Chapter 7: Evolution – Long Answer Type Questions

CBSE Class 12 Biology Long Answer Questions (NCERT): Evolution

Course & Examination Details

Course: CBSE Class 12 Biology
Unit: Unit II – Genetics and Evolution
Chapter: Chapter 7 – Evolution
Prescribed Textbook: NCERT Biology Class XII
Examination: CBSE Class 12 Board Examination
Question Type: Long Answer Type
Answer Length: 120–150 words each

Section A: Origin of Life

Q1. Explain the theory of chemical evolution for the origin of life.

Answer:
The theory of chemical evolution explains that life originated from non-living matter through a series of chemical reactions on primitive Earth. According to this theory, proposed by A. I. Oparin and J. B. S. Haldane, the early Earth had a reducing atmosphere containing methane, ammonia, hydrogen, and water vapour. Energy from lightning, UV radiation, and volcanic activity led to the formation of simple organic molecules. These molecules accumulated in oceans, forming a “primordial soup.” Gradually, they polymerised into macromolecules such as proteins and nucleic acids. Aggregation of these molecules resulted in protocells, which eventually developed metabolic reactions and self-replication, giving rise to the first living organisms.

Q2. Describe the significance of the Miller–Urey experiment.

Answer:
The Miller–Urey experiment provided experimental evidence supporting the theory of chemical evolution. In this experiment, Stanley Miller and Harold Urey simulated early Earth conditions using a closed apparatus containing methane, ammonia, hydrogen, and water vapour. Electric sparks were passed to mimic lightning. After several days, organic compounds such as amino acids were formed. This demonstrated that essential building blocks of life could be synthesised abiotically from inorganic substances. The experiment confirmed that the primitive Earth environment was suitable for the formation of organic molecules, strengthening the scientific basis of the origin of life through chemical evolution.

Section B: Evidences for Evolution

Q3. Explain homologous and analogous organs as evidences of evolution.

Answer:
Homologous organs are organs that share the same basic structural plan and origin but perform different functions. For example, the forelimbs of humans, whales, and bats have similar bone structure but different functions, indicating common ancestry and divergent evolution. Analogous organs, on the other hand, have different origins and structures but perform similar functions. Wings of birds and insects are analogous, showing convergent evolution. Together, homologous and analogous organs provide strong morphological evidence for evolution by explaining both common ancestry and adaptation to similar environments.

Q4. Describe palaeontological evidence in support of evolution.

Answer:
Palaeontological evidence is based on the study of fossils, which are preserved remains or impressions of ancient organisms found in sedimentary rocks. Fossils provide a chronological record of life on Earth, showing gradual changes in organisms over time. Transitional fossils, such as Archaeopteryx, display characteristics of two different groups, linking reptiles and birds. Fossil records demonstrate extinction of old forms and emergence of new ones, supporting the concept of evolution. They also help trace evolutionary lineages and establish the sequence of evolutionary events.

Q5. How does molecular biology provide evidence for evolution?

Answer:
Molecular evidence for evolution comes from comparative studies of DNA, RNA, and protein sequences. Closely related organisms show greater similarity in their genetic material, while distant organisms show more differences. Proteins like cytochrome c have been compared across species to determine evolutionary relationships. Similarities in molecular sequences indicate common ancestry and evolutionary divergence over time. Molecular evidence is considered highly reliable because genetic material directly reflects hereditary relationships, making it a powerful tool in evolutionary studies.

Section C: Adaptive Radiation

Q6. Explain adaptive radiation with suitable examples.

Answer:
Adaptive radiation is the evolutionary process in which multiple species arise from a common ancestor and adapt to different ecological niches. A classic example is Darwin’s finches of the Galapagos Islands, where finches evolved different beak shapes according to food sources. Another example is Australian marsupials, which diversified into forms adapted to various habitats. Adaptive radiation demonstrates divergent evolution driven by natural selection, geographical isolation, and availability of ecological opportunities, explaining the origin of biodiversity.

Q7. Differentiate between divergent and convergent evolution.

Answer:
Divergent evolution occurs when organisms from a common ancestor evolve into different species due to adaptation to different environments. It results in homologous organs and adaptive radiation. Convergent evolution occurs when unrelated organisms evolve similar traits due to similar environmental pressures, resulting in analogous organs. Divergent evolution highlights common ancestry, while convergent evolution highlights functional adaptation. Both processes demonstrate the role of natural selection in shaping organisms.

Section D: Theories of Evolution

Q8. Explain Lamarck’s theory of inheritance of acquired characters.

Answer:
Lamarck proposed that organisms acquire traits during their lifetime due to use and disuse of organs, and these acquired traits are passed to offspring. For example, giraffes were believed to develop long necks by stretching to reach leaves. Lamarckism attempted to explain adaptation and evolution but lacked experimental support. Later studies showed that acquired characters do not affect germ cells and are not inherited. Hence, Lamarck’s theory is considered scientifically incorrect.

Q9. Describe Darwin’s theory of natural selection.

Answer:
Darwin’s theory of natural selection explains evolution as a result of differential survival and reproduction. According to Charles Darwin, organisms produce more offspring than can survive. Individuals show variations, and those with favourable traits survive the struggle for existence. These individuals reproduce and pass advantageous traits to the next generation. Over time, accumulation of such traits leads to adaptation and formation of new species. Natural selection is the central mechanism of evolution.

Q10. Compare Lamarckism and Darwinism.

Answer:
Lamarckism is based on inheritance of acquired characters due to use and disuse, whereas Darwinism is based on natural selection acting on pre-existing variations. Lamarckism assumes environmental influence directly changes traits, while Darwinism emphasises survival of the fittest. Lamarckism lacks experimental evidence, whereas Darwinism is widely accepted and supported by genetics, fossils, and molecular biology.

Section E: Mechanism of Evolution

Q11. Explain mutation as an evolutionary force.

Answer:
Mutation is a sudden heritable change in DNA sequence that creates new alleles. It introduces genetic variation, which is essential for evolution. Mutations may be beneficial, harmful, or neutral. Although mutation rates are low, their cumulative effect over generations contributes to evolutionary change. Mutations provide raw material for natural selection and play a key role in adaptation and speciation.

Q12. Describe gene flow and its evolutionary significance.

Answer:
Gene flow refers to the movement of individuals or gametes between populations, resulting in transfer of alleles. It changes allele frequencies and increases genetic variation within populations. Gene flow can introduce new traits, reduce differences between populations, and counteract the effects of genetic drift. It plays an important role in evolution by influencing population structure and adaptation.

Q13. Explain genetic drift with examples.

Answer:
Genetic drift is a random change in allele frequencies, significant in small populations. It may lead to loss or fixation of alleles irrespective of their adaptive value. The founder effect occurs when a small group establishes a new population, carrying limited genetic variation. The bottleneck effect occurs when population size is drastically reduced due to disasters. Genetic drift reduces genetic diversity and can lead to rapid evolutionary changes.

Q14. Describe natural selection as a mechanism of evolution.

Answer:
Natural selection favours individuals with traits that enhance survival and reproduction. These individuals contribute more offspring to the next generation, increasing frequency of favourable alleles. Over time, populations adapt to their environment, leading to evolutionary change. Natural selection operates in different forms, including stabilising, directional, and disruptive selection, shaping population characteristics.

Section F: Hardy–Weinberg Principle

Q15. Explain the Hardy–Weinberg principle and its significance.

Answer:
The Hardy–Weinberg principle states that allele frequencies in a population remain constant across generations if no evolutionary forces act. It is expressed by the equation p² + 2pq + q² = 1. The principle provides a mathematical model to study population genetics. Any deviation from equilibrium indicates that evolution is occurring. It helps identify the role of evolutionary forces such as mutation, selection, and gene flow.

Q16. What conditions are required for Hardy–Weinberg equilibrium?

Answer:
Hardy–Weinberg equilibrium requires a large population size, random mating, no mutation, no migration, and no natural selection. If any of these conditions are violated, allele frequencies change, indicating evolution. These conditions help identify evolutionary influences acting on populations.

Section G: Human Evolution

Q17. Explain the major stages in human evolution.

Answer:
Human evolution began with ape-like ancestors such as Dryopithecus and Ramapithecus. Homo habilis was the earliest tool-using human. Homo erectus showed upright posture, larger brain, and use of fire. Homo neanderthalensis exhibited cultural behaviour. Modern humans, Homo sapiens, show advanced intelligence, language, and social organisation. This sequence highlights gradual anatomical and behavioural evolution.

Q18. Describe the trends observed in human evolution.

Answer:
Major trends in human evolution include increased brain size, reduced jaw and teeth size, bipedal locomotion, development of speech, and cultural advancement. These changes improved survival, communication, and social interaction, making humans highly adaptable.

Q19. Explain the African origin of modern humans.

Answer:
Fossil and molecular evidence suggest that modern humans originated in Africa and later migrated to other continents. Genetic studies show greater diversity in African populations, supporting this theory. Migration led to global distribution of Homo sapiens.

Section H: Patterns of Natural Selection

Q20. Explain stabilising selection with an example.

Answer:
Stabilising selection favours intermediate phenotypes and eliminates extremes, maintaining population stability. For example, average birth weight in humans is favoured, while very low or very high weights are selected against.

Q21. Describe directional selection.

Answer:
Directional selection favours individuals with traits at one extreme, shifting population characteristics over time. Example: evolution of antibiotic-resistant bacteria due to selective pressure of antibiotics.

Q22. Explain disruptive selection and its evolutionary importance.

Answer:
Disruptive selection favours both extreme phenotypes over intermediate forms. It increases variation and can lead to speciation by splitting populations into distinct groups.

Section I: Integrated Concepts

Q23. Why is evolution considered a population-level phenomenon?

Answer:
Evolution involves changes in allele frequencies within populations over generations. Individuals do not evolve; populations do. Hence, evolution is considered a population-level phenomenon.

Q24. How does isolation contribute to evolution and speciation?

Answer:
Isolation prevents gene flow between populations, allowing them to diverge genetically due to mutation, selection, and drift. Over time, reproductive isolation leads to formation of new species.

Q25. Why is the study of evolution important in biology?

Answer:
Evolution explains the origin, diversity, and adaptation of organisms. It integrates genetics, ecology, and palaeontology, forming the foundation of modern biology and helping understand life’s history.