Q.Explain antibiotic resistance observed in bacteria in light of Darwinian selection theory.
Concept understanding — Darwinian Natural Selection
Imagine you walk into a crowded room. Some people are naturally louder, some are quieter. Some are taller, some shorter. Now imagine that, for some reason, the room is suddenly plunged into darkness, and everyone has to find the exit by touch alone. Who do you think will get out first? Probably not the loudest talker, but the person who happens to have the most sensitive fingertips or the best memory of where the door was.
That simple scenario captures the core of Darwinian natural selection. It is not about being "better" in some moral or absolute sense. It is about being a better fit for the specific situation you are in.
The Everyday Intuition: "Survival of the Fittest" — But What Does "Fittest" Mean?
The phrase "survival of the fittest" is often misunderstood. It does not mean the strongest, fastest, or most aggressive individual wins. In biology, "fitness" has a very specific meaning: the ability to survive long enough to reproduce and leave offspring.
Think of it this way: in a game of musical chairs, the "fittest" player isn't the one who dances the best. It's the one who, when the music stops, is standing on a chair. The chair is the environment. The player's ability to grab that chair is their fitness.
Darwin never used the phrase "survival of the fittest" in his first edition of On the Origin of Species. It was coined by Herbert Spencer, a philosopher, and Darwin adopted it in later editions. Darwin's own term was "natural selection," which is a more accurate description of the process.
The Precise Meaning: How Natural Selection Works
Natural selection is not a conscious force. It is a blind, automatic process that follows from three simple facts that are always true in any population of living things:
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Variation: Individuals in a population are not identical. Even within a species, there is a range of traits — different beak sizes in birds, different fur colours in rabbits, different heights in humans. This variation is the raw material.
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Inheritance: Many of these variations are passed from parents to offspring. A tall parent is more likely to have tall children. A fast-running cheetah is more likely to have fast-running cubs.
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Differential Survival and Reproduction: More individuals are born than can possibly survive. Resources (food, water, shelter, mates) are limited. This creates a "struggle for existence." Individuals with traits that give them even a slight advantage in this struggle are more likely to survive and, crucially, to reproduce.
The result: Over many generations, the traits that helped those individuals survive and reproduce become more common in the population. The traits that were less helpful become rarer. The population gradually changes to become better suited to its environment.
Natural selection does not create new traits. It only acts on the variation that already exists. It is like a sieve, not a sculptor. The sieve lets through the grains of sand that are the right size for the hole, and holds back the rest. The environment is the sieve.
Why It Matters: The Engine of Adaptation
Natural selection is the mechanism that explains how life becomes adapted to its surroundings. It is why:
- A cactus has spines instead of leaves (to reduce water loss in a desert).
- A polar bear has white fur (to blend in with snow and ice).
- A human has an opposable thumb (to grip tools).
It is not a plan or a goal. It is a consequence of simple, observable facts playing out over vast stretches of time. The NCERT textbook for Class 12 Biology (Chapter 6, "Evolution") states this clearly: "Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny."
A Few Key Points to Remember
- It acts on individuals, but it changes populations. An individual does not evolve. The population as a whole changes over generations.
- It is not random. The variation that arises is random (mutations), but the selection itself is not. The environment "selects" which variations are helpful.
- It has no direction or purpose. It does not make organisms "better" in any absolute sense. It only makes them better for their current environment. If the environment changes, what was once a helpful trait can become a disadvantage.
- It is slow. Significant change usually takes many generations, often thousands or millions of years.
A common exam question asks: "Is natural selection the same as evolution?" The answer is no. Evolution is the fact that populations change over time. Natural selection is one of the mechanisms that causes that change. There are other mechanisms (like genetic drift and gene flow), but natural selection is the most important one for explaining adaptation.
So, the next time you see a perfectly camouflaged insect or a bird with a beak shaped exactly for the seeds it eats, remember: you are looking at the result of a blind, relentless, and beautiful process — a process that has been running for billions of years, with no goal other than the simple, powerful logic of leaving more offspring behind.
On search engines, students preparing for exams frequently type in "Darwinian Natural Selection class 12 biology", "Darwinian Natural Selection NEET questions", or "Darwinian Natural Selection: Definition, Diagram & Examples". This concept is directly part of the Evolution chapter in the NCERT/CBSE Class 12 Biology syllabus, and it is also an important topic for NEET and state medical/CET entrance exams, making it worth mastering for both board and competitive-exam preparation.
Antibiotic resistance in bacteria is a textbook example of Darwinian natural selection in action. When a population of bacteria is exposed to an antibiotic, most individuals die, but a few may already possess a random, pre-existing mutation that confers resistance. These resistant bacteria survive and reproduce, passing the resistance trait to their offspring. Over successive generations, the resistant strain becomes the dominant form in the population because it has a higher fitness in the presence of the antibiotic. The antibiotic acts as the selecting agent, and the environment (the drug) does not create the variation — it simply selects from the variation that already exists. This is exactly what Darwin described: heritable variation, differential survival and reproduction, and a change in the population over time.
Antibiotic resistance in bacteria illustrates Darwinian natural selection because pre-existing genetic variation for resistance is selected for by the antibiotic, leading to the differential survival and reproduction of resistant individuals and a consequent change in the bacterial population over time.
Antibiotic resistance in bacteria is a real-time, observable example of Darwinian natural selection, where pre-existing genetic variation is acted upon by the selective pressure of antibiotics, leading to the survival and spread of resistant strains.
To understand antibiotic resistance through Darwin's lens, we must first set the scene. Darwinian natural selection is not a force that creates new traits on demand. It works on variation that already exists within a population. In a large bacterial population, there is enormous genetic diversity. Some bacteria, by random mutation, already possess genes that allow them to survive in the presence of an antibiotic. These mutations might alter the antibiotic's target site, produce enzymes that break down the drug, or pump the drug out of the cell. Crucially, these resistant individuals are present before the antibiotic ever arrives.
Now, introduce the antibiotic. This is the selective pressure — the environmental challenge. For the vast majority of bacteria that lack the resistance gene, the antibiotic is lethal. They are eliminated. But the few resistant bacteria survive. They are not "created" by the antibiotic; they were already there, and the antibiotic simply removes their competition.
This is a common point of confusion. The antibiotic does not cause the resistance mutation. The mutation occurs randomly, and the antibiotic selects for it. This is the core of Darwin's insight: variation is random, but selection is not.
The surviving resistant bacteria now have a huge advantage. With the susceptible bacteria gone, there is less competition for resources and space. The resistant individuals reproduce, passing on their resistance genes to their offspring. Over many generations (which, for bacteria, can happen in hours or days), the entire population becomes dominated by resistant bacteria. What we observe as "antibiotic resistance" is simply the outcome of this differential survival and reproduction.
The key takeaway from Darwin's theory is that natural selection does not produce perfection, only adaptation to the current environment. If the antibiotic is removed, the resistant bacteria might be at a slight disadvantage compared to susceptible ones (the resistance gene often carries a "fitness cost"), and the population could shift back. But as long as the antibiotic is present, the resistant form is the fittest.
This process is not a metaphor. It is a direct, measurable, and rapid demonstration of natural selection in action. Every time a course of antibiotics is not completed, we are providing a perfect laboratory for this selection to occur: we kill the weakest bacteria, leaving the most resistant ones to survive and multiply.
In short, antibiotic resistance in bacteria is a textbook case of Darwinian natural selection: pre-existing genetic variation for resistance is favoured by the selective pressure of the antibiotic, leading to the survival and proliferation of resistant bacteria, while susceptible ones are eliminated.
Rather than starting from bacteria, run the argument in general terms first: any case of Darwinian selection needs three ingredients — heritable variation that already exists, a selective agent that kills or spares individuals based on that variation, and differential reproduction that shifts the population's makeup over generations. Once this three-part checklist is in mind, slot in antibiotic resistance as one example among several (industrial melanism in moths, pesticide resistance in insects) rather than treating it as a special case — the same three-step logic explains all of them.
Showing the 12 most recent of 30 on this concept.
- CBSE 2026Set 57/1/11 markMCQQ.Assertion (A) : The population of melanized moths increased in industrial areas after Industrial Revolution. Reason (R) : In Industrial environment lichen covered trees were replaced by soot-covered trees offering better camouflage to dark coloured moths. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.
›Reveal solutionSolution
The assertion and reason are both true, and the reason correctly explains why melanized moths became more common in industrial areas after the Industrial Revolution.
This question draws on one of the most famous and well-documented examples of natural selection in action — the peppered moth (Biston betularia) in England. The story is a classic illustration of how environmental change can drive evolutionary change, and it is directly covered in the NCERT Class 12 Biology textbook under the chapter on Evolution.
Before the Industrial Revolution, the typical peppered moth had a light, speckled appearance that blended beautifully with the lichen-covered bark of trees. This camouflage protected them from predatory birds. A dark, melanic form of the same species existed but was very rare, because it stood out starkly against the pale lichen and was easily spotted and eaten.
The Industrial Revolution changed the landscape dramatically. Soot and smoke from coal-burning factories coated trees and killed the lichen, turning the bark dark. Now the situation reversed: the light-coloured moths became highly visible against the soot-blackened trees, while the dark (melanized) moths were well-camouflaged. Birds now ate more of the light moths, and the dark moths survived and reproduced in greater numbers. Over several decades, the population shifted from mostly light to mostly dark in industrial areas.
NoteThis phenomenon is called industrial melanism. It is a textbook case of natural selection acting on a heritable trait, not a change acquired during the moth's lifetime.
Now look at the Assertion (A): "The population of melanized moths increased in industrial areas after Industrial Revolution." This is exactly what happened — the dark form became dominant in polluted regions.
The Reason (R) states: "In Industrial environment lichen covered trees were replaced by soot-covered trees offering better camouflage to dark coloured moths." This is also accurate. The key link is that the change in tree colour directly altered which moths survived predation. The dark moths were better hidden on soot-covered bark, so they lived longer and left more offspring.
Therefore, the reason correctly explains the assertion. The camouflage advantage was the mechanism behind the population shift.
ImportantThe NCERT textbook explicitly uses this example to explain natural selection. It states that the change in moth colour was due to better survival of the dark variety in the sooty environment — not because the moths deliberately changed colour.
Option (A) is the correct choice: both statements are true, and the reason is the correct explanation for the assertion.
✓Final answerIn short, the increase in melanized moths after the Industrial Revolution was directly caused by the improved camouflage they gained on soot-darkened trees, making the reason the correct explanation for the assertion.
- CBSE 2026Set 57/3/11 markMCQQ.Appearance of antibiotic-resistant bacteria is an example of evolution due to : (A) Adaptive radiation (B) Divergent evolution (C) Artificial selection (D) Anthropogenic action
›Reveal solutionSolution
Antibiotic-resistant bacteria evolve through anthropogenic action — human use of antibiotics creates the selective pressure that favours resistant strains, a direct example of evolution driven by human activity.
Darwin's theory of natural selection rests on a simple but powerful idea: organisms with traits better suited to their environment survive and reproduce more successfully, passing those advantageous traits to the next generation. Over time, populations change. The environment acts as the selective agent, and the result is evolution.
Now consider what happens when we introduce antibiotics into bacterial populations. Before antibiotics, a bacterial colony contains natural genetic variation — most individuals are susceptible to the drug, but a tiny fraction carry random mutations that confer resistance. These resistant bacteria have no particular advantage in the absence of antibiotics; they're just different.
The moment we administer an antibiotic, the landscape shifts entirely. The drug kills or inhibits the susceptible bacteria, but the resistant ones survive. Suddenly, resistance is no longer a neutral quirk — it's a life-or-death advantage. The resistant bacteria reproduce freely in the now-emptied niche, and within generations the population is dominated by resistant strains. The antibiotic didn't create the resistance; it simply selected for bacteria that already possessed it.
This is evolution in fast-forward, and the selective pressure is unmistakably human in origin. We manufacture the antibiotics, we prescribe them, we use them in agriculture, and we often misuse them by stopping treatment early or deploying them unnecessarily. Each use is an evolutionary experiment, and bacteria — with their rapid generation times — respond with breathtaking speed.
Let's see why the other options don't fit:
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Adaptive radiation describes the rapid diversification of a single ancestral species into multiple forms, each adapted to different ecological niches — think Darwin's finches spreading across the Galápagos. Antibiotic resistance doesn't involve branching into diverse new species; it's a single trait spreading through existing populations.
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Divergent evolution occurs when related species accumulate differences over time as they adapt to different environments, eventually becoming distinct. Again, we're not watching bacterial species diverge into separate lineages here; we're watching one trait sweep through a population.
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Artificial selection is the deliberate breeding of organisms by humans to enhance desired traits — crop improvement, dog breeds, livestock. We choose which individuals reproduce. With antibiotic resistance, we're not selecting which bacteria to breed; we're simply killing the susceptible ones and leaving the resistant ones to multiply on their own.
ImportantThe key distinction is intent and mechanism. Artificial selection involves deliberate human choice of breeding pairs. Anthropogenic action is broader — any human activity that alters selective pressures, whether intentional or not.
The term anthropogenic means "originating from human activity." Antibiotic resistance is a textbook case: humans introduced a novel environmental challenge (the drug), and natural selection did the rest. We didn't breed resistant bacteria; we created the conditions under which resistance became advantageous. The evolution is natural, but the selective pressure is entirely man-made.
✓Final answerIn short, antibiotic-resistant bacteria exemplify evolution by anthropogenic action (D) — human use of antibiotics imposes a selective pressure that favours pre-existing resistant variants, driving rapid evolutionary change in bacterial populations.
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- CBSE 2026Set 57/3/11 markMCQQ.Identify the statements that correctly describe Darwin's Theory of Evolution.(i) Overproduction of organisms leads to competition.(ii) Variation is inherited and causes evolution.(iii) Acquired characters are inherited.(iv) Survival depends on favourable traits.(v) New species arise due to accumulation of favourable variations. Choose the correct option. (A) (i), (ii),(iv) and(v) are correct (B) (ii),(iii) and(iv) are correct (C) All are correct (D) Only(i) and(iv) are correct
›Reveal solutionSolution
Darwin’s theory rests on overproduction, inherited variation, survival of the fittest, and the gradual accumulation of favourable variations — but it explicitly rejects the inheritance of acquired characters.
Darwin’s theory of evolution by natural selection is one of the most elegant and powerful ideas in biology. It explains how life changes over time without needing any guiding hand or purpose. The core logic is simple: organisms produce more offspring than can possibly survive, those offspring vary in their traits, and the environment “selects” the variants best suited to it. Over many generations, this process can give rise to entirely new species.
Let’s examine each statement in the question against what Darwin actually proposed.
Statement (i): Overproduction of organisms leads to competition.
This is absolutely correct. Darwin observed that every species has the potential to produce far more young than the environment can support. A single pair of elephants, for example, could in theory produce millions of descendants in a few centuries — yet elephant numbers remain roughly stable. The inevitable result is a struggle for existence: competition for food, shelter, mates, and safety. Overproduction sets the stage for natural selection.
Statement (ii): Variation is inherited and causes evolution.
Also correct. Darwin knew that individuals within a species differ from one another, and that many of these differences are passed from parents to offspring. He called this “inherited variation.” Without heritable variation, natural selection would have nothing to act upon — all individuals would be identical, and no change could occur. Evolution happens precisely because favourable variations are inherited and spread through a population.
Statement (iii): Acquired characters are inherited.
This is the one statement that does not belong to Darwin’s theory. The idea that traits acquired during an organism’s lifetime (like a blacksmith’s strong arm or a giraffe’s stretched neck) can be passed to offspring was proposed by Jean-Baptiste Lamarck, not Darwin. Darwin himself was unsure about the mechanism of inheritance, but he never made the inheritance of acquired characters a pillar of his theory. Modern genetics has shown that such acquired traits are not inherited — changes in body cells do not affect the DNA in eggs or sperm.
Watch outA common exam mistake is to confuse Lamarck’s idea with Darwin’s. Remember: Darwin said variation arises by chance and is then selected; Lamarck said organisms change in response to need and pass those changes on. They are fundamentally different.
Statement (iv): Survival depends on favourable traits.
Correct. This is the heart of natural selection. Individuals with traits that give them an advantage in their environment — sharper teeth, better camouflage, stronger roots — are more likely to survive and reproduce. Those without such advantages are more likely to die before reproducing. Darwin called this “survival of the fittest,” where “fittest” means best adapted to the local conditions, not necessarily the strongest.
Statement (v): New species arise due to accumulation of favourable variations.
Correct. Darwin argued that natural selection does not work overnight. Favourable variations are tiny and gradual. Over thousands or millions of generations, these small advantages accumulate. When populations become so different that they can no longer interbreed, a new species has formed. This is the process of descent with modification — the slow, steady transformation that produces the diversity of life.
ImportantDarwin’s theory is often summarised as: overproduction → variation → struggle for existence → natural selection → inheritance of favourable variations → evolution of new species. Statement (iii) is the odd one out — it belongs to Lamarck, not Darwin.
Now, looking at the options:
- Option (A) lists (i), (ii), (iv), and (v) — all correct.
- Option (B) includes (iii), which is wrong.
- Option (C) says all are correct — but (iii) is incorrect.
- Option (D) leaves out (ii) and (v), which are essential parts of Darwin’s theory.
✓Final answerThe correct option is (A): statements (i), (ii), (iv), and (v) accurately describe Darwin’s theory of evolution, while statement (iii) — the inheritance of acquired characters — is a Lamarckian idea, not a Darwinian one.
- CBSE 2026Set ANNUAL1 markQ.Which theory was given by Charles Darwin?
›Reveal solutionSolution
Darwin's theory of natural selection explains that organisms with heritable traits better suited to their environment survive and reproduce more successfully, gradually driving evolutionary change.
Charles Darwin, along with Alfred Russel Wallace, proposed the theory of Natural Selection, formally published by Darwin in 1859 in his book On the Origin of Species.
The key ideas of this theory are:
- Individuals within a population show heritable variations.
- Populations tend to produce more offspring than the environment can support, leading to a struggle for existence.
- Individuals with variations better suited ('fitter') to their environment are more likely to survive and reproduce — 'survival of the fittest'.
- Over successive generations, these favourable, heritable variations accumulate in the population, gradually leading to evolutionary change and the emergence of new species.
This theory remains the central mechanism explaining how evolution occurs, later reinforced by genetics as the modern synthesis.
✓Final answerCharles Darwin proposed the theory of Natural Selection (evolution by natural selection), published in 'On the Origin of Species'.
- CBSE 2026Set ANNUAL1 markMCQQ.For a long time, it was believed that the organisms were fixed and unchanging. Which theory was proposed by Charles Darwin to challenge this belief?(a) Theory of Pangenesis(b) Theory of Use and Disuse(c) Theory of Natural Selection(d) Theory of Inheritance of Acquired Characters
›Reveal solutionSolution
Darwin's Theory of Natural Selection (published in 'On the Origin of Species', 1859) proposed that organisms change over generations through differential survival and reproduction of the fittest variants.
Darwin observed that organisms produce more offspring than the environment can support, that individuals show heritable variations, and that individuals better suited to the environment survive and reproduce more successfully ('survival of the fittest'). Over generations, favourable variations accumulate, driving evolutionary change. This directly challenged the older belief that species were fixed and unchanging.
The other options belong to different, largely discredited theories: Pangenesis (Darwin's own separate, incorrect idea of heredity) and Use and Disuse / Inheritance of Acquired Characters are Lamarck's theory, which proposed organs develop/degenerate with use/disuse and that such acquired traits are inherited — not the theory that challenged fixity of species in the way Natural Selection did.
✓Final answer(c) Theory of Natural Selection.
- CBSE 2026Set ANNUAL1 markMCQQ.Identify the effect brought about by single step large mutation in evolution from the options given below :(a) Saltational speciation(b) Founder effect(c) Gene migration(d) Evolution by special creation
›Reveal solutionSolution
Speciation brought about by a single-step large mutation is called saltation (saltational speciation).
While Darwinian evolution emphasises gradual accumulation of small heritable variations, Hugo de Vries (working on Oenothera, the evening primrose) argued that mutation caused evolution and that large, sudden, single-step mutations — which he called saltation — could produce a new species in one step, rather than by slow, minor variations. Thus the effect of a single-step large mutation is saltational speciation.
Founder effect and gene migration relate to changes in allele frequency in populations, and "evolution by special creation" is a non-evolutionary religious idea — none of these describe a single large mutation.
✓Final answer(A) Saltational speciation.
- CBSE 2025Set A1 markQ.Match the correct pair and write the match for 'Darwin'. Column I:(i) Apomixis(ii) Darwin(iii) Toddy(iv) Agarose(v) Detritivorous. Column II:(a) Galapagos Islands(b) Southern India(c) Sea weeds(d) Earthworm(e) Grass family.
›Reveal solutionSolution
Darwin matches with (a) Galapagos Islands, the site of his famous finch observations during the voyage of the Beagle.
Charles Darwin, during his voyage aboard HMS Beagle, observed several closely related but distinct species of finches on the different islands of the Galapagos archipelago, each with beak shapes adapted to the specific food sources available on its island. These observations of adaptive radiation from a common ancestor became a cornerstone example supporting his theory of evolution by natural selection, published in 'On the Origin of Species' (1859).
✓Final answer(ii) Darwin → (a) Galapagos Islands.
- CBSE 2025Set ANNUAL1 markMCQQ.The essence of Darwinian theory about evolution is -(a) Mutation(b) Spontaneous generation(c) Natural selection(d) Chemical change
›Reveal solutionSolution
Darwin proposed that heritable variations giving a survival/reproductive advantage are naturally selected and become more common over generations.
Darwin's theory rests on the observations that populations show heritable variation, more offspring are produced than can survive, and individuals best suited to the environment survive and reproduce more successfully ('survival of the fittest'). This differential, non-random survival and reproduction of variants — natural selection — is the essence of Darwinism, driving gradual evolutionary change. Mutation is a source of variation (emphasised later by neo-Darwinism, not Darwin's own core idea), spontaneous generation is an outdated, disproved concept about the origin of life, and chemical change relates to the origin of life on early Earth, not to Darwin's mechanism of evolution.
✓Final answer(c) Natural selection.
- CBSE 2025Set ANNUAL1 markMCQQ.Assertion (A): Evolution is not occurring at present. Reason (R): Evolution takes a very long time to occur.(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).(c) Assertion (A) is true but Reason (R) is false.(d) Both Assertion (A) and Reason (R) are false.
›Reveal solutionSolution
Evolution is a continuous, ongoing process happening in populations right now, so the Assertion is false; and while large-scale (macro)evolutionary change is typically slow, evolutionary/genetic change at the population level (microevolution) can be observed within a few generations, so the blanket Reason is also not correct.
The Assertion claims evolution is not occurring at present — this is factually false. Evolution is a continuous process driven by ongoing mutation, genetic recombination, gene flow, genetic drift, and natural selection acting on populations every generation. Directly observable modern examples include the rapid rise of antibiotic-resistant bacteria, pesticide-resistant insects, and the classic industrial melanism seen in the peppered moth (Biston betularia) population within a human lifetime — all clear evidence that evolutionary change is happening today, not just in the deep geological past.
The Reason states that evolution always takes a very long time — while large morphological/macroevolutionary changes (e.g., the origin of a new phylum) do typically unfold over long geological timescales, this is not a universal rule: microevolutionary changes in allele frequencies within a population can occur rapidly, within just a handful of generations, whenever selection pressure is strong. So the reason, taken as a general/absolute statement, is also not correct.
Since neither the Assertion nor the (as-stated, absolute) Reason holds up, the correct choice is that both are false.
✓Final answer(d) Both Assertion (A) and Reason (R) are false.
- CBSE 2025Set ZOOLOGY1 markMCQQ.Which of the following is the modern concept of origin of life?(i) Special creation(ii) Spontaneous generation(iii) Biogenesis(iv) Chemical evolution
›Reveal solutionSolution
The modern concept of the origin of life is chemical evolution — life arose abiotically from simple molecules under early-Earth conditions.
The modern concept, the theory of chemical evolution proposed by Oparin and Haldane, holds that the first forms of life arose from non-living inorganic and organic molecules (chemogeny). Under the conditions of the primitive Earth — reducing atmosphere, energy from UV rays, lightning and heat — simple molecules combined to form complex organic molecules, then aggregates (coacervates/protobionts), and finally the first cells. The Miller–Urey experiment provided experimental support.
Special creation (i) is a religious/traditional view, spontaneous generation/abiogenesis (ii) was disproved by Redi/Pasteur, and biogenesis (iii) states life comes only from pre-existing life but does not explain the very first origin.
✓Final answer(iv) Chemical evolution.
- CBSE 2024Set 57/3/11 markMCQQ.During the 1850s in the pre-industrialisation era in England, the expected effect of natural selection on the number of dark-winged moths as compared to white-winged moths was : (A) more in number (B) less in number (C) both were equal in number (D) both were less in number
›Reveal solutionSolution
Before industrialisation, tree bark was light-coloured and lichen-covered, making dark-winged moths conspicuous to predators while white-winged moths were camouflaged. Natural selection favoured the white form, so dark-winged moths were less in number.
This question tests your understanding of industrial melanism in Biston betularia (the peppered moth), one of the most elegant real-world demonstrations of natural selection in action. The key is to think about the environment before the soot and pollution arrived.
Natural selection operates through differential survival: individuals whose traits make them better camouflaged against predators leave more offspring. The trait frequencies in a population shift toward whatever the environment favours at that moment.
In pre-industrial England during the 1850s, the countryside was unpolluted. Tree trunks and branches were covered with light-coloured lichens, creating a pale, mottled background. Now picture two moths resting on such a tree:
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The white-winged (light) moth blends beautifully into the lichen-covered bark. Birds hunting by sight struggle to spot it.
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The dark-winged (melanic) moth stands out starkly against the pale background—a conspicuous target for any insectivorous bird.
Predation pressure acts as the selective force. Birds preferentially eat the moths they can see most easily. The dark moths suffer higher predation rates, so fewer survive to reproduce. Over generations, the dark allele remains rare in the population because its carriers are removed before they can pass it on.
TipRemember the rule: camouflage = survival. Whichever morph matches the background at the time will be favoured by selection.
The situation reversed dramatically during industrialisation (post-1850s). Soot blackened the trees and killed the lichens, turning the environment dark. Suddenly the dark-winged moths had the camouflage advantage, and their frequency soared—a shift documented by collectors and later confirmed experimentally. But in the 1850s, before that environmental change, the light background meant light moths dominated.
Watch outDon't confuse the timeline. The question specifies the 1850s pre-industrialisation era—this is before the famous increase in melanic forms. The dark moths were still rare at this point.
✓Final answerThe correct option is (B): dark-winged moths were less in number than white-winged moths during the pre-industrialisation era.
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- CBSE 2024Set E1 markMCQQ.At which level is natural selection operated to evolve the desired traits?(a) Individual level(b) Population level(c) Community level(d) Ecosystem level
›Reveal solutionSolution
Natural selection operates at the POPULATION level, shifting the frequency of favourable traits over generations.
An individual organism cannot evolve within its own lifetime; it simply lives or dies, reproduces or does not. What changes over time is the genetic make-up of the whole POPULATION:
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Individuals carrying advantageous heritable variations survive and reproduce more successfully (differential reproduction).
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Over many generations the proportion (frequency) of those favourable alleles rises in the population.
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Thus the observable evolution of a desired trait is a property of the population, not of a single individual, community or ecosystem.
✓Final answer(b) Population level.
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