Chapter 7: Evolution – Study Modules with Revision Notes
CBSE Class 12 Biology Study Module (NCERT Based): Evolution – Unit II Genetics and 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 (Theory, MCQs, Case-Based & Competency-Based Questions)
Introduction to the Chapter
The chapter Evolution explains the origin of life on Earth, the gradual change in organisms over time, and the processes responsible for biological diversity. It integrates concepts from genetics, palaeontology, embryology, and molecular biology to explain how life evolved and diversified. Evolution is a highly conceptual chapter in CBSE Class 12 Biology and is frequently tested through assertion–reason questions, numericals based on Hardy–Weinberg equilibrium, case-based questions, and long answers.
1. Origin of Life
The origin of life refers to the process by which living organisms arose from non-living matter.
1.1 Special Creation Theory
- Life was created by a supernatural power.
- Does not explain fossil records or biological diversity.
- Not accepted scientifically.
1.2 Spontaneous Generation Theory
- Proposed that life originated from non-living matter suddenly.
- Disproved by experiments of Redi, Spallanzani, and Pasteur.
1.3 Theory of Chemical Evolution
Proposed by A. I. Oparin and J. B. S. Haldane.
Key Points
- Early Earth had a reducing atmosphere (CH₄, NH₃, H₂, H₂O).
- Simple inorganic molecules formed organic compounds.
- Organic molecules aggregated to form protocells.
1.4 Miller–Urey Experiment
- Demonstrated abiotic synthesis of organic molecules.
- Supported chemical evolution theory.
1.5 Modern View
- Life originated around 4 billion years ago.
- First organisms were anaerobic and heterotrophic.
- Photosynthetic organisms released oxygen, changing atmosphere.
2. Evidences for Evolution
Evolution is supported by multiple lines of evidence.
2.1 Morphological and Anatomical Evidences
Homologous Organs
- Same basic structure, different functions.
- Indicate common ancestry.
- Example: Forelimbs of mammals.
Analogous Organs
- Different origin, similar function.
- Indicate convergent evolution.
- Example: Wings of birds and insects.
2.2 Vestigial Organs
- Reduced, non-functional structures.
- Example: Appendix, coccyx in humans.
2.3 Embryological Evidence
- Early embryos of vertebrates show similarities.
- Suggest common ancestry.
2.4 Palaeontological Evidence
- Fossils provide direct evidence of evolution.
- Show transitional forms like Archaeopteryx.
2.5 Molecular Evidence
- Similarities in DNA, RNA, and protein sequences.
- Cytochrome c comparison supports evolutionary relationships.
3. Adaptive Radiation
Adaptive radiation refers to the evolution of multiple species from a common ancestor, each adapted to a different ecological niche.
Key Features
- Rapid diversification
- Occupation of varied habitats
- Driven by natural selection
Examples
- Darwin’s finches in Galapagos Islands
- Australian marsupials
Significance
- Explains biological diversity
- Shows divergent evolution
4. Biological Evolution
Biological evolution is the gradual change in the genetic makeup of populations over generations.
4.1 Lamarckism
Proposed by Jean-Baptiste Lamarck.
Principles
- Use and disuse of organs
- Inheritance of acquired characters
Limitations
- Acquired traits are not inherited.
- Experimentally disproved.
4.2 Darwinism (Natural Selection)
Proposed by Charles Darwin.
Key Postulates
- Overproduction
- Variation
- Struggle for existence
- Survival of the fittest
- Natural selection
Significance
- Explains adaptation and speciation
- Foundation of modern evolutionary biology
5. Mechanism of Evolution
Evolution occurs due to changes in allele frequencies within populations.
5.1 Mutation
- Sudden heritable changes in DNA.
- Creates new alleles.
5.2 Gene Flow (Migration)
- Movement of individuals between populations.
- Alters gene frequency.
5.3 Genetic Drift
- Random changes in gene frequency.
- Significant in small populations.
- Examples: Founder effect, Bottleneck effect.
5.4 Natural Selection
- Differential survival and reproduction.
- Types:
- Stabilising selection
- Directional selection
- Disruptive selection
5.5 Recombination
- Produces new gene combinations.
- Increases variation.
6. Hardy–Weinberg Principle
Proposed by G. H. Hardy and Wilhelm Weinberg.
Statement
Allele frequencies in a population remain constant from generation to generation in the absence of evolutionary forces.
Equation
- p² + 2pq + q² = 1
- p + q = 1
Conditions for Equilibrium
- Large population
- Random mating
- No mutation
- No migration
- No natural selection
Evolutionary Forces
Any deviation from equilibrium indicates evolution.
7. Human Evolution
Human evolution explains the origin and development of modern humans.
7.1 Early Ancestors
- Dryopithecus – ape-like
- Ramapithecus – man-like
7.2 Genus Homo
- Homo habilis – tool user
- Homo erectus – upright posture, fire use
- Homo neanderthalensis – cultural behaviour
- Homo sapiens – modern humans
7.3 Key Trends
- Increase in brain size
- Reduced jaw size
- Development of speech
- Social organisation
7.4 African Origin
- Humans originated in Africa.
- Migrated to other continents.
CBSE Board Examination Focus
High-Weightage Areas
- Evidences of evolution
- Darwinism vs Lamarckism
- Hardy–Weinberg numerical problems
- Mechanism of evolution
- Human evolution sequence
Frequently Asked Question Types
- Assertion–Reason
- Case-based questions
- Numerical problems
- Long answer comparisons
- MCQs based on concepts
Quick Revision Notes
- Life originated through chemical evolution.
- Homologous organs indicate common ancestry.
- Adaptive radiation explains diversification.
- Natural selection is the driving force of evolution.
- Hardy–Weinberg equilibrium explains genetic stability.
- Human evolution shows gradual anatomical and behavioural changes.
Conclusion
The chapter Evolution provides a comprehensive understanding of how life originated, diversified, and adapted over millions of years. It connects genetic variation with natural selection and population genetics, offering a scientific explanation for biodiversity and human origin. Mastery of this chapter is essential for scoring well in CBSE Class 12 Biology and for building a strong foundation in evolutionary biology. This NCERT-aligned study module ensures concept clarity, exam relevance, and effective revision.
✔ Strictly NCERT-Based
✔ CBSE Board Examination Aligned
✔ Ideal for Theory, Numericals & Case-Based Questions