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Evolution

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Chapter Notes

Overview
This chapter traces the story of life on Earth, from its chemical origin to the evolution of complex biodiversity, including humans. It explains the mechanisms of evolution, primarily natural selection, and the evidence supporting it from fossils, comparative anatomy, embryology, and biochemistry. The one big idea a student must walk away with is that evolution is a change in allele frequencies in populations over time, driven by natural selection, genetic drift, and other factors, leading to the descent of all life from common ancestors.
Key Concepts (23)
Evolution
The study of the history of life forms on Earth; the gradual change in the characteristics of a population over successive generations.
e.g. The change in the frequency of dark-winged moths in industrial areas of England over a century.
Big Bang Theory
A theory that explains the origin of the universe as a singular huge explosion, following which the universe expanded and cooled, leading to the formation of hydrogen, helium, and later galaxies.
e.g. The universe is about 13.8 billion years old and is still expanding.
Panspermia
The hypothesis that life came from outside Earth, with early Greek thinkers believing units of life called spores were transferred to different planets.
e.g. Some astronomers still consider the idea that life was seeded on Earth from space.
Theory of Spontaneous Generation
The belief that life arose from decaying and rotting matter like straw and mud; disproved by Louis Pasteur's experiments.
e.g. The idea that maggots arise spontaneously from rotting meat.
Chemical Evolution
The formation of diverse organic molecules (e.g., RNA, proteins) from inorganic constituents, proposed by Oparin and Haldane as a precursor to life.
e.g. Miller's experiment produced amino acids from methane, ammonia, hydrogen, and water vapour under electric discharge.
Theory of Special Creation
The conventional religious belief that all living organisms were created as they are today, diversity has always been the same, and Earth is about 4000 years old.
e.g. The idea that each species was individually created and does not change over time.
Natural Selection
The mechanism of evolution proposed by Darwin, where individuals with heritable traits better suited to the environment survive and reproduce more, leaving more progeny.
e.g. In a polluted area, dark-winged moths survive better than white-winged moths because they are camouflaged against soot-covered tree trunks.
Fitness (Reproductive Fitness)
According to Darwin, fitness refers ultimately and only to reproductive fitness — the ability of an individual to leave more progeny than others in a population.
e.g. A bacterium that can survive an antibiotic and reproduce is said to have higher fitness under those conditions.
Fossils
Remains of hard parts of life-forms found in rocks, providing paleontological evidence for evolution.
e.g. Fossils of dinosaurs found in sedimentary rock layers indicate they lived millions of years ago and are now extinct.
Homologous Organs
Organs with similar anatomical structure but different functions, indicating divergent evolution from a common ancestor.
e.g. The forelimbs of whales, bats, cheetahs, and humans all have humerus, radius, ulna, carpals, metacarpals, and phalanges.
Analogous Organs
Organs with different anatomical structures but similar functions, indicating convergent evolution due to similar habitats.
e.g. Wings of a butterfly and wings of a bird; eye of the octopus and eye of mammals.
Adaptive Radiation
The process of evolution of different species in a given geographical area starting from a point and radiating to other habitats.
e.g. Darwin's finches on the Galapagos Islands evolved from a seed-eating ancestor into insectivorous and vegetarian forms with different beaks.
Divergent Evolution
Evolution where the same structure develops along different directions due to adaptations to different needs, resulting in homologous organs.
e.g. The forelimbs of mammals evolving into wings in bats, flippers in whales, and arms in humans.
Convergent Evolution
Evolution where different structures evolve for the same function, resulting in analogous organs.
e.g. The flippers of penguins (birds) and dolphins (mammals) are analogous structures.
Mutation
Large, sudden, random, and directionless changes in the genetic material of an organism, proposed by Hugo de Vries as the cause of evolution (saltation).
e.g. A single-step mutation in evening primrose leading to a new species.
Saltation
The theory proposed by Hugo de Vries that mutation causes speciation in a single step, as opposed to Darwin's gradual evolution.
e.g. De Vries believed that a single large mutation could produce a new species instantly.
Gene Pool
The total of all genes and their alleles in a population.
e.g. In a population of pea plants, the gene pool includes all alleles for flower colour, seed shape, etc.
Genetic Equilibrium
A state where allele frequencies in a population remain constant from generation to generation, as described by the Hardy-Weinberg principle.
e.g. If a population is not evolving, the frequencies of alleles A and a remain stable over time.
Gene Flow (Gene Migration)
The movement of alleles from one population to another due to migration of individuals, changing gene frequencies in both populations.
e.g. Pollen carried by wind from one plant population to another introduces new alleles.
Genetic Drift
A change in allele frequency in a population due to chance events, especially significant in small populations.
e.g. A random storm kills most of a population, leaving a small group with a different allele frequency than the original.
Founder Effect
A type of genetic drift where a small group of individuals (founders) establishes a new population, resulting in a different allele frequency from the original population.
e.g. A few birds from a mainland population colonise an island; the island population has different allele frequencies due to the small sample.
Speciation
The formation of new and distinct species in the course of evolution.
e.g. The evolution of different finch species from a common ancestor on the Galapagos Islands.
Hominid
A member of the family Hominidae, which includes humans and their ancestors, characterised by upright walking and larger brains.
e.g. Homo habilis, Homo erectus, and Homo sapiens are all hominids.
Formulas & Equations (2)
p² + 2pq + q² = 1
Reads as: p squared plus 2 p q plus q squared equals one
Variables: p = frequency of allele A; q = frequency of allele a; p² = frequency of AA individuals; 2pq = frequency of Aa individuals; q² = frequency of aa individuals
Use when: Use this formula to calculate genotype frequencies in a population that is in Hardy-Weinberg equilibrium, or to detect if evolution is occurring by comparing observed frequencies to expected.
⚠ Students often forget that p + q = 1, and then incorrectly calculate p² or q² without first confirming the sum of allele frequencies is 1.
p + q = 1
Reads as: p plus q equals one
Variables: p = frequency of dominant allele A; q = frequency of recessive allele a
Use when: Use this to find the frequency of one allele when the other is known, before applying the Hardy-Weinberg equation.
⚠ Students sometimes use this equation for genotype frequencies instead of allele frequencies.
How It Works
Miller's Experiment (Chemical Evolution)
  1. Step 1: Create a closed flask containing methane (CH₄), hydrogen (H₂), ammonia (NH₃), and water vapour.
  2. Step 2: Maintain the flask at 800°C.
  3. Step 3: Pass electric discharge through the mixture to simulate lightning.
  4. Step 4: After a period, analyse the contents of the flask.
  5. Step 5: Observe the formation of amino acids.
💡 Students often forget the exact gases used: CH₄, H₂, NH₃, and water vapour. Also, the temperature is 800°C, not room temperature.
Natural Selection (Darwinian Mechanism)
  1. Step 1: Populations have built-in variation in characteristics.
  2. Step 2: Natural resources are limited, leading to competition for survival.
  3. Step 3: Individuals with characteristics that enable better survival in the environment (better fitness) outbreed others.
  4. Step 4: These advantageous characteristics are heritable and passed to the next generation.
  5. Step 5: Over many generations, the population's characteristics change, and new forms appear.
💡 Fitness in Darwinian terms is reproductive fitness — leaving more progeny — not physical strength or health.
Hardy-Weinberg Principle (Genetic Equilibrium)
  1. Step 1: Determine the frequency of each allele in a population (e.g., p for A, q for a).
  2. Step 2: Assume the population is large, random mating occurs, no mutation, no gene flow, and no natural selection.
  3. Step 3: The allele frequencies remain constant from generation to generation (p + q = 1).
  4. Step 4: The genotype frequencies are given by p² + 2pq + q² = 1.
  5. Step 5: If observed frequencies differ from expected, evolution is occurring.
💡 Students often forget that Hardy-Weinberg equilibrium assumes NO evolution is happening. Any deviation indicates evolution.
Factors Affecting Hardy-Weinberg Equilibrium (Mechanisms of Evolution)
  1. Step 1: Gene migration (gene flow) — movement of alleles between populations changes frequencies.
  2. Step 2: Genetic drift — random change in allele frequency, especially in small populations (e.g., founder effect).
  3. Step 3: Mutation — introduces new alleles, changing frequencies.
  4. Step 4: Genetic recombination — during gametogenesis, creates new combinations of alleles.
  5. Step 5: Natural selection — differential survival and reproduction of individuals with certain alleles.
💡 All five factors disturb genetic equilibrium and lead to evolution. Students often miss 'genetic recombination' as a factor.
Evolution of Man (Hominid Evolution)
  1. Step 1: About 15 mya, Dryopithecus (more ape-like) and Ramapithecus (more man-like) existed.
  2. Step 2: About 3-4 mya, man-like primates walked upright in eastern Africa (Australopithecines).
  3. Step 3: About 2 mya, Homo habilis (first human-like being, brain 650-800 cc) appeared, used stone tools.
  4. Step 4: About 1.5 mya, Homo erectus (brain ~900 cc) appeared, ate meat.
  5. Step 5: Between 1,00,000-40,000 years ago, Neanderthal man (brain ~1400 cc) lived, used hides, buried dead.
  6. Step 6: About 75,000-10,000 years ago, modern Homo sapiens arose, developed cave art (e.g., Bhimbetka).
💡 The sequence of brain size increase: Australopithecines (small) → Homo habilis (650-800 cc) → Homo erectus (~900 cc) → Neanderthal (~1400 cc) → Homo sapiens (~1400+ cc). Students often mix up the order.
Exam Traps — Don't Make These Mistakes
⚠ Confusing Homologous and Analogous Organs
✗ Wings of birds and wings of insects are homologous because they both help in flying.
→ Homology is based on similar anatomical structure and common ancestry, not similar function. Bird wings and insect wings have different structures and origins.
✓ Wings of birds and wings of insects are analogous organs because they have different anatomical structures but perform the same function (flight), indicating convergent evolution.
⚠ Misinterpreting Darwinian Fitness
✗ The fittest organisms are the strongest, fastest, or most intelligent.
→ Darwinian fitness refers specifically to reproductive fitness — the ability to leave more progeny. Strength or speed is only relevant if it leads to greater reproductive success.
✓ Fitness, according to Darwin, refers ultimately and only to reproductive fitness — those who are better fit in an environment leave more progeny than others.
⚠ Confusing Genetic Drift and Gene Flow
✗ Genetic drift is the movement of alleles due to migration of individuals.
→ Genetic drift is a change in allele frequency due to chance events, not migration. Migration of individuals causes gene flow (gene migration).
✓ Genetic drift is a random change in allele frequency due to chance, especially in small populations. Gene flow (gene migration) is the movement of alleles between populations due to migration of individuals.
⚠ Misstating the Hardy-Weinberg Equation
✗ p² + q² = 1
→ The Hardy-Weinberg equation for genotype frequencies is p² + 2pq + q² = 1. The term 2pq (frequency of heterozygotes) is often omitted by students.
✓ The correct equation is p² + 2pq + q² = 1, where p² is the frequency of AA, 2pq is the frequency of Aa, and q² is the frequency of aa.
⚠ Confusing Divergent and Convergent Evolution
✗ The wings of birds and bats are an example of convergent evolution because they have similar function.
→ Wings of birds and bats are actually homologous as forelimbs (both are modified vertebrate forelimbs), showing divergent evolution. Convergent evolution involves different structures evolving similar functions.
✓ The forelimbs of birds and bats are homologous organs, showing divergent evolution from a common ancestor. The eye of the octopus and the eye of mammals are analogous, showing convergent evolution.
⚠ Forgetting the Conditions for Hardy-Weinberg Equilibrium
✗ Hardy-Weinberg equilibrium is maintained in all natural populations.
→ Hardy-Weinberg equilibrium is an ideal state that requires specific conditions: large population size, random mating, no mutation, no gene flow, and no natural selection. Natural populations rarely meet all these conditions.
✓ Hardy-Weinberg equilibrium is maintained only in an ideal population with no evolution, requiring large population size, random mating, no mutation, no gene flow, and no natural selection. In nature, these conditions are rarely met, so evolution usually occurs.
Compare & Contrast
Structural similarity and function
Homologous Organs
Analogous Organs
Homologous organs have similar anatomical structure but different functions (divergent evolution, common ancestry). Analogous organs have different anatomical structures but similar functions (convergent evolution, no common ancestry).
Cause of change in allele frequency
Genetic Drift
Gene Flow (Gene Migration)
Genetic drift is a random change in allele frequency due to chance events (e.g., natural disaster). Gene flow is the movement of alleles between populations due to migration of individuals.
Nature of variation
Darwinian Variation
Mutation (de Vries)
Darwinian variations are small, gradual, and directional. Mutations (as per de Vries) are large, sudden, random, and directionless, leading to saltation (single-step speciation).
Exam Focus
Marks pattern: Typically 2-3 MCQs (1 mark each), 1-2 Very Short Answer Questions (1-2 marks), 1 Short Answer Question (3 marks), and 1 Long Answer Question (5 marks). Total weightage: approximately 10-12 marks.
Most asked: Evidences for evolution (fossils, homologous and analogous organs, embryology) · Hardy-Weinberg principle and its applications · Natural selection and its types (stabilising, directional, disruptive) · Adaptive radiation (Darwin's finches, Australian marsupials) · Origin of life (Miller's experiment, chemical evolution) · Evolution of man (hominid sequence)
Don't skip: Explain Hardy-Weinberg principle with the equation and factors affecting it (5 marks).; Differentiate between homologous and analogous organs with examples (3 marks).; Describe the experiment of Miller and Urey and its significance (3 marks).; Explain adaptive radiation with examples of Darwin's finches and Australian marsupials (3 marks).; Trace the evolution of man from Dryopithecus to Homo sapiens (5 marks).
One-Line Revision (read aloud before the exam)
  1. The Big Bang theory explains the origin of the universe about 13.8 billion years ago.
  2. Earth formed about 4.5 billion years ago, and life appeared about 4 billion years ago.
  3. Louis Pasteur disproved the theory of spontaneous generation by showing life comes only from pre-existing life.
  4. Oparin and Haldane proposed chemical evolution, and Miller's experiment confirmed the formation of amino acids from inorganic molecules.
  5. Darwin's theory of natural selection is based on variation, limited resources, competition, and reproductive fitness.
  6. Fossils, homologous organs, and analogous organs are key evidences for evolution.
  7. Homologous organs have similar structure but different functions (divergent evolution).
  8. Analogous organs have different structures but similar functions (convergent evolution).
  9. Adaptive radiation is the evolution of different species from a common ancestor in a geographical area, e.g., Darwin's finches.
  10. The Hardy-Weinberg principle states that allele frequencies remain constant in a population in the absence of evolution.
  11. The Hardy-Weinberg equation is p² + 2pq + q² = 1, where p and q are allele frequencies.
  12. Five factors disturb Hardy-Weinberg equilibrium: gene flow, genetic drift, mutation, genetic recombination, and natural selection.
  13. Genetic drift is a random change in allele frequency, and the founder effect is a special case.
  14. Natural selection can be stabilising, directional, or disruptive.
  15. The first cellular forms of life appeared about 2000 million years ago.
  16. The first mammals were shrew-like, and mammals became dominant after the decline of reptiles.
  17. Dryopithecus and Ramapithecus were early primates from about 15 mya.
  18. Homo habilis (650-800 cc brain) was the first human-like being, followed by Homo erectus (~900 cc).
  19. Neanderthal man had a brain size of 1400 cc and lived 1,00,000-40,000 years ago.
  20. Modern Homo sapiens arose in Africa and developed cave art about 18,000 years ago, as seen at Bhimbetka.

MCQs — 10 Questions with Answers

Q1. In Miller-Urey's 1953 experiment, which compound was NOT present in the early Earth's simulated atmosphere inside the flask?

  • A. Methane (CH₄)
  • B. Ammonia (NH₃)
  • C. Oxygen (O₂) ✓
  • D. Water vapour

Answer: C — Early Earth had a reducing atmosphere lacking free oxygen; Miller-Urey used only CH₄, NH₃, H₂O, and H₂ to simulate early conditions, proving organic molecules could form abiotically.

Q2. Which statement best explains why Darwin's theory of natural selection was revolutionary in the 19th century?

  • A. It proved that God does not exist
  • B. It showed that life forms gradually evolved from common ancestors over vast geological time through non-random survival of heritable traits ✓
  • C. It demonstrated that all organisms are identical at birth
  • D. It revealed that mutations were the only mechanism of evolution

Answer: B — Darwin's theory explained how diversity arose through gradual change, variation, and differential reproductive success—challenging the special creation theory's timescale and mechanism.

Q3. Homologous organs in mammals are those that:

  • A. Perform the same function in different animals
  • B. Share the same embryological origin and bone structure but perform different functions due to different evolutionary pressures ✓
  • C. Evolved independently to solve the same environmental problem
  • D. Have no relationship to common ancestry

Answer: B — Homologous organs (bat wing, human arm, horse leg) share common skeletal elements but diverged in form and function, proving common descent and adaptation to different lifestyles.

Q4. In the Hardy-Weinberg equation p² + 2pq + q² = 1, if the frequency of allele 'A' (p) is 0.6, what is the frequency of heterozygotes (Aa) in the population?

  • A. 0.36
  • B. 0.48 ✓
  • C. 0.16
  • D. 0.64

Answer: B — Heterozygotes are represented by 2pq; if p = 0.6, then q = 0.4, so 2pq = 2(0.6)(0.4) = 0.48.

Q5. Directional natural selection is most clearly demonstrated by the case of:

  • A. Peppered moths becoming darker in polluted industrial regions of 19th-century England ✓
  • B. Human birth weight staying around 3.5 kg despite variation in parents
  • C. Finch beak size showing bimodal distribution with both large and small beaks favoured
  • D. All organisms in a population surviving equally well regardless of trait variation

Answer: A — Dark moths had higher survival in sooty environments (directional selection), while stabilising selects middle values and disruptive selects extremes; answer B and C show other selection types.

Q6. Which piece of evidence for evolution directly shows that life forms alive today share common ancestors with extinct organisms?

  • A. Analogous organs in different species
  • B. Presence of similar embryonic structures in different vertebrate species
  • C. Fossil record showing gradual changes in hard body parts across geological layers ✓
  • D. Geographic distribution of species on different islands

Answer: C — Fossils are actual remains of extinct organisms preserved in rock layers, showing direct evidence of past life forms and gradual transformation through time periods.

Q7. Which condition would NOT violate the Hardy-Weinberg principle and would result in no evolutionary change?

  • A. Random mating with no mutation, selection, migration, or genetic drift occurring ✓
  • B. Mutation introducing new alleles at a constant rate
  • C. Natural selection eliminating harmful recessive alleles
  • D. Migration of individuals from nearby populations bringing in new alleles

Answer: A — Hardy-Weinberg requires no mutations, selection, migration, or drift plus random mating; any violation of these conditions causes allele frequency change, leading to evolution.

Q8. Read the following statements: Assertion (A): Fitness in evolutionary biology refers to an organism's physical strength and ability to fight competitors. Reason (R): Natural selection favours traits that increase reproductive success and passing of alleles to offspring. Which is correct?

  • A. Both A and R are true, and R explains A
  • B. Both A and R are true, but R does not explain A
  • C. A is false; R is true and correctly defines fitness ✓
  • D. Both A and R are false

Answer: C — Fitness is reproductive output (number of viable fertile offspring), not physical strength; the misconception in A is a classic Class 12 exam trap.

Q9. Which of the following observations would provide the STRONGEST evidence that two mammal species share a recent common ancestor?

  • A. Both species have wings for flying
  • B. Both species have similar bone structure in limbs, nearly identical embryos in early stages, and similar DNA sequences ✓
  • C. Both species are found on the same continent
  • D. Both species eat the same type of food

Answer: B — Homologous bone structures, embryological similarity, and genetic similarity (high DNA match) all indicate common ancestry; sympatry and diet are due to convergence or competition, not ancestry.

Q10. If a population of insects experiences heavy insecticide spraying that kills 90% of light-coloured insects but only 10% of dark-coloured insects, and the population size remains stable over generations, which type of natural selection is occurring?

  • A. Stabilising selection favouring the middle phenotype
  • B. Disruptive selection favouring both extreme phenotypes
  • C. Directional selection favouring the dark phenotype ✓
  • D. Artificial selection imposed by humans for pest control

Answer: C — Selective pressure (insecticide) consistently favours one extreme (dark colour), shifting allele frequencies in one direction—the definition of directional selection.

Flashcards

What is the Oparin-Haldane theory?

First life arose from non-living organic molecules (RNA, proteins) through chemical evolution under early Earth's reducing atmosphere with high temperature, lightning, and volcanic activity.

Why did Miller-Urey's 1953 experiment matter for evolution?

It demonstrated that amino acids (life's building blocks) could form spontaneously from inorganic molecules (CH₄, NH₃, H₂O, H₂) under conditions resembling early Earth, supporting chemical evolution.

Define natural selection in Darwin's terms.

Organisms with heritable traits better suited to their environment survive and reproduce more successfully, passing those advantageous traits to offspring, causing population change over time.

What is the difference between homologous and analogous organs?

Homologous organs have the same evolutionary origin and bone structure but different functions (bat wing, human arm); analogous organs have different origins but similar functions (bird and butterfly wings).

What does the Hardy-Weinberg equation p² + 2pq + q² = 1 predict?

Allele frequencies in a population remain constant generation to generation if there is no mutation, selection, migration, or genetic drift, meaning no evolution occurs.

Name three types of natural selection.

Directional selection (one extreme trait favoured), stabilising selection (middle trait favoured, extremes eliminated), and disruptive selection (both extreme traits favoured, middle eliminated).

What embryological evidence supports common ancestry?

Embryos of fish, reptiles, and mammals look nearly identical in early stages and develop similarly, showing they share a common ancestor despite later divergence into different forms.

How did the fossil record help Darwin?

Fossils showed that extinct life forms existed in the past, species changed gradually over geological time, and Earth must be billions of years old, not 4000 years.

What does 'fitness' mean in evolutionary biology?

Fitness refers only to reproductive success—how many viable, fertile offspring an organism produces—not physical strength or intelligence.

Why is the ozone layer formation important in Earth's early history?

The ozone layer blocked UV radiation from the sun, allowing life to move from water to land by protecting cells from UV damage that would kill unprotected organisms.

Important Board Questions

Define natural selection. How does it lead to evolution of a population? [2 marks]

State: heritable variation exists in population; environment selects organisms better suited to survive; they reproduce more, passing advantageous traits to offspring; allele frequencies change over generations = evolution. Include one example like dark peppered moths.

Explain the Oparin-Haldane theory with reference to Miller-Urey's experiment. What did this experiment prove about the origin of life? [5 marks]

Explain: early Earth had reducing atmosphere (CH₄, NH₃, H₂O, H₂), high temperature, lightning, volcanic activity → these conditions allowed chemical evolution of organic molecules. Miller-Urey replicated these conditions in lab (flask, electric discharge, 800°C) and produced amino acids, proving that life's building blocks could form non-biologically from inorganic matter, supporting the idea that life did not require divine creation but arose from chemistry. Mention that such compounds were also found in meteorites, confirming the process occurs in space.

Using the Hardy-Weinberg principle and the concept of natural selection, explain how and why allele frequencies change in a real population over time. Give an example of how one factor violates Hardy-Weinberg equilibrium and leads to evolution. [6 marks]

State Hardy-Weinberg equation (p² + 2pq + q² = 1) and explain it predicts constant allele frequencies if no forces act. List five violations: mutation (introduces new alleles), natural selection (favours some alleles over others), genetic drift (random change in small populations), migration (adds/removes alleles), non-random mating (changes genotype frequencies). Detailed example: if peppered moths face insecticide spraying killing light moths, frequency of dark allele increases each generation because dark moths survive and reproduce more successfully (directional selection); this is evolution. Show with numbers: if initial dark allele frequency (p) = 0.3, after 5 generations of selection p might = 0.7, violating Hardy-Weinberg and proving the population evolved. Conclude: real populations always have one or more violated conditions, so evolution is inevitable.

Assertion-Reason Questions for Evolution

Q1. Assertion (A):

The theory of spontaneous generation proposed that life arose from non-living matter such as decaying straw and mud.

Reason (R):

Louis Pasteur's experiments with pre-sterilised flasks demonstrated that life can only come from pre-existing life.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The assertion correctly describes spontaneous generation theory. The reason explains why this theory was rejected—Pasteur's experiments proved life only comes from pre-existing life. The reason directly explains why spontaneous generation was dismissed, making it the correct explanation for why the assertion's theory was wrong.

Q2. Assertion (A):

Earth's early atmosphere was a reducing atmosphere containing methane, ammonia, hydrogen, and water vapour.

Reason (R):

The reducing atmosphere was essential because it provided the necessary conditions for the chemical evolution of organic molecules as proposed by Oparin and Haldane.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true. Early Earth did have a reducing atmosphere (assertion is correct). This reducing atmosphere was specifically necessary for chemical evolution to occur, as energy sources like UV rays and electric discharge could act on these inorganic compounds to form organic molecules (reason is correct and explains the assertion).

Q3. Assertion (A):

S.L. Miller's 1953 experiment demonstrated the formation of amino acids under simulated early Earth conditions.

Reason (R):

This experiment proved that life had spontaneously generated in the laboratory flask.

Answer: C — A is true but R is false — The assertion is true—Miller did demonstrate amino acid formation. However, the reason is false because Miller's experiment demonstrated chemical evolution (formation of organic molecules), not spontaneous generation of life. The experiment created organic building blocks, not living organisms. Therefore, assertion is true but reason is false.

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Case-Based Questions (CBQ) for Evolution

Case Study:

A research team studying meteorite samples found organic compounds similar to those produced in S.L. Miller's experiments. They also analysed the composition of early Earth's atmosphere using geological evidence and found it to be rich in methane, ammonia, and water vapour. The team then simulated these conditions in a controlled laboratory environment and observed the formation of amino acids and other organic molecules over several weeks. These findings support the hypothesis that similar chemical processes might occur on other planets with similar atmospheric conditions.

Q1. Which theory is most strongly supported by the evidence presented in this research passage?

  • A. The theory of spontaneous generation
  • B. The Oparin-Haldane theory of chemical evolution ✓
  • C. The theory of special creation
  • D. The panspermia hypothesis

Answer: B

Q2. The presence of organic compounds in meteorites is significant because it suggests:

  • A. Life currently exists on meteorites
  • B. Chemical evolution processes similar to those on early Earth may occur elsewhere in space ✓
  • C. Meteorites are the original source of all life on Earth
  • D. Miller's experiment was incorrect in its findings

Answer: B

Q3. According to the passage, what was the most critical factor enabling the formation of organic molecules in the laboratory simulation?

  • A. The reproduction of early Earth's atmospheric composition (CH₄, NH₃, H₂O) under laboratory conditions ✓
  • B. The presence of living microorganisms in the flask
  • C. The use of modern technological equipment
  • D. The absence of oxygen in the laboratory

Answer: A

Q4. The research team's findings that similar chemical processes might occur on other planets implies which of the following?

  • A. Life must have originated on multiple planets simultaneously
  • B. All planets with organic molecules currently support living organisms
  • C. Chemical evolution could be a universal process under appropriate atmospheric conditions ✓
  • D. Meteorites are the only source of organic compounds in the universe

Answer: C

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Previous Year Questions (PYQ) for Evolution

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 57/1/1 · 3 marks

(a) How is Hardy-Weinberg expression (p2 + 2pq + q2) = 1 derived? (2 marks) (b) List any two factors that disturb the genetic equilibrium. (1 mark)

CBSE 2026 · Set 57/1/3 · 3 marks

How is Hardy-Weinberg expression (p2 + 2pq + q2) = 1 derived? List any two factors that disturb the genetic equilibrium.

CBSE 2026 · Set 57/3/1 · 2 marks

(b) The allele for brown eyes (B) is dominant over blue eyes (b). In a population, 36% of individuals have blue eyes. Assuming Hardy-Weinberg equilibrium, calculate: (i) Frequency of allele B (ii) Percentage of heterozygous individuals (iii) Percentage of individuals that are homozygous dominant (Working to be shown)

CBSE 2026 · Set 57/3/2 · 1 mark

Assertion (A) : Hardy-Weinberg principle says that allele frequencies in a population are stable. Reason (R) : Hardy-Weinberg principle is applicable only when genetic drift does not occur.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 57/3/2 · 2 marks

(b) The allele for brown eyes (B) is dominant over blue eyes (b). In a population, 36% of individuals have blue eyes. Assuming Hardy-Weinberg equilibrium, calculate : (i) Frequency of allele B (ii) Percentage of heterozygous individuals (iii) Percentage of individuals that are homozygous dominant (Working to be shown)

CBSE 2026 · Set 57/1/1 · 1 mark

The idea of use and disuse of organs for evolution of organism was proposed by

  • Charles Darwin
  • Thomas Malthus
  • Hugo De Vries
  • Lamarck

CBSE 2026 · Set 57/1/1 · 1 mark

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.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 57/1/2 · 1 mark

Assertion (A): When we see stars, we are apparently peeping into the past. Reason (R): What we see today is an object whose emitted light started its journey millions of years back reaching our eyes now.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 57/1/3 · 1 mark

Which one of the following evidences does not support Darwin's theory of natural selection?

  • Branching Descent
  • Small & directional variations
  • Appearance of new traits by mutations
  • Survival of the fittest

CBSE 2026 · Set 57/1/3 · 1 mark

Assertion (A): Fossils provide direct evidence for evolution. Reason (R): Fossils help to trace evolutionary relationships among organisms over time.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 57/2/1 · 1 mark

What conclusion can be drawn if the forelimbs of an aquatic animal and a terrestrial animal have similar anatomical structure ?

  • Live in same habitat
  • Share common ancestors
  • Forelimbs perform same functions
  • Forelimbs are analogous

CBSE 2026 · Set 57/2/1 · 3 marks

S.L. Miller conducted an experiment in 1953 to simulate the conditions of early earth in a laboratory setup. Answer the following questions based on the experiment : (a) What was the aim of his experiment ? (b) Name the gases used and the source of energy.

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Class 12 Biology Previous Year Questions, Chapter-wise

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