Section 15 — Board Exam-Pattern Questions (CBSE Class 12 Biology · Chapter 6)
Chapter 6 (Evolution) is a steady scorer in the CBSE Class 12 Biology Board exam. Its parent unit (Genetics and Evolution) carries about 20 marks, and Evolution contributes a steady share — MCQs and short answers on the theories and evidences, a frequent 3-mark question on adaptive radiation or the types of selection, and often a 5-mark question on Darwin's natural selection, Hardy–Weinberg or human evolution.
This section curates 24 high-yield Board exam-pattern questions (modelled on the CBSE style — not year-tagged official questions) in CBSE answer-writing format and mark-weighting:
| Marks | Number of Qs | Indicative frequency in papers |
|---|---|---|
| 1 mark | 7 questions (Q1–Q7) | 2–3 / paper |
| 2 marks | 6 questions (Q8–Q13) | 2 / paper |
| 3 marks | 6 questions (Q14–Q19) | 1–2 / paper |
| 5 marks | 5 questions (Q20–Q24) | 1 / paper (high probability) |
| Total | 24 questions | 62 marks of practice |
How to use: Cover the answer, attempt it yourself scaled to the marks, then check against the model answer.
No quiz at the end: Pair with Sections 14 (Solved Examples) and 16 (NEET-Pattern Practice Questions).
What CBSE Keeps Asking
Most CBSE questions on Chapter 6 fall into 6 recurring themes:
1. Origin of life (1- or 2-mark) — the reducing atmosphere, Miller's experiment, Oparin–Haldane, Pasteur's disproof of spontaneous generation.
2. The theories (2- or 3-mark) — Lamarck vs Darwin, natural selection, the role of Wallace and Malthus.
3. Evidences for evolution (2-, 3- or 5-mark) — fossils and dating, homologous vs analogous organs, divergent vs convergent evolution, embryology.
4. Adaptive radiation & selection in action (3- or 5-mark) — Darwin's finches, Australian marsupials, industrial melanism, resistance.
5. Mechanism & types of selection (2- or 3-mark) — de Vries' mutation theory, and the stabilising / directional / disruptive graphs.
6. Hardy–Weinberg & human evolution (3- or 5-mark) — the equation and its five disturbing factors; the Dryopithecus-to-sapiens sequence with brain sizes.
1-Mark Questions (Q1–Q7)
Q1. [1 mark] Who disproved the theory of spontaneous generation, and what was his conclusion?
Answer: Louis Pasteur, who showed that life comes only from pre-existing life.
Q2. [1 mark] Name the two scientists who independently proposed natural selection as the mechanism of evolution.
Answer: Charles Darwin and Alfred Russel Wallace.
Q3. [1 mark] What type of evolution do homologous organs represent?
Answer: Divergent evolution (from a common ancestor).
Q4. [1 mark] Which type of natural selection favours the average value of a trait and removes both extremes?
Answer: Stabilising selection.
Q5. [1 mark] Write the Hardy–Weinberg equation for genotype frequencies.
Answer: (with ).
Q6. [1 mark] Approximately when did the dinosaurs become extinct?
Answer: About 65 million years ago.
Q7. [1 mark] Name the plant Hugo de Vries worked on to develop his mutation theory.
Answer: The evening primrose (Oenothera lamarckiana).
2-Mark Questions (Q8–Q13)
Q8. [2 marks] Distinguish between homologous and analogous organs, giving one example of each.
Answer: Homologous organs share the same basic structure but perform different functions and indicate common ancestry (divergent evolution) — e.g. the forelimbs of a whale, bat and human. Analogous organs have different structures but perform the same function and indicate convergent evolution — e.g. the wing of a bird and the wing of a butterfly.
Q9. [2 marks] How does industrial melanism in the peppered moth illustrate natural selection?
Answer: Before industrialisation, pale moths were camouflaged on lichen-covered bark and were common. When soot killed the lichen and blackened the bark, dark moths became better hidden from bird predators and survived to breed, while light moths were eaten. The population shifted from light to dark — natural selection acting on existing variation.
Q10. [2 marks] State any two points of Lamarck's theory of evolution.
Answer: (i) Use and disuse of organs — organs used more become better developed while unused organs shrink. (ii) Inheritance of acquired characters — features gained during an individual's lifetime are passed to the offspring (e.g. the giraffe's lengthening neck).
Q11. [2 marks] What is adaptive radiation? Name one example.
Answer: Adaptive radiation is the evolution of several different species from a single ancestral form when it spreads into a new area offering varied conditions. Example: Darwin's finches on the Galapagos Islands, which developed different beaks suited to different diets (also the Australian marsupials).
Q12. [2 marks] Name the five factors that can disturb Hardy–Weinberg genetic equilibrium.
Answer: Gene flow (gene migration), genetic drift, mutation, genetic recombination and natural selection. When any of these operates, allele frequencies change and the population evolves.
Q13. [2 marks] Compare Darwin's and de Vries' views on the pace of evolution.
Answer: Darwin held that evolution is slow and gradual, built up from small variations selected over many generations. de Vries argued that evolution occurs by large, sudden mutations (saltation) that can produce a new species in a single step.
3-Mark Questions (Q14–Q19)
Q14. [3 marks] Explain the three types of natural selection and the effect each has on a trait's distribution.
Answer: (i) Stabilising selection favours the average (mean) value and eliminates both extremes, so variation is reduced and the mean stays the same. (ii) Directional selection favours one extreme, so the mean shifts towards that end (as in industrial melanism). (iii) Disruptive selection favours both extremes and selects against the mean, which can split the population into two groups.
Q15. [3 marks] State Darwin's theory of natural selection in terms of variation, over-reproduction and fitness.
Answer: Individuals in a population vary in their traits. Populations tend to produce more offspring than the environment can support (Malthus' idea), so there is a struggle for existence. Individuals with variations that suit them better survive and reproduce more successfully — they have higher reproductive fitness — so favourable variations become more common over generations. This differential survival and reproduction is natural selection.
Q16. [3 marks] Describe how fossils serve as evidence for evolution and how their age is determined.
Answer: Fossils are the preserved remains, impressions or casts of organisms that lived in the past, usually found in sedimentary rock. Studying them (palaeontology) shows that life-forms have changed over time and that different organisms existed in different ages — series such as the horse fossils reveal gradual change. Their age is estimated from the relative position of the rock strata (deeper = older) and by radioactive (radiometric) dating of the rocks.
Q17. [3 marks] With one example each, distinguish between divergent and convergent evolution.
Answer: Divergent evolution is when a single ancestral structure is modified to perform different functions in different descendants, producing homologous organs — e.g. the forelimbs of mammals (whale flipper, bat wing, human hand). Convergent evolution is when unrelated groups facing similar environments independently evolve similar features, producing analogous organs — e.g. the marsupial (Tasmanian) wolf of Australia and the placental wolf, which look and hunt alike despite distant ancestry.
Q18. [3 marks] In a Hardy–Weinberg population, a recessive disorder occurs in 4% of individuals. Find the frequency of the recessive allele, the dominant allele and the carriers.
Answer: The recessive phenotype gives , so . Then . The carrier (heterozygote) frequency is , i.e. 32%. So the recessive allele frequency is 0.2, the dominant allele frequency is 0.8, and 32% of the population are carriers.
Q19. [3 marks] Outline the main stages in the evolution of man from Australopithecus to Homo sapiens, with brain sizes where relevant.
Answer: Australopithecus (~2 million years ago) lived in East African grasslands, walked upright, hunted with stone weapons but mainly ate fruit. Homo habilis was the first hominid, brain 650–800 cc, probably not a meat-eater. Homo erectus (~1.5 million years ago, fossils found in Java in 1891) had a brain of about 900 cc and probably ate meat. Homo neanderthalensis had a brain of about 1400 cc. Modern Homo sapiens arose during the ice age, about 75,000 to 10,000 years ago. The overall trend is increasing brain size, upright posture and greater tool use.
5-Mark Questions (Q20–Q24)
Q20. [5 marks] Describe Miller's experiment and explain how it supports the Oparin–Haldane hypothesis of the origin of life.
Answer:
Oparin and Haldane proposed that the first forms of life arose through chemical evolution — organic molecules building up from the inorganic gases of the early, reducing atmosphere.
Miller's experiment (1953): Stanley Miller built a closed apparatus to imitate early-earth conditions. He sealed in the gases of the primitive atmosphere — methane (CH₄), hydrogen (H₂), ammonia (NH₃) and water vapour — and kept them at about 800 °C. He passed electric discharges through the mixture to imitate lightning, while a flask of boiling water represented the early ocean and a condenser returned the products to the water.
Result: After about a week, amino acids — the building blocks of proteins — had formed in the water. Later experiments along the same lines produced sugars, nitrogen bases and fats, and similar molecules were found in meteorites.
Significance: The experiment showed that simple inorganic gases, given energy, can assemble into the organic molecules of life — exactly as Oparin and Haldane had predicted — giving chemical evolution firm experimental support.
Q21. [5 marks] Explain Darwin's theory of natural selection. How does it differ from Lamarck's theory of evolution?
Answer:
Darwin's theory of natural selection rests on a few observations and conclusions:
- Individuals within a population show heritable variation.
- Populations tend to over-reproduce — more offspring are born than can survive (Malthus' idea) — leading to a struggle for existence.
- Individuals with variations better suited to the environment survive and reproduce more successfully; this is higher reproductive fitness ('survival of the fittest').
- Over many generations, favourable variations accumulate, so populations change and, through branching descent, all organisms share common ancestors.
Darwin reached this conclusion together with Alfred Russel Wallace, who arrived at the same idea independently.
How it differs from Lamarck: Lamarck explained evolution by the use and disuse of organs and the inheritance of acquired characters — traits gained during an individual's life (like a stretched neck) being passed on. Darwin rejected this: he held that evolution acts on naturally occurring heritable variation through differential survival and reproduction, not on characters acquired during a lifetime, which are in fact not inherited.
Q22. [5 marks] State the Hardy–Weinberg principle. In a population of 1000 people at equilibrium, 90 show a recessive trait. Calculate the allele and genotype frequencies, and explain what a departure from these values would mean.
Answer:
Hardy–Weinberg principle: in a large, randomly mating population free of disturbing factors, the allele and genotype frequencies remain constant from generation to generation — the gene pool is in genetic equilibrium. It is expressed as and , where = frequency of , = frequency of and = frequency of .
Calculation: The recessive phenotype () frequency is , so and .
- Dominant allele frequency ; recessive allele frequency .
- (490 people), (420 people), (90 people). Check: .
Departure from these values: If the measured frequencies differ from those predicted, the population is not at equilibrium — its allele frequencies are changing across generations. Since that is precisely the definition of evolution, the size of the departure measures the extent of evolutionary change taking place, driven by factors such as gene flow, drift, mutation, recombination or natural selection.
Q23. [5 marks] Describe the various evidences for organic evolution under the headings palaeontological, comparative anatomical and embryological.
Answer:
Palaeontological (fossil) evidence: Fossils are preserved remains of past organisms in sedimentary rocks. Deeper strata hold older fossils, and radioactive dating fixes their age. Fossil series (such as those of the horse) show a gradual change of forms over time, and many fossils represent life-forms that no longer exist — direct proof that life has changed.
Comparative anatomical evidence:
- Homologous organs — same basic structure, different functions (forelimbs of whale, bat, human) — show divergent evolution from a common ancestor.
- Analogous organs — different structure, same function (wings of bird and butterfly; eye of octopus and mammal) — show convergent evolution.
- Vestigial organs (reduced, functionless structures) also point to descent with modification.
Embryological evidence: Haeckel observed that the early embryos of different vertebrates are remarkably similar, sharing features such as gill slits, before diverging into their adult forms. von Baer refined this, showing embryos share only early features and never pass through the adult stages of other animals. This common early development points to shared ancestry.
Together, these independent lines of evidence build a strong case for organic evolution.
Q24. [5 marks] Describe the origin and evolution of man from the first man-like primates to modern Homo sapiens, noting the trends in brain size and behaviour.
Answer:
Dryopithecus and Ramapithecus (~15 million years ago): two hairy, ape-like primates that walked like present-day gorillas and chimpanzees; Ramapithecus was more man-like and Dryopithecus more ape-like.
Australopithecus (~2 million years ago): lived in the East African grasslands, walked upright, was about 4 feet tall, hunted with stone weapons but mainly ate fruit.
Homo habilis: the first hominid; brain capacity 650–800 cc; probably did not eat meat.
Homo erectus (~1.5 million years ago): fossils found in Java in 1891; brain about 900 cc; probably ate meat.
Homo neanderthalensis: brain about 1400 cc; lived in the near east and central Asia (~100,000–40,000 years ago); used hides for protection and buried their dead.
Homo sapiens: arose in Africa and spread across the continents, forming distinct races; modern Homo sapiens arose during the ice age, about 75,000 to 10,000 years ago. Cave art (as at Bhimbetka, Madhya Pradesh) appeared ~18,000 years ago and agriculture ~10,000 years ago.
Overall trends: a steady increase in brain size, a shift to upright posture, and an ever-greater use of tools — the three threads running through the whole story of human evolution.
End of Section 15
You have now worked through 24 high-yield CBSE Board questions spanning the origin of life, the theories of Lamarck and Darwin, the evidences for evolution, adaptive radiation, industrial melanism, the types of selection, the Hardy–Weinberg principle and human evolution. If you can answer the five 5-markers (Q20–Q24) from memory with their key points, you have effectively secured the chapter's contribution to your Board paper.
Final tip: On Board day, draw and label diagrams (Miller's apparatus, homologous vs analogous limbs, the three selection graphs, the human-evolution sequence) wherever the question allows — examiners give marks for clear, labelled structure. For Hardy–Weinberg problems, show every step: find , take its square root, then , and state each frequency.