CBSE Important Questions Class 12 Biology Chapter 6 – Evolution

Class 12 Biology Chapter 6 Evolution is the sixth chapter of the NCERT Class 12 Biology textbook. It explains how life began on earth, the evidence for evolution (fossils, homologous and analogous organs, embryology), Darwin's theory of natural selection, Lamarck's idea of acquired characters, the three types of natural selection, the Hardy-Weinberg principle, and the stages of human evolution from Australopithecus to Homo sapiens.

These CBSE important questions for Class 12 Biology Chapter 6 cover the order in which molecules appeared on early earth, fossils and Archaeopteryx, Homo habilis, stabilising and directional selection graphs, geological eras, natural selection in beetles, the Hardy-Weinberg calculation, embryological evidence, and assertion-reason questions on homologous organs and Lamarck's theory. The set has multiple-choice, short-answer and long-answer questions with answers for board exam revision.

Q1. During the course of evolution of life on earth, the substances appeared in the sequence as
(a) glucose, amino acids, nucleic acids, proteins.
(b) ammonia, amino acids, proteins, nucleic acids.
(c) water, amino acids, nucleic acids, enzymes.
(d) amino acids, ammonia, phosphates, nucleic acids.

Answer: (b) ammonia, amino acids, proteins, nucleic acids.
In the primitive atmosphere of early earth, ammonia was one of the main gases. Under reducing conditions with electric discharges (as shown in the Miller-Urey experiment), simple amino acids formed. These amino acids joined to form proteins, and finally nucleotides assembled to form nucleic acids (RNA and DNA), which carry genetic information.

Q2. What are fossils? How do they form? Describe their evolutionary aspect with an example.

Answer: Fossils are the remnants or impressions of organisms that lived in the past. They are preserved through natural processes over many years: after death, organisms get buried under sediments, and conditions such as high pressure, low oxygen, high temperature and dehydration prevent decay and allow minerals to replace the organic tissues. Fossils are helpful in understanding the phylogeny and evolutionary history of organisms. The fossil records help in building a broad historical sequence of biological evolution.
Example: Archaeopteryx had both reptilian characters (teeth, long bony tail) and avian characters (feathers, wishbone). It is the connecting link between reptiles and birds. This indicates that birds have evolved from reptiles.

Q3. Chronic alcohol abuse affects multiple organs and systems in the body. Which of the following is a major consequence of long-term excessive alcohol consumption?
(a) Development of fatty liver and cirrhosis due to liver damage
(b) Increased production of antibodies leading to stronger immunity
(c) Enhanced reflexes and improved motor coordination
(d) Permanent increase in alertness and concentration

Answer: (a) Development of fatty liver and cirrhosis due to liver damage
Ethanol is broken down in the liver. Long-term heavy drinking causes fat to collect in the liver cells (fatty liver). This leads to inflammation, scarring (fibrosis) and finally cirrhosis of the liver. Alcohol does not improve immunity, reflexes or alertness.

Q4. A team of paleoanthropologists discovered fossil remains in East Africa dated to approximately 2 million years ago. The skeletal structure showed a relatively larger brain cavity than earlier hominids and evidence of tool usage. Based on these findings and known evolutionary traits, identify the most likely species and its significance in human evolution.
(a) Australopithecus afarensis
(b) Homo habilis
(c) Homo erectus
(d) Homo sapiens

Answer: (b) Homo habilis
The fossil evidence points to a species with a larger brain capacity and tool usage, which are the key traits of Homo habilis. Homo habilis lived around 2.4 to 1.4 million years ago and is considered the first true human ancestor in the genus Homo. Australopithecus afarensis walked upright but did not use tools, Homo erectus came later and used fire, and Homo sapiens emerged much later with complex language and culture.

Q5. In an evolutionary study, two types of natural selection were observed in a population. Analyse the given graphs representing Type I and Type II natural selection and identify the correct interpretation:

Graphs of Type I (stabilising) and Type II (directional) natural selection showing conserved and eliminated regions

(a) In Type I natural selection, traits farthest from the mean are conserved while those near to it are eliminated; whereas in Type II, traits near to one extreme are conserved.
(b) In both types of natural selection shown, traits closest to the mean are always eliminated.
(c) In Type I natural selection, traits closest to both extremes are eliminated; whereas in Type II, only one extreme's traits are eliminated while conserving those near to it.
(d) Both types show that all variations except for those exactly at the mean value get eliminated over time.

Answer: (c) In Type I natural selection, traits closest to both extremes are eliminated; whereas in Type II, only one extreme's traits are eliminated while conserving those near to it.
Type I is stabilising selection: individuals with trait values close to the mean are conserved and those at both extremes are eliminated (shaded tails on both sides). Type II is directional selection: individuals at one extreme are eliminated and the population mean shifts towards the other extreme (shading on one side only).

Q6. Match the geological era with the major evolutionary event.

Column I (Era) Column II (Event)
a. Paleozoic Era 1. Origin of mammals
b. Mesozoic Era 2. Explosion of invertebrate diversity
c. Cenozoic Era 3. Age of reptiles
d. Precambrian Time 4. Origin of unicellular life

(a) a–2, b–3, c–1, d–4
(b) a–3, b–2, c–1, d–4
(c) a–1, b–2, c–3, d–4
(d) a–2, b–1, c–3, d–4

Answer: (a) a–2, b–3, c–1, d–4
Paleozoic Era: known for the Cambrian explosion, which led to the diversification of invertebrates. Mesozoic Era: dominated by reptiles, especially dinosaurs, hence called the Age of Reptiles. Cenozoic Era: marked by the rise of mammals and eventually humans. Precambrian Time: the earliest era, in which unicellular life originated.

Q7. A population of beetles shows variation in body colour – green and brown. Over several generations, the frequency of brown beetles increases. Researchers collected data over 5 generations to study the impact of predation and camouflage on survival and reproduction.

Generation Green Beetles Brown Beetles Total Population
1 400 100 500
2 350 150 500
3 280 220 500
4 200 300 500
5 120 380 500

Based on the data, which of the following is the most likely reason for the increase in brown beetles?
(a) Brown beetles reproduce faster than green beetles
(b) Brown beetles are better camouflaged and escape predation
(c) Green beetles mutate into brown beetles
(d) Brown beetles are artificially selected by humans

Answer: (b) Brown beetles are better camouflaged and escape predation
The share of brown beetles rises steadily from 100/500 = 0.20 in generation 1 to 220/500 = 0.44 in generation 3 and 380/500 = 0.76 in generation 5. A steady rise like this shows natural selection: brown beetles are better camouflaged, so fewer of them are eaten by predators, and they survive and reproduce more than green beetles.

Q8. In a population under Hardy-Weinberg equilibrium, the frequency of allele A is 0.7 and allele a is 0.3. What percentage of the population is expected to be heterozygous?
(a) 21%
(b) 42%
(c) 49%
(d) 9%

Answer: (b) 42%
Heterozygous genotype frequency = 2pq, where p = 0.7 and q = 0.3.
2pq = 2 × 0.7 × 0.3 = 0.42, i.e. 42% of the population is expected to be heterozygous (Aa).

Q9. Embryological evidence shows that vertebrate embryos share common features like pharyngeal gill slits and notochord during early development. Why are embryological similarities considered strong evidence for evolution?
(a) They show that all organisms develop identical adult structures
(b) They prove that environmental factors do not influence development
(c) They indicate that organisms share a common developmental blueprint
(d) They suggest that acquired traits are inherited

Answer: (c) They indicate that organisms share a common developmental blueprint
During early embryonic stages, all vertebrates show structures like pharyngeal gill slits and a notochord. These shared features suggest that the different vertebrate groups have inherited common developmental genes from a shared ancestor, which supports the theory of evolution.

Q10. Which of the following statements about fossils is incorrect?
(a) Fossils provide direct evidence of past life forms
(b) Fossils help in tracing evolutionary relationships
(c) Fossils are found only in igneous rocks
(d) Fossils can indicate the age of the rock layers

Answer: (c) Fossils are found only in igneous rocks
Fossils form when organisms are buried by sediments that slowly harden into sedimentary rocks. Igneous rocks form from cooled magma or lava at very high temperatures that destroy organic remains, so fossils are mostly found in sedimentary rocks, not igneous rocks.

Q11. In the given question, a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the choices given below the question.
Assertion (A): The forelimbs of a frog, lizard, bird and human show structural similarity but perform different functions.
Reason (R): These forelimbs are analogous organs that evolved due to similar environmental pressures.
(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.

Answer: (c) A is true, but R is false.
The forelimbs of frog, lizard, bird and human share the same basic skeletal plan, which shows a common ancestry. Such organs are homologous organs: they have a similar structure but perform different functions, and they arise by divergent evolution. Analogous organs have different structures but similar functions and arise by convergent evolution, so the reason is false.

Q12. In the given question, a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the choices given below the question.
Assertion (A): Lamarck's theory explains the origin of new species through natural selection.
Reason (R): Lamarck emphasised the role of environmental influence and individual effort in evolution.
(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 false, but R is true.
(d) A is true, but R is false.

Answer: (c) A is false, but R is true.
Lamarck's theory does not involve natural selection; that concept was given by Darwin. Lamarck said that environmental changes and individual effort (such as stretching or using an organ) lead to acquired traits, which are then inherited. So the assertion is false, but the reason is true.

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