CBSE Class 12 Biology (2026–27)
Chapter 6: Evolution
20 Important 2–3 Marks Questions and Answers
The CBSE Class 12 Biology syllabus for 2026–27 includes Origin of Life, evidences of evolution, Darwin’s theory, modern synthetic theory, natural selection, gene flow, genetic drift, Hardy–Weinberg principle, adaptive radiation, and human evolution.
1. What is biological evolution?
Answer:
Biological evolution is the gradual change in inherited characteristics of populations over successive generations. It results in the formation of new species and the diversity of life forms seen today. Evolution occurs due to genetic variations produced by mutation and recombination. Natural selection acts on these variations and favors organisms better adapted to their environment. Over long periods, beneficial traits accumulate in populations, leading to evolutionary changes. Evidence from fossils, comparative anatomy, embryology, and molecular biology supports the concept of evolution. Evolution explains the origin of species and their adaptation to changing environmental conditions.
2. Explain the Oparin–Haldane theory of origin of life.
Answer:
The Oparin–Haldane theory states that life originated from non-living organic molecules through chemical evolution. According to this theory, the primitive Earth had a reducing atmosphere containing methane, ammonia, hydrogen, and water vapour. Energy from lightning, volcanic eruptions, and ultraviolet radiation caused these simple molecules to react and form complex organic compounds. These compounds accumulated in oceans, creating a “primordial soup.” Over time, complex molecules formed aggregates called protobionts, which eventually developed into primitive living cells. This theory was experimentally supported by the Miller–Urey experiment, which synthesized amino acids under simulated primitive Earth conditions.
3. What was the significance of the Miller–Urey experiment?
Answer:
The Miller–Urey experiment, conducted in 1953, provided experimental support for the chemical origin of life. The scientists created conditions similar to those believed to exist on primitive Earth by using methane, ammonia, hydrogen, and water vapour in a closed apparatus. Electric sparks simulated lightning. After a few days, several organic compounds, including amino acids, were formed. Amino acids are the building blocks of proteins and are essential for life. The experiment demonstrated that complex organic molecules could arise spontaneously from simple inorganic substances under suitable conditions, supporting the Oparin–Haldane theory of chemical evolution.
4. What are fossils? How do they provide evidence for evolution?
Answer:
Fossils are preserved remains, impressions, or traces of organisms that lived in the past. They are usually found in sedimentary rocks. Fossils provide direct evidence for evolution because they reveal the existence of organisms that lived millions of years ago and show gradual changes in life forms over time. Fossil records indicate transitional forms connecting ancestral and modern species. By studying the age and structure of fossils, scientists can trace evolutionary relationships and the sequence of appearance of different organisms. Thus, paleontological evidence strongly supports the concept that present-day species evolved from earlier forms.
5. Differentiate between homologous and analogous organs.
Answer:
Homologous organs have the same basic structural plan and origin but perform different functions. They indicate divergent evolution and common ancestry. Examples include the forelimbs of humans, whales, bats, and horses. Analogous organs perform similar functions but differ in structure and origin. They indicate convergent evolution, where unrelated organisms develop similar adaptations. Examples include the wings of birds and insects. Homologous organs provide strong evidence for evolution because they suggest descent from a common ancestor, whereas analogous organs demonstrate adaptation to similar environmental conditions despite different evolutionary origins.
6. What is adaptive radiation? Give an example.
Answer:
Adaptive radiation is the process by which a single ancestral species rapidly diversifies into multiple species adapted to different ecological niches. This occurs when organisms encounter new habitats with varied environmental conditions. Each descendant species develops unique adaptations suited to a particular lifestyle or habitat. A classic example is Darwin’s finches of the Galápagos Islands. These birds evolved from a common ancestor but developed different beak shapes and sizes according to their feeding habits. Adaptive radiation demonstrates how environmental factors and natural selection can promote species diversification and evolutionary change.
7. State Darwin’s theory of natural selection.
Answer:
Darwin’s theory of natural selection explains evolution through the survival and reproduction of individuals with favorable variations. Organisms produce more offspring than can survive due to limited resources. Individuals within a population show variations, some of which provide advantages in a particular environment. Those possessing beneficial traits survive, reproduce, and pass these traits to their offspring. Less adapted individuals are eliminated. Over generations, favorable characteristics accumulate in the population, resulting in evolutionary changes and the formation of new species. Darwin called this process “survival of the fittest,” which forms the basis of modern evolutionary theory.
8. Explain industrial melanism as evidence of natural selection.
Answer:
Industrial melanism refers to the increase in dark-colored individuals in polluted industrial areas. The classic example involves the peppered moth. Before industrialization, light-colored moths were common because they blended with lichen-covered trees. During industrialization, soot darkened tree trunks and destroyed lichens. Dark-colored moths became better camouflaged and escaped predation, while light-colored moths were more easily seen by birds. Consequently, dark moths increased in number. This change demonstrated natural selection, where environmental conditions favored one trait over another, leading to changes in population characteristics over time.
9. What is genetic drift?
Answer:
Genetic drift is the random change in allele frequencies within a population due to chance events rather than natural selection. It is more significant in small populations. Genetic drift can cause certain alleles to become more common or disappear entirely, regardless of their adaptive value. Over time, this may reduce genetic variation and contribute to the formation of new species. Founder effect and bottleneck effect are common examples of genetic drift. Since changes occur by chance, genetic drift can influence evolution even when environmental conditions remain unchanged.
10. What is gene flow? How does it affect evolution?
Answer:
Gene flow is the transfer of alleles from one population to another through migration and interbreeding. When individuals move between populations and reproduce, they introduce new genetic material into the recipient population. Gene flow increases genetic variation and may alter allele frequencies. It can reduce genetic differences between populations and prevent speciation by maintaining genetic similarity. In some cases, gene flow introduces advantageous genes that improve adaptation. Thus, gene flow is an important evolutionary force influencing genetic diversity and population structure.
11. State the Hardy–Weinberg principle.
Answer:
The Hardy–Weinberg principle states that allele and genotype frequencies in a population remain constant from generation to generation if no evolutionary forces act on the population. The equilibrium is represented by the equation:
p^2+2pq+q^2=1
Here, p and q represent allele frequencies. The principle assumes a large population, random mating, absence of mutation, migration, natural selection, and genetic drift. Any deviation from this equilibrium indicates that evolution is occurring. The Hardy–Weinberg principle serves as a useful tool for studying population genetics and understanding evolutionary changes.
12. Mention factors that disturb Hardy–Weinberg equilibrium.
Answer:
Hardy–Weinberg equilibrium is disturbed by several evolutionary forces. These include mutation, which introduces new alleles; gene flow, which transfers alleles between populations; genetic drift, which randomly changes allele frequencies; natural selection, which favors certain traits; and recombination, which creates new gene combinations. Non-random mating can also alter genotype frequencies. When these factors operate, allele frequencies change over generations, causing evolution. Therefore, deviation from Hardy–Weinberg equilibrium serves as evidence that evolutionary processes are influencing the population.
13. What is mutation? Why is it important in evolution?
Answer:
Mutation is a sudden, heritable change in the DNA sequence of an organism. Mutations may occur naturally or due to environmental factors such as radiation and chemicals. They create new alleles and increase genetic variation within populations. Most mutations are neutral or harmful, but some may provide advantages that improve survival and reproduction. Natural selection acts upon these beneficial mutations, allowing them to spread through populations. Thus, mutations serve as the ultimate source of new genetic variation and play a crucial role in the evolutionary process and the origin of new species.
14. Explain divergent evolution with an example.
Answer:
Divergent evolution occurs when related organisms evolve different characteristics due to adaptation to different environments. It results in the development of homologous organs. These structures share a common origin but perform different functions. A classic example is the forelimbs of vertebrates such as humans, bats, whales, and horses. Although these limbs have the same basic skeletal structure, they are adapted for grasping, flying, swimming, and running respectively. Divergent evolution indicates that these organisms evolved from a common ancestor and gradually became different due to varying environmental pressures.
15. Explain convergent evolution with an example.
Answer:
Convergent evolution occurs when unrelated organisms independently evolve similar adaptations because they live in similar environments or face similar selection pressures. This process leads to the formation of analogous organs. For example, the wings of birds and insects perform the same function of flight but differ in structure and evolutionary origin. Similarly, streamlined bodies in sharks and dolphins evolved independently for efficient swimming. Convergent evolution demonstrates how similar environmental conditions can shape organisms in comparable ways despite their different ancestral backgrounds. It provides important evidence for the role of natural selection in evolution.
16. What are molecular evidences of evolution?
Answer:
Molecular evidence for evolution comes from similarities in DNA, RNA, and protein sequences among different organisms. Species sharing a recent common ancestor have more similar genetic material than distantly related species. For example, humans and chimpanzees possess highly similar DNA sequences. Comparative studies of proteins such as cytochrome c also reveal evolutionary relationships. Molecular biology allows scientists to construct evolutionary trees and trace ancestry with great accuracy. Such evidence strongly supports the theory that all living organisms originated from common ancestors and diversified through evolutionary processes.
17. Describe stabilizing natural selection.
Answer:
Stabilizing selection is a type of natural selection that favors average individuals while eliminating extreme variations. As a result, the population mean remains relatively constant, and variation decreases. This form of selection is common in stable environments where intermediate traits provide the highest fitness. An example is human birth weight. Babies with very low or very high birth weights have lower survival rates, while those with average weights survive better. Stabilizing selection helps maintain established adaptations and reduces the occurrence of unfavorable extreme characteristics within populations.
18. What is directional selection?
Answer:
Directional selection occurs when natural selection favors individuals at one extreme of a trait distribution. As a result, the population average shifts toward that extreme over time. This type of selection usually occurs when environmental conditions change. A well-known example is industrial melanism in peppered moths, where dark-colored moths became more common in polluted areas. Directional selection promotes adaptation by increasing the frequency of advantageous traits and can lead to significant evolutionary changes if environmental pressures persist for many generations.
19. Write a short note on human evolution.
Answer:
Human evolution describes the gradual development of modern humans from primate ancestors. Early ancestors included Dryopithecus and Ramapithecus. Later forms such as Australopithecus exhibited upright walking. Homo habilis showed tool-making abilities, while Homo erectus possessed a larger brain and used fire. Neanderthals demonstrated advanced social behavior and hunting skills. Finally, Homo sapiens evolved with highly developed intelligence, language, and culture. Fossil records, comparative anatomy, and molecular studies provide evidence for human evolution. This evolutionary journey reflects the gradual accumulation of adaptive traits over millions of years.
20. Explain antibiotic resistance in bacteria using Darwin’s theory.
Answer:
Antibiotic resistance illustrates Darwin’s theory of natural selection. In a bacterial population, some individuals possess genetic variations that make them resistant to antibiotics. When an antibiotic is applied, sensitive bacteria are killed, while resistant bacteria survive. These surviving bacteria reproduce rapidly and pass the resistance genes to their offspring. Over time, the population becomes dominated by resistant bacteria. The antibiotic does not create resistance; rather, it selects individuals already carrying resistant traits. This example demonstrates how natural selection acts on existing variations, leading to evolutionary changes in populations.