequency of 0.6 and allele a has a frequency of 0.4. Using the Hardy-Weinberg principle, calculate the expected frequency of heterozygous individuals.

According to the Hardy-Weinberg principle, if p = frequency of allele A = 0.6 and q = frequency of allele a = 0.4, then genotype frequencies are: AA = p² = 0.36, Aa = 2pq, aa = q² = 0.16.
The frequency of heterozygous individuals (Aa) = 2pq = 2 × 0.6 × 0.4 = 0.48. Therefore, 48% of the population is expected to be heterozygous (Aa).
In a population of 1000 individuals, this means 480 individuals are expected to be heterozygous. If the observed frequency differs significantly from 0.48, it indicates that one or more of the five Hardy-Weinberg disturbing factors (gene flow, genetic drift, mutation, recombination, or natural selection) is acting on this locus, causing evolutionary change.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 8

Applied Concept

A comparative anatomist studying the forelimbs of a whale, a bat, a cheetah, and a human finds they all contain the same bones arranged similarly, despite serving very different functions. What evolutionary conclusion can be drawn, and what term is used for this type of structure?

The presence of the same bones (humerus, radius, ulna, carpals, metacarpals, phalanges) in the same relative arrangement in the forelimbs of these four mammals, despite performing entirely different functions (swimming, flying, running, manipulating), is strong evidence of common ancestry.
These are called homologous structures — organs that share a similar anatomical plan due to descent from a common ancestor, even though they perform different functions in different organisms. Homology is a product of divergent evolution, where the same ancestral structure was modified differently in each lineage.
The anatomical similarity cannot be explained by coincidence or by similar function (analogy) since the structures look the same even though the functions are vastly different. The most parsimonious explanation is that all four mammals inherited this limb structure from a common mammalian ancestor, and each lineage subsequently modified it through natural selection to suit its ecological niche.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 9

Applied Concept

Scientists collected moths in England in 1850 and found predominantly white-winged moths. A collection from the same area in 1920 showed predominantly dark-winged moths. What evolutionary mechanism does this demonstrate and how?

This is the classic example of industrial melanism demonstrating natural selection in action. Before industrialisation, trees were covered with white lichen; white-winged moths were camouflaged against this background and escaped predators, while dark-winged moths were visible and eaten.
After industrialisation, soot darkened tree trunks, eliminating the lichen; now white-winged moths were conspicuous and were preferentially preyed upon, while dark-winged moths were camouflaged and survived. This change in selection pressure reversed the population frequency within 70 years.
The observation is further supported by the fact that in rural, non-industrialised areas, the count of dark moths remained low — confirming that the environment (not chance) drove the change. This is a direct demonstration of natural selection: in a mixed population, those that better adapt to the current environment survive, reproduce more, and increase in frequency over generations.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 10

Applied Concept

A palaeontologist discovers fossils of organisms with features intermediate between fish and amphibians — with both fins and rudimentary limbs. How do such transitional fossils support the theory of evolution?

Transitional fossils show intermediate morphological features between two known groups, providing direct evidence of evolutionary linkage. The described organism, with both fins and rudimentary limbs, could represent the lobefin fish described in the chapter — fish with stout, strong fins that could move on land around 350 mya.
According to palaeontological evidence, different-aged rock sediments contain fossils of different life-forms; the presence of organisms with transitional features in sediments of the appropriate geological age confirms the predicted evolutionary sequence from fish to amphibians.
Special creation theory cannot explain transitional fossils — if species were separately created, intermediates would not exist. Their existence is elegantly explained by evolution: as fish populations gradually moved onto land, natural selection favoured stronger fins that could support weight, leading over many generations to limb-like appendages and eventually true amphibian limbs.

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Chapter sections

Chapter 6: Evolution | Application Questions

A doctor notices that a common antibiotic is no longer effective against a bacterial strain that was previously susceptible. With reference to Darwin's theory of natural selection, explain how this resistance could have developed.

A biologist studying two isolated islands finds that both islands have a predatory carnivore — one is a placental mammal and the other is a marsupial — but they look strikingly similar. Explain this observation using the concepts of adaptive radiation and convergent evolution.

A population genetics study finds that in a small island population of birds, the frequency of a particular wing-colour allele changed dramatically after a cyclone drastically reduced the population. Identify the evolutionary force at work and explain its consequence.

Fossil evidence from different sedimentary rock layers of a given region shows that early rock layers contain only simple marine invertebrates, while upper layers contain fish, then amphibians, and finally reptiles. How does this pattern support the theory of evolution?

A farmer uses the same pesticide every year and initially finds it effective. After a few years, the pesticide no longer controls the pest population. Explain this observation using the principles of evolution discussed in the chapter.

With reference to Darwin's finches, explain how a single ancestral species colonising the Galapagos Islands could give rise to multiple species with different ecological roles.

In a population of 1000 individuals, allele A has a frequency of 0.6 and allele a has a frequency of 0.4. Using the Hardy-Weinberg principle, calculate the expected frequency of heterozygous individuals.

A comparative anatomist studying the forelimbs of a whale, a bat, a cheetah, and a human finds they all contain the same bones arranged similarly, despite serving very different functions. What evolutionary conclusion can be drawn, and what term is used for this type of structure?

Scientists collected moths in England in 1850 and found predominantly white-winged moths. A collection from the same area in 1920 showed predominantly dark-winged moths. What evolutionary mechanism does this demonstrate and how?

A palaeontologist discovers fossils of organisms with features intermediate between fish and amphibians — with both fins and rudimentary limbs. How do such transitional fossils support the theory of evolution?

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