Chapter 7 — Evolution — traces the story of life from origin to the diverse species we see today. It covers theories of evolution, evidences, Hardy-Weinberg principle, and human evolution. Carries 5-7 marks in Board exams.
Key Concepts
Origin of Life
- Oparin-Haldane theory: Life originated from pre-existing non-living organic molecules (chemical evolution)
- Miller-Urey experiment (1953): Simulated primitive atmosphere (CH₄, NH₃, H₂, H₂O) + electric discharge → amino acids formed. Proved abiogenic origin of life.
- First life: ~3.5 billion years ago; were probably chemoheterotrophs
Theories of Evolution
| Theory | Proponent | Key Idea |
|---|---|---|
| Use and Disuse | Lamarck | Organs used more develop; acquired characters inherited (disproved) |
| Natural Selection | Darwin | Survival of the fittest; variations + struggle → adaptation |
| Mutation Theory | Hugo de Vries | Large sudden heritable changes (mutations) drive evolution |
Evidences for Evolution
| Evidence | Description | Example |
|---|---|---|
| Homologous organs | Same origin, different function (divergent evolution) | Forelimbs of whale, bat, horse, human |
| Analogous organs | Different origin, same function (convergent evolution) | Wings of bird vs butterfly |
| Vestigial organs | Reduced/non-functional remnants | Appendix, wisdom teeth, nictitating membrane in humans |
| Fossils | Preserved remains of ancient organisms | Archaeopteryx (link between reptiles and birds) |
| Embryology | Similar embryonic development across vertebrates | Gill slits in all vertebrate embryos |
| Molecular evidence | DNA/protein sequence similarities | Cytochrome c similarity across species |
Hardy-Weinberg Principle
p² + 2pq + q² = 1 (genotype frequencies)
p + q = 1 (allele frequencies, where p = dominant, q = recessive)
Allele frequencies remain constant in a population IF: no mutation, no selection, random mating, no migration, large population
Factors that disturb equilibrium: mutation, selection, genetic drift, migration, non-random mating
p + q = 1 (allele frequencies, where p = dominant, q = recessive)
Allele frequencies remain constant in a population IF: no mutation, no selection, random mating, no migration, large population
Factors that disturb equilibrium: mutation, selection, genetic drift, migration, non-random mating
Types of Natural Selection
- Stabilising: Favours average phenotype → reduces variation (e.g., birth weight in humans)
- Directional: Favours one extreme phenotype (e.g., industrial melanism in peppered moth)
- Disruptive: Favours both extremes → may lead to speciation
Human Evolution (in order)
Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Homo sapiens neanderthalensis → Homo sapiens sapiens (modern humans, ~100,000 years ago, Africa)
Key milestones: Australopithecus (bipedal, 2 mya); Homo habilis (first tool maker); Homo erectus (used fire); Neanderthals (large brain, buried dead); Modern humans originated in Africa.
Solved Examples
Example 1
Q: In a population, frequency of allele ‘a’ is 0.4. Find genotype frequencies.
Solution: q = 0.4, p = 1 − 0.4 = 0.6
AA (p²) = 0.36, Aa (2pq) = 0.48, aa (q²) = 0.16
Quick Revision Points
- Origin: Oparin-Haldane (chemical evolution); Miller-Urey (proved amino acid synthesis)
- Darwin: natural selection; Lamarck: use/disuse (disproved)
- Homologous: same origin, different function; Analogous: different origin, same function
- Hardy-Weinberg: p² + 2pq + q² = 1; disturbed by 5 factors
- Industrial melanism: directional selection (peppered moth)
- Modern humans: Homo sapiens sapiens; originated in Africa
Chapter Navigation
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Next: Human Health and Disease Class 12 Notes
Related Chapters in Class 12 Biology
- Principles of Inheritance and Variation Class 12 Notes
- Molecular Basis of Inheritance Class 12 Notes
- Ecosystem Class 12 Notes
Practice What You Learned
Test yourself with our NEET Mock Test Set 1 to see how well you’ve mastered the concepts.