Evolution Class 12: Concepts, Hardy-Weinberg Problems and Board Answers

What Evolution is about

Weightage: This chapter is part of Unit VII, Genetics and Evolution, which carries 20 of the 70 theory marks together with Principles of Inheritance and Variation and Molecular Basis of Inheritance.Source: CBSE Biology (044) syllabus 2026-27, course structure, Unit VII Genetics and Evolution

In real board papers: asked in 8 of the 8 board papers we hold (2023 to 2026), on average 5.1 marks per paper. See the questions

Key concepts

Origin of the universe, earth and life

NCERT section 6.1 Origin of Life

Universe: about 13.8 billion years old. The Big Bang theory says it began with a single huge explosion; as it expanded, it cooled; hydrogen and helium formed; gases condensed under gravity into galaxies.

Early earth (formed about 4.5 billion years ago in the solar system of the Milky Way):
1. No atmosphere at first; water vapour, methane, carbon dioxide and ammonia released from the molten mass covered the surface.
2. UV rays split water into hydrogen and oxygen; the lighter H₂ escaped.
3. Oxygen combined with ammonia and methane to form water, CO₂ and others; the ozone layer formed.
4. As earth cooled, water vapour fell as rain and filled depressions to form oceans.
5. Life appeared about 500 million years after earth formed, almost 4 billion years ago.

Ideas about the origin of life:

• Panspermia: units of life ('spores') came to earth from other planets (early Greek thinkers; still liked by some astronomers).
• Spontaneous generation: life arises from decaying matter (straw, mud). Louis Pasteur disproved it: in pre-sterilised flasks, no life came from killed yeast; in a flask open to air, new organisms arose. So life comes only from pre-existing life.
• Chemical evolution (Oparin of Russia and Haldane of England): the first life came from pre-existing non-living organic molecules (e.g. RNA, protein), formed from inorganic constituents. Early conditions: high temperature, volcanic storms, a reducing atmosphere with CH₄, NH₃ etc.

Miller's experiment (S.L. Miller, 1953): in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800 °C, he passed an electric discharge and observed the formation of amino acids. Others later obtained sugars, nitrogen bases, pigments and fats. Meteorites contain similar compounds, suggesting such processes occur elsewhere in space. So chemical evolution was more or less accepted.

After chemical evolution: how the first self-replicating, metabolic 'capsule' arose is unknown. The first non-cellular forms may have appeared about 3 billion years ago (giant molecules: RNA, protein, polysaccharides). The first cellular forms probably did not appear until about 2000 million years ago; they were probably single cells, all in water. This view, that life arose slowly from non-living molecules (abiogenesis), is accepted by the majority.

Worked example

Describe Miller's experiment. What did it show, and why is it said to support the Oparin–Haldane hypothesis? (3 marks)

Try it first, then show the solutionSolution

1. Miller created early-earth-like conditions: a closed flask with CH₄, H₂, NH₃ and water vapour at 800 °C, with electric discharge (like lightning). (1)
2. Amino acids formed; similar experiments later produced sugars, nitrogen bases, pigments and fats. (1)
3. Oparin and Haldane proposed that organic molecules formed from inorganic ones under such reducing conditions (chemical evolution) before life arose; Miller showed this can happen. (1)

Answer: Electric discharge through CH₄, H₂, NH₃ and water vapour at 800 °C produced amino acids, showing that organic molecules can form abiotically, as chemical evolution proposes.

Darwin's theory and earlier ideas

NCERT section 6.2 Evolution of Life Forms – A Theory; 6.5 Biological Evolution; 6.6 Mechanism of Evolution

Special creation (from religious literature) has three ideas: (1) all living organisms were created as they are; (2) diversity has always been the same and will stay the same; (3) earth is about 4000 years old. These were strongly challenged in the 19th century.

Darwin's observations (voyage on H.M.S. Beagle): living forms share similarities among themselves and with forms that lived millions of years ago; many past forms are extinct; new forms arose at different times; life forms have evolved gradually.

Darwin's reasoning (natural selection):
1. Facts: natural resources are limited; populations are fairly stable in size except for seasonal changes; members of a population vary (no two are alike); most variations are inherited.
2. Populations could grow exponentially if all reproduced maximally (as in bacteria), but real sizes are limited, so there must be competition for resources.
3. Individuals with heritable variations that use resources better (better adapted) survive and leave more progeny.
4. Over many generations, the population's characteristics change, and new forms appear.

Fitness, for Darwin, means reproductive fitness: leaving more progeny. Fitness depends on inherited characters, so there must be a genetic basis for selection.

Two key concepts: branching descent and natural selection. Alfred Wallace, working in the Malay Archipelago, reached similar conclusions at about the same time. Thomas Malthus' work on populations may have influenced Darwin.

Rate of evolution depends on life span: bacteria multiply into millions in hours, so a variant (B) that survives a change in the medium can outgrow the rest (A) in days; in fish or fowl, with life spans of years, the same would take millions of years.

Lamarck (French naturalist, before Darwin): evolution driven by use and disuse of organs and inheritance of acquired characters; example, giraffes stretching their necks to reach leaves and passing on long necks. Nobody accepts this now.

Hugo de Vries (evening primrose, early 20th century): mutations, large differences arising suddenly, cause evolution. Mutations are random and directionless; Darwin's variations are small and directional. Darwin's evolution was gradual; de Vries' mutations cause speciation in a single step, called saltation. Population genetics later reconciled these views.

Worked example

Differentiate between Darwin's variations and de Vries' mutations. Why is Lamarck's explanation of the giraffe's long neck rejected? (3 marks)

Try it first, then show the solutionSolution

1. Darwin: variations are small, heritable and directional; evolution is gradual. (1)
2. de Vries: mutations are large, sudden, random and directionless; a single step can create a new species (saltation). (1)
3. Lamarck said giraffes stretched their necks and passed on the acquired longer neck; acquired (non-genetic) characters are not inherited, because only changes in the genetic material pass to offspring. (1)

Answer: Darwin: small directional variations, gradual change; de Vries: large random mutations, saltation; Lamarck rejected because acquired characters are not inherited.

Evidences for evolution

NCERT section 6.3 What are the Evidences for Evolution?

1. Palaeontological evidence (fossils): fossils are remains of hard parts of life forms found in rocks. Sedimentary rocks form layers, one above the other; different-aged layers hold fossils of different life forms (e.g. extinct dinosaurs). Life forms changed over time and some are restricted to certain geological periods, so new forms arose at different times. Fossil age is found by radioactive dating.

2. Embryological evidence: Ernst Haeckel proposed that all vertebrate embryos, including humans, show features absent in adults, such as a row of vestigial gill slits behind the head (functional only in fish). Karl Ernst von Baer disapproved of this: embryos never pass through the adult stages of other animals.

3. Comparative anatomy and morphology:

• Homologous organs: same basic structure, different functions; result of divergent evolution; indicate common ancestry.
• Forelimbs of whales, bats, cheetah and humans: all have humerus, radius, ulna, carpals, metacarpals and phalanges.
• Vertebrate hearts and brains.
• Plants: thorn of Bougainvillea and tendril of Cucurbita.
• Analogous organs: different structure, similar function; result of convergent evolution (similar habitats select similar adaptive features).
• Wings of butterfly and bird.
• Eye of octopus and of mammals.
• Flippers of penguins and dolphins.
• Sweet potato (root modification) and potato (stem modification).

4. Molecular (biochemical) evidence: similar proteins and genes performing a given function in diverse organisms point to common ancestry.

5. Artificial selection: humans have bred plants and animals (e.g. many dog breeds) that are still the same kind. If humans can create new breeds in hundreds of years, nature could do the same over millions of years.

6. Natural selection observed: industrial melanism in England.

• 1850s (before industrialisation): more white-winged moths. 1920 (after industrialisation): more dark-winged (melanised) moths in the same area.
• Explanation: predators spot a moth against a contrasting background. Before industrialisation, tree trunks were covered by whitish lichens, so white moths were camouflaged and dark ones eaten. Industrial smoke and soot killed the lichens and darkened trunks, so dark moths survived.
• Supporting fact: in rural (unindustrialised) areas melanic moths stayed rare. No variant is completely wiped out. Lichens are pollution indicators: they do not grow in polluted areas.

7. Evolution by anthropogenic action: excess use of herbicides and pesticides selects resistant varieties; antibiotics and drugs select resistant microbes and cells, within months or years, not centuries. This shows evolution is stochastic (based on chance events and chance mutations), not directed.

Worked example

Classify as homologous or analogous, giving the type of evolution: (a) forelimbs of whale and bat, (b) wings of butterfly and bird, (c) thorn of Bougainvillea and tendril of Cucurbita, (d) sweet potato and potato. (2 marks)

Try it first, then show the solutionSolution

1. (a) Homologous: same bones (humerus to phalanges), different functions; divergent evolution. (½)
2. (b) Analogous: different structures, same function (flight); convergent evolution. (½)
3. (c) Homologous: both are modified stems/axillary structures with different functions; divergent evolution. (½)
4. (d) Analogous: root modification versus stem modification, both storing food; convergent evolution. (½)

Answer: (a) homologous, divergent (b) analogous, convergent (c) homologous, divergent (d) analogous, convergent.

Adaptive radiation

NCERT section 6.4 What is Adaptive Radiation?

Adaptive radiation: the evolution of different species in a given geographical area, starting from a point (an ancestral stock) and literally radiating into other habitats.

Darwin's finches (Galapagos Islands): Darwin found many varieties of small black birds on the same island. He suggested all evolved on the island itself from an original seed-eating form; different beaks arose, producing insectivorous and vegetarian finches. This is one of the best examples.

Australian marsupials: many different marsupials (e.g. Tasmanian wolf, marsupial mole, koala, kangaroo, sugar glider and others) evolved from one ancestral stock, all within the Australian island continent.

Convergent evolution between radiations: when more than one adaptive radiation occurs in an isolated area (in different habitats), we can call it convergent evolution. In Australia, placental mammals also radiated, and many look similar to a corresponding marsupial, e.g. placental wolf and Tasmanian wolf (marsupial).

Why Australian marsupials survived: due to continental drift, Australia was isolated, so its pouched mammals faced no competition from other mammals. (By contrast, South American mammals resembling horse, hippopotamus, bear and rabbit were overridden by North American fauna when the two continents joined.)

Flow chart: ancestral stock → isolation in one area → different habitats/food → different adaptations → many species (adaptive radiation).

Worked example

Explain adaptive radiation with reference to Darwin's finches. Is the similarity between a Tasmanian wolf and a placental wolf an example of homology or of convergence? (3 marks)

Try it first, then show the solutionSolution

1. Adaptive radiation: evolution of different species in one geographical area from a common ancestor, radiating into different habitats. (1)
2. Darwin's finches on the Galapagos evolved from an original seed-eating form; beaks became adapted to different foods, producing insectivorous and vegetarian finches. (1)
3. The two wolves belong to different lines (marsupial and placental) that radiated separately; their similarity arose because they adapted to similar ways of life: convergent evolution. (1)

Answer: Finches radiated from a seed-eating ancestor into different beak types; the two wolves show convergent evolution.

Mechanism of evolution and types of natural selection

NCERT section 6.6 Mechanism of Evolution; 6.7 (gene flow, genetic drift, types of natural selection)

Sources of variation: mutation (random, sudden changes in DNA) and recombination during gametogenesis. Microbial experiments show that pre-existing advantageous mutations, when selected, produce new phenotypes and, over generations, speciation.

Factors that change allele frequencies:
1. Gene migration / gene flow: when part of a population moves to another place and population, allele frequencies change in both. New alleles join the new population and are lost from the old. Repeated migration = gene flow.
2. Genetic drift: the same kind of change in allele frequency, but by chance. Sometimes the drifted sample is so different that it becomes a new species; the original drifted population are the founders and this is the founder effect.
3. Mutation and 4. Genetic recombination: create new variation.
5. Natural selection: heritable variations that improve survival and reproduction get passed on more.
Coupled with greater reproductive success, these make a population look different over time.

Three types of natural selection (on a bell-shaped curve of a trait):

• Stabilising: more individuals acquire the mean (average) value; extremes are removed; the curve gets narrower and taller around the mean.
• Directional: more individuals acquire a value other than the mean (one extreme); the peak shifts in one direction.
• Disruptive: more individuals acquire the peripheral values at both ends; the middle is selected against; the curve may become two-peaked.

Evolution: process or result? When we describe the world, evolution is a process; when we describe life on earth, it is the result of natural selection. Whether to regard these as processes or end results of unknown processes is still not clear.

Worked example

In a bird population, birth weight shows a bell-shaped curve. Over many years, very light and very heavy chicks die more often, and the curve becomes taller and narrower at the same mean. Name the type of selection, and sketch in words how the curve would change under directional and disruptive selection. (3 marks)

Try it first, then show the solutionSolution

1. This is stabilising selection: intermediate (mean) weights are favoured, extremes removed, so the curve narrows around the same mean. (1)
2. Directional: one extreme is favoured; the peak shifts towards that end (mean value changes). (1)
3. Disruptive: both extremes are favoured and the middle removed; the curve flattens and may form two peaks. (1)

Answer: Stabilising; directional shifts the peak to one side; disruptive splits it towards both ends.

Hardy–Weinberg principle

NCERT section 6.7 Hardy-Weinberg Principle

Principle: in a population, the allele frequencies at a locus stay constant from generation to generation. The gene pool (all genes and alleles in a population) stays constant: this is genetic equilibrium (Hardy–Weinberg equilibrium).

Algebra (two alleles A and a):

• Frequency of A = p; frequency of a = q; p + q = 1.
• Frequency of AA = p² (probability that A is on both chromosomes); aa = q²; Aa = 2pq.
• p² + 2pq + q² = 1, the binomial expansion of (p + q)².

Meaning: if measured frequencies differ from expected ones, the direction and size of the difference indicate the extent of evolutionary change. A change in allele frequency = evolution.

Five factors that disturb the equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection.

Method for problems:
1. Start from the homozygous recessive frequency, because only aa can be recognised by phenotype: q = √(q²).
2. p = 1 − q.
3. AA = p², Aa = 2pq; multiply by population size for numbers.
4. Check that p² + 2pq + q² = 1.

Worked check (two methods): if 16% of a population shows the recessive trait, q² = 0.16, so q = 0.4 and p = 0.6. AA = 0.36, Aa = 2 × 0.6 × 0.4 = 0.48, aa = 0.16. Sum = 1.00. Second method: allele frequency from genotypes: p = AA + ½Aa = 0.36 + 0.24 = 0.60, matching.

Worked example

In a population of 1000 people in Hardy–Weinberg equilibrium, 90 have a recessive trait (aa). Calculate p, q and the expected numbers of AA and Aa individuals. (3 marks)

Try it first, then show the solutionSolution

1. q² = 90/1000 = 0.09, so q = 0.3 and p = 1 − 0.3 = 0.7. (1)
2. AA = p² = 0.49 → 490 people; Aa = 2pq = 2 × 0.7 × 0.3 = 0.42 → 420 people. (1)
3. Check 1: 490 + 420 + 90 = 1000. Check 2 (allele count): A alleles = 2 × 490 + 420 = 1400 of 2000 → p = 0.7. (1)

Answer: p = 0.7, q = 0.3; AA = 490, Aa = 420 (aa = 90).

A brief account of evolution of life forms

NCERT section 6.8 A Brief Account of Evolution

Timeline (mya = million years ago):

• ~2000 mya: first cellular forms. How non-cellular aggregates of giant molecules became membrane-bound cells is unknown. Some cells could release O₂, probably like the light reaction of photosynthesis (splitting water with solar energy). Single cells later became multicellular.
• ~500 mya: invertebrates formed and active.
• ~350 mya: jawless fish probably evolved.
• ~320 mya: sea weeds and a few plants existed. The first organisms to invade land were plants; they were widespread when animals came on land.
• ~350 mya: fish with stout, strong fins could move onto land and go back to water. These lobefins evolved into the first amphibians. A Coelacanth, thought extinct, was caught in South Africa in 1938.
• Amphibians (ancestors of frogs and salamanders) → reptiles, which lay thick-shelled eggs that do not dry up in the sun. Modern descendants: turtles, tortoises, crocodiles.
• For the next ~200 million years, reptiles of many shapes and sizes dominated. Giant ferns (pteridophytes) fell and slowly formed coal deposits. Some reptiles returned to water as fish-like reptiles about 200 mya (e.g. Ichthyosaurs). Land reptiles were the dinosaurs; the biggest, Tyrannosaurus rex, was about 20 feet tall.
• ~65 mya: dinosaurs suddenly disappeared; the reason is unknown (climatic change, or many evolved into birds; perhaps both).
• First mammals were like shrews; viviparous and more intelligent in sensing and avoiding danger. When reptiles declined, mammals took over.
• Continental drift: when South and North America joined, South American mammals were overridden by North American fauna; Australian marsupials survived because of no competition.
• Some mammals live wholly in water: whales, dolphins, seals, sea cows.

Plant line (NCERT sketch): from early unicellular forms through algae, bryophytes, pteridophytes and seed ferns to gymnosperms and angiosperms over geological periods.

Worked example

Arrange in the order in which they appeared: reptiles, jawless fish, first cellular life, lobefins/amphibians, invertebrates, mammals. Give one reason why reptiles could live fully on land. (2 marks)

Try it first, then show the solutionSolution

1. First cellular life (~2000 mya) → invertebrates (~500 mya) → jawless fish (~350 mya) → lobefins and amphibians → reptiles → mammals. (1)
2. Reptiles lay thick-shelled eggs that do not dry up in the sun, so they need not return to water to breed. (1)

Answer: Cellular life → invertebrates → jawless fish → lobefins/amphibians → reptiles → mammals; shelled eggs freed reptiles from water.

Origin and evolution of man

NCERT section 6.9 Origin and Evolution of Man

Sequence (oldest first):
1. Dryopithecus and Ramapithecus (~15 mya): hairy primates that walked like gorillas and chimpanzees. Ramapithecus was more man-like; Dryopithecus more ape-like.
2. Man-like primates in eastern Africa (~3–4 mya): fossils from Ethiopia and Tanzania show hominid features; probably not taller than 4 feet but walked upright.
3. Australopithecines (~2 mya): lived in East African grasslands; hunted with stone weapons but mainly ate fruit.
4. Homo habilis: the first human-like being (hominid); brain 650–800 cc; probably did not eat meat.
5. Homo erectus (~1.5 mya): fossils found in Java in 1891; brain about 900 cc; probably ate meat.
6. Neanderthal man: brain 1400 cc; lived in the Near East and Central Asia between 1,00,000 and 40,000 years ago; used hides to protect the body and buried their dead.
7. Homo sapiens: arose in Africa, moved across continents and developed into distinct races. Modern Homo sapiens arose during the ice age, 75,000–10,000 years ago.

Cultural milestones: prehistoric cave art about 18,000 years ago (e.g. Bhimbetka rock shelter, Raisen district, Madhya Pradesh); agriculture about 10,000 years ago, with human settlements.

Skull comparison (NCERT figure): the skull of a baby chimpanzee is more like an adult human skull than like an adult chimpanzee skull.

Trends: increasing brain size (650 → 900 → 1400 cc), upright walking, change in diet (fruit → meat), tool use, care of the dead, art, language and self-consciousness. The evolution of modern man appears to parallel the evolution of the human brain and language.

Worked example

Arrange in order of evolution and give the brain capacity where known: Homo erectus, Neanderthal man, Homo habilis, Ramapithecus. Which of them first buried their dead? (3 marks)

Try it first, then show the solutionSolution

1. Order: Ramapithecus (~15 mya) → Homo habilis → Homo erectus (~1.5 mya) → Neanderthal man (1,00,000–40,000 years ago). (1)
2. Brain capacity: Homo habilis 650–800 cc; Homo erectus about 900 cc; Neanderthal 1400 cc (none given for Ramapithecus). (1)
3. Neanderthal man buried their dead (and used hides for protection). (1)

Answer: Ramapithecus → Homo habilis (650–800 cc) → Homo erectus (900 cc) → Neanderthal (1400 cc); Neanderthals buried their dead.

Evolution in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Adaptive Radiation and Convergent Evolution
• Natural Selection and Its Types
• Hardy-Weinberg Principle

Evolution: key facts

Hardy–Weinberg equations

$p+q=1;p^2+2pq+q^2=1$

p² = AA, 2pq = Aa, q² = aa.

Finding q from the recessive phenotype

$q=\sqrt{\frac{\text{number of aa}}{\text{total}}};p=1-q$

Start with aa, because AA and Aa look the same.

Allele frequency from genotype counts

$p=\frac{2(AA)+(Aa)}{2N};q=\frac{2(aa)+(Aa)}{2N}$

Use when the population may not be in equilibrium; N = number of individuals.

Key dates

$\text{universe}\approx 13.8\text{billion yr};\text{earth}\approx 4.5\text{billion yr};\text{life}\approx 4\text{billion yr ago}$

First cellular forms about 2000 mya; dinosaurs vanished about 65 mya.

Brain capacity in human evolution

$\text{H. habilis}650\text{–}800\text{cc}\to \text{H. erectus}900\text{cc}\to \text{Neanderthal}1400\text{cc}$

Homo erectus fossils: Java, 1891.

Homology versus analogy

$\text{homologous: same structure, different function (divergent)};\text{analogous: different structure, same function (convergent)}$

Homology shows common ancestry.

Evolution: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Evolution

The question patterns to expect, and the method that works for each.

Tricks to remember Evolution

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Homo = Same build, different job; Ana = Another build, same job

  Homologous: same structure, different function, divergent, common ancestry. Analogous: different structure, same function, convergent.

  Use it when: Classifying organs.

• Miller's soup: 'Mighty Hot Natural Water' + spark

  Methane (CH₄), Hydrogen (H₂), Ammonia (NH₃), Water vapour, at 800 °C, with electric discharge → amino acids.

  Use it when: Miller's experiment.

• Five disturbers of equilibrium: 'Many Girls Never Run Slow'

  Migration (gene flow), Genetic drift, Natural selection, Recombination, Spontaneous mutation.

  Use it when: Factors affecting Hardy–Weinberg equilibrium.

• Selection shapes: S-D-D

  Stabilising squeezes to the middle; Directional drags to one side; Disruptive divides to both ends.

  Use it when: Graph questions on types of selection.

• Human line: 'Dry Rams Ate Hot Eggs, Never Sour'

  Dryopithecus, Ramapithecus, Australopithecus, Homo habilis, Homo erectus, Neanderthal, Homo sapiens.

  Use it when: Order of human evolution.

• Brain grows 650 → 900 → 1400

  habilis 650–800 cc, erectus 900 cc, Neanderthal 1400 cc.

  Use it when: Brain capacity questions.

Evolution: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Evolution

The slips examiners see most often, and what to write instead.

• ✗Mistake: Starting a Hardy–Weinberg problem from the dominant phenotype

  Wrong working

  84% show the dominant trait, so p² = 0.84 and p = 0.92.

  Right working

  84% dominant means 16% recessive: q² = 0.16, q = 0.4, p = 0.6. The dominant 84% includes both AA (36%) and Aa (48%).

  ✓Do this: Always start from q² (the recessive phenotype).

  Usually costs: Most of the marks in the numerical.

• ✗Mistake: Calling the thorn of Bougainvillea and tendril of Cucurbita analogous

  ✓Do this: They are homologous (same origin, different functions), showing divergent evolution.

  Usually costs: 1 mark.

• ✗Mistake: Saying industrial pollution caused moths to mutate and turn dark

  ✓Do this: Both forms already existed; pollution changed which form was camouflaged, so predators selected against the white form (natural selection).

  Usually costs: 1 mark.

• ✗Mistake: Confusing gene flow with genetic drift

  ✓Do this: Gene flow is change by migration (repeated movement of alleles between populations); genetic drift is change by chance, especially in small populations (founder effect).

  Usually costs: 1 mark.

• ✗Mistake: Writing that Homo habilis ate meat and Homo erectus did not

  ✓Do this: Homo habilis probably did not eat meat; Homo erectus probably did.

  Usually costs: ½ mark.

• ✗Mistake: Mixing up adaptive radiation and convergent evolution

  ✓Do this: Adaptive radiation: many species from one ancestor in one area. Convergent evolution: unrelated groups becoming similar (e.g. marsupial and placental wolves).

  Usually costs: 1 mark.

NCERT exercise guide: Evolution

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Evolution: previous year questions

• Asked in 8 of the 8 board papers we hold (2023 to 2026), on average 5.1 marks per paper; 19 questions in all, counting OR alternatives. In 4 of 4 sample papers.

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

Solved board questions from Evolution

Real questions from recent papers. Try each one first, then open our step-by-step solution with marks per step.

Every Evolution question, by year

Practice questions on Evolution

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which gases did Miller use in his experiment?
   (a) CH₄, H₂, NH₃, water vapour (b) CO₂, O₂, N₂, water vapour (c) CH₄, O₂, NH₃, CO₂ (d) H₂, O₂, N₂, CH₄

   Hint

   A reducing atmosphere.

   Answer

   (a) CH₄, H₂, NH₃, water vapour

2. Q2 · 1 mark · Assertion-reasonBoard level

   Assertion (A): Homologous organs indicate common ancestry.
   Reason (R): Homologous organs perform the same function in different organisms.
   (a) Both A and R are true and R is the correct explanation of A.
   (b) Both A and R are true but R is not the correct explanation of A.
   (c) A is true but R is false.
   (d) A is false but R is true.

   Hint

   Same structure, different function.

   Answer

   (c) A is true but R is false: homologous organs have the same structure but different functions.

3. Q3 · 1 mark · MCQHarder board level

   If the frequency of the dominant allele is 0.7 in a population in equilibrium, the frequency of heterozygotes is:
   (a) 0.21 (b) 0.42 (c) 0.49 (d) 0.09

   Hint

   2pq.

   Answer

   (b) 0.42 (2 × 0.7 × 0.3)

4. Q4 · 2 marks · Very short answerBoard level

   Distinguish between divergent and convergent evolution with one example each. (2 marks)

   Hint

   Homology versus analogy.

   Answer

   Divergent: the same ancestral structure develops in different directions for different needs, giving homologous organs, e.g. forelimbs of whale, bat, cheetah and human (1). Convergent: different structures evolve for the same function in similar habitats, giving analogous organs, e.g. wings of butterfly and bird (1).

5. Q5 · 2 marks · Very short answerHarder board level

   Give two reasons why Darwin said that fitness means reproductive fitness, and why selection needs a genetic basis. (2 marks)

   Hint

   Leaving progeny; inheritance.

   Answer

   Survival matters to evolution only if the survivor leaves more progeny, so the better-fit leave more offspring and their characters spread (1). Only inherited characters pass to the progeny, so the variation selected must have a genetic basis for the population to change (1).

6. Q6 · 2 marks · Very short answerHarder board level

   Explain von Baer's objection to Haeckel's embryological evidence. (2 marks)

   Hint

   Adult stages.

   Answer

   Haeckel proposed that vertebrate embryos show features of other adults, such as vestigial gill slits behind the head (functional only in fish) (1). Von Baer showed by careful study that embryos never pass through the adult stages of other animals, so this proposal was disapproved (1).

7. Q7 · 3 marks · Short answerBoard level

   Trace the evolution of man from Dryopithecus to modern Homo sapiens with one feature of each stage. (3 marks)

   Hint

   Seven stages.

   Answer

   Dryopithecus (ape-like) and Ramapithecus (man-like), ~15 mya, hairy, walked like gorillas; man-like primates in East Africa ~3–4 mya, upright, under 4 feet (1). Australopithecines ~2 mya, stone weapons, ate fruit; Homo habilis, 650–800 cc, no meat; Homo erectus ~1.5 mya, 900 cc, ate meat (1). Neanderthal, 1400 cc, used hides, buried dead; Homo sapiens arose in Africa; modern forms in the ice age 75,000–10,000 years ago; cave art ~18,000 years ago, agriculture ~10,000 years ago (1).

8. Q8 · 3 marks · Short answerHarder board level

   In a population of 400 snails, 36 have a recessive shell colour. Assuming equilibrium, find the number of heterozygous snails. Verify your answer by an allele count. (3 marks)

   Hint

   q² = 36/400.

   Answer

   q² = 36/400 = 0.09, q = 0.3, p = 0.7 (1). Aa = 2pq × 400 = 0.42 × 400 = 168; AA = 0.49 × 400 = 196; total 196 + 168 + 36 = 400 (1). Allele check: a alleles = 2 × 36 + 168 = 240 of 800 = 0.3 (1).

9. Q9 · 5 marks · Long answerHarder board level

   (a) Describe the three types of natural selection with graphs. (3)
   (b) Explain how gene flow and genetic drift change allele frequencies. (2) (5 marks)

   Hint

   Mean, one side, both ends.

   Answer

   (a) Stabilising: individuals with the mean value are favoured; the curve becomes narrower and taller at the same mean (1). Directional: individuals with a value other than the mean (one extreme) are favoured; the peak shifts (1). Disruptive: individuals at both ends are favoured; the middle is removed and two peaks may form (graphs of each) (1). (b) Gene flow: migration of part of a population changes allele frequencies in both the old and new populations; repeated migration is gene flow (1). Genetic drift: chance changes in allele frequency, especially in small groups; a small drifted founder group may become a new species (founder effect) (1).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose farmers spray the same insecticide every season and find that after five years it kills very few insects. Explain using natural selection, and suggest one way to slow this down. (3 marks)

    Hint

    Resistant variants already exist.

    Answer

    The insect population had variation; a few insects were resistant by chance mutation (1). Each spraying killed sensitive insects, leaving resistant ones to breed; over many short generations the population became mostly resistant: evolution by anthropogenic action within years (1). Rotating different insecticides or reducing unnecessary spraying lowers the selection pressure for one resistance (1).

11. Q11 · 2 marks · Very short answerHOTS / competency

    Why could bacteria form a 'new type' within days while the same change in fish would take millions of years? (2 marks)

    Hint

    Life span.

    Answer

    The rate at which new forms appear depends on the life cycle: bacteria divide fast and become millions within hours, so a variant favoured by new conditions outgrows the rest in days (1). Fish have life spans of years, so the same number of generations, and hence the same change, takes millions of years (1).

12. Q12 · 2 marks · Very short answerHarder board level

    Why are lichens called pollution indicators, and how does this relate to the moth story? (2 marks)

    Hint

    They don't grow in polluted air.

    Answer

    Lichens do not grow in polluted areas, so their absence indicates pollution (1). Industrial pollution killed the whitish lichens on tree trunks, exposing dark bark, which made dark moths camouflaged and white moths visible to predators (1).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.How old is the universe according to NCERT?Show answer

   Almost 13.8 billion years.

2. 2.Who disproved spontaneous generation and how?Show answer

   Louis Pasteur: no life arose from killed yeast in pre-sterilised flasks, but it did in flasks open to air.

3. 3.What did Miller observe in his experiment?Show answer

   Formation of amino acids.

4. 4.Where did Wallace work?Show answer

   In the Malay Archipelago.

5. 5.Who objected to Haeckel's embryological evidence?Show answer

   Karl Ernst von Baer: embryos never pass through the adult stages of other animals.

6. 6.Give two examples of analogous organs.Show answer

   Wings of butterfly and bird; eye of octopus and mammals; flippers of penguins and dolphins; sweet potato and potato (any two).

7. 7.What is saltation?Show answer

   De Vries' idea of a single-step large mutation causing speciation.

8. 8.What is the founder effect?Show answer

   When a small drifted group's allele frequencies differ so much that it forms a different population or species; the drifted population are the founders.

9. 9.Which fish thought to be extinct was caught in 1938?Show answer

   A Coelacanth, caught in South Africa.

10. 10.What was the brain capacity of Neanderthal man?Show answer

    About 1400 cc.

11. 11.Where can prehistoric cave paintings be seen in India?Show answer

    Bhimbetka rock shelter, Raisen district, Madhya Pradesh.

12. 12.When did modern Homo sapiens arise?Show answer

    During the ice age, between 75,000 and 10,000 years ago.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you describe early earth conditions and Miller's experiment?
• ☐Can you state the four observations behind Darwin's natural selection and what fitness means?
• ☐Can you give three examples each of homologous and analogous organs?
• ☐Can you explain industrial melanism and why lichens matter?
• ☐Can you describe adaptive radiation with two examples and explain convergent evolution in Australia?
• ☐Can you solve a Hardy–Weinberg problem and list the five factors that disturb equilibrium?
• ☐Can you draw and explain stabilising, directional and disruptive selection?
• ☐Can you write the order of human evolution with brain capacities and features?

Evolution: frequently asked questions