Chapter 7: Evolution – Short Answer Type Questions
CBSE Class 12 Biology Short 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: Short Answer Type
- Answer Length: 60–80 words each
Section A: Origin of Life
Q1. Explain the theory of chemical evolution.
Answer:
The theory of chemical evolution explains that life originated from non-living matter through gradual chemical changes on early Earth. In a reducing atmosphere, simple inorganic molecules formed organic compounds, which polymerised into complex macromolecules. These molecules aggregated to form protocells, eventually giving rise to the first living organisms. This theory provides a scientific explanation for the origin of life.
Q2. Describe the significance of the Miller–Urey experiment.
Answer:
The Miller–Urey experiment demonstrated that organic molecules like amino acids could be synthesised abiotically under simulated primitive Earth conditions. By passing electric sparks through a mixture of gases, the experiment supported the theory of chemical evolution and provided experimental evidence for the origin of organic compounds from inorganic substances.
Q3. Why is the theory of spontaneous generation rejected?
Answer:
The theory of spontaneous generation suggested that life arises suddenly from non-living matter. It was rejected after experiments by Redi, Spallanzani, and Pasteur showed that living organisms arise only from pre-existing life, establishing the principle of biogenesis.
Q4. Describe the nature of the primitive Earth atmosphere.
Answer:
The primitive Earth atmosphere was reducing in nature and lacked free oxygen. It consisted mainly of methane, ammonia, hydrogen, and water vapour. This environment favoured chemical reactions leading to the formation of organic molecules essential for the origin of life.
Q5. Why were the earliest organisms anaerobic?
Answer:
The earliest organisms were anaerobic because free oxygen was absent in the primitive atmosphere. They derived energy through anaerobic metabolic pathways until photosynthetic organisms released oxygen, leading to an aerobic environment.
Section B: Evidences for Evolution
Q6. Explain homologous organs with significance.
Answer:
Homologous organs have the same basic structural plan and origin but perform different functions in different organisms. For example, the forelimbs of mammals. They indicate common ancestry and support divergent evolution, providing strong evidence for evolution.
Q7. Differentiate between homologous and analogous organs.
Answer:
Homologous organs share a common origin and structure but differ in function, whereas analogous organs have different origins and structures but perform similar functions. Homologous organs indicate divergent evolution, while analogous organs indicate convergent evolution.
Q8. What are vestigial organs? Explain their evolutionary significance.
Answer:
Vestigial organs are reduced, non-functional structures present in organisms, such as the human appendix. They represent remnants of organs that were functional in ancestors, providing evidence of evolutionary change over time.
Q9. Explain embryological evidence for evolution.
Answer:
Embryological evidence shows that early embryos of vertebrates resemble each other closely, suggesting a common ancestry. These similarities indicate that vertebrates evolved from a common ancestor.
Q10. How do fossils provide evidence for evolution?
Answer:
Fossils are preserved remains of ancient organisms. They provide a chronological record of life forms, showing gradual changes and transitional forms, thus offering direct evidence of evolutionary processes.
Section C: Adaptive Radiation
Q11. Explain adaptive radiation with an example.
Answer:
Adaptive radiation is the evolution of multiple species from a common ancestor, each adapted to different ecological niches. Darwin’s finches of the Galapagos Islands evolved different beak shapes suited to various food sources.
Q12. What is divergent evolution?
Answer:
Divergent evolution is the process by which organisms from a common ancestor evolve into different species with distinct traits due to adaptation to different environments.
Q13. Explain adaptive radiation in Australian marsupials.
Answer:
Australian marsupials evolved from a common ancestral stock into diverse forms adapted to various habitats. This radiation occurred due to geographical isolation and natural selection.
Q14. Why is adaptive radiation important in evolution?
Answer:
Adaptive radiation explains the diversification of species and the origin of biodiversity. It highlights the role of natural selection in shaping species to occupy different ecological niches.
Q15. How does adaptive radiation differ from convergent evolution?
Answer:
Adaptive radiation involves divergence from a common ancestor into multiple species, whereas convergent evolution involves unrelated species developing similar traits due to similar environmental pressures.
Section D: Theories of Evolution
Q16. Explain Lamarck’s theory of inheritance of acquired characters.
Answer:
Lamarck proposed that traits acquired during an organism’s lifetime due to use or disuse are inherited by offspring. For example, giraffes developing long necks. However, this theory lacks experimental support.
Q17. Why is Lamarckism considered incorrect?
Answer:
Lamarckism is incorrect because acquired traits are not inherited genetically. Experiments have shown that only changes in germ cells are passed to offspring, not somatic changes.
Q18. Explain Darwin’s theory of natural selection.
Answer:
Natural selection states that organisms produce more offspring than can survive. Due to variations, some individuals are better adapted and survive to reproduce, passing favourable traits to the next generation.
Q19. What is survival of the fittest?
Answer:
Survival of the fittest refers to the survival and reproduction of organisms best adapted to their environment, leading to the continuation of advantageous traits.
Q20. State the significance of Darwinism.
Answer:
Darwinism explains adaptation, speciation, and the diversity of life. It forms the foundation of modern evolutionary biology.
Section E: Mechanism of Evolution
Q21. Explain mutation as a mechanism of evolution.
Answer:
Mutation is a sudden heritable change in DNA that creates new alleles. It introduces genetic variation, which is essential for evolution and natural selection.
Q22. What is gene flow and its role in evolution?
Answer:
Gene flow is the movement of individuals between populations, causing transfer of alleles. It alters gene frequencies and increases genetic variation.
Q23. Explain genetic drift with example.
Answer:
Genetic drift is random change in allele frequencies, significant in small populations. Founder effect and bottleneck effect are examples that can reduce genetic diversity.
Q24. Describe natural selection as an evolutionary force.
Answer:
Natural selection favours individuals with advantageous traits, leading to increased survival and reproduction. Over time, it changes allele frequencies in populations.
Q25. Explain recombination in evolution.
Answer:
Recombination creates new gene combinations during meiosis, increasing genetic variation and providing material for natural selection.
Section F: Hardy–Weinberg Principle
Q26. State the Hardy–Weinberg principle.
Answer:
The Hardy–Weinberg principle states that allele frequencies in a population remain constant from generation to generation if no evolutionary forces act.
Q27. Write the Hardy–Weinberg equation and explain it.
Answer:
The equation p² + 2pq + q² = 1 represents genotype frequencies in a population. Any deviation indicates evolution.
Q28. List the conditions required for Hardy–Weinberg equilibrium.
Answer:
Large population size, random mating, no mutation, no migration, and no natural selection are required to maintain equilibrium.
Q29. What does deviation from Hardy–Weinberg equilibrium indicate?
Answer:
Deviation indicates that evolutionary forces such as selection, mutation, or drift are acting on the population.
Q30. Why is Hardy–Weinberg principle important?
Answer:
It provides a mathematical model to study population genetics and detect evolutionary changes.
Section G: Human Evolution
Q31. Explain the concept of human evolution.
Answer:
Human evolution describes the gradual development of modern humans from ancestral primates through anatomical, physiological, and behavioural changes.
Q32. Describe the significance of Dryopithecus.
Answer:
Dryopithecus was an ape-like ancestor showing arboreal habits, representing an early stage in human evolution.
Q33. What was the importance of Homo habilis?
Answer:
Homo habilis was the earliest tool-using human, marking a significant advancement in cognitive and manual skills.
Q34. Explain the role of Homo erectus in evolution.
Answer:
Homo erectus showed upright posture, larger brain, and use of fire, indicating advanced survival and social behaviour.
Q35. Describe Homo sapiens.
Answer:
Homo sapiens are modern humans characterised by high intelligence, language, culture, and advanced tool use.
Section H: Trends and Patterns
Q36. What are the major trends in human evolution?
Answer:
Major trends include increased brain size, reduced jaw size, bipedalism, speech development, and complex social behaviour.
Q37. Explain bipedalism in human evolution.
Answer:
Bipedalism allowed humans to walk upright, freeing hands for tool use and improving vision, aiding survival and adaptation.
Q38. What evidence supports African origin of humans?
Answer:
Fossil records and molecular studies indicate that modern humans originated in Africa and migrated globally.
Q39. How did culture influence human evolution?
Answer:
Culture, language, and tool use accelerated human adaptation and survival, influencing evolutionary success.
Q40. Why is human evolution considered continuous?
Answer:
Human evolution is continuous due to ongoing genetic variation, adaptation, and cultural evolution.
Section I: Natural Selection Patterns
Q41. Explain stabilising selection.
Answer:
Stabilising selection favours intermediate phenotypes and eliminates extreme traits, maintaining population stability.
Q42. Describe directional selection.
Answer:
Directional selection favours individuals with traits at one extreme, shifting population characteristics over time.
Q43. Explain disruptive selection.
Answer:
Disruptive selection favours extreme phenotypes over intermediate ones, potentially leading to speciation.
Q44. How does natural selection lead to speciation?
Answer:
Accumulation of favourable traits and reproductive isolation can result in the formation of new species.
Q45. Why is variation important for evolution?
Answer:
Variation provides raw material for natural selection, enabling adaptation and survival in changing environments.
Section J: Integrated Concepts
Q46. What is convergent evolution?
Answer:
Convergent evolution occurs when unrelated organisms develop similar traits due to similar environmental pressures.
Q47. How does molecular evidence support evolution?
Answer:
Similarities in DNA and protein sequences reflect evolutionary relationships and common ancestry.
Q48. What role does isolation play in evolution?
Answer:
Isolation prevents gene flow, allowing populations to diverge genetically and form new species.
Q49. Why is evolution considered a population-level phenomenon?
Answer:
Evolution involves changes in allele frequencies within populations, not individuals.
Q50. Why is the study of evolution important in biology?
Answer:
Evolution explains the origin, diversity, adaptation, and interrelationships of living organisms.
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