Evolution Class 12 Notes विकास — कक्षा 12 जीव विज्ञान अध्याय 6

Revise these first (prerequisites)

• Chapter 4–5: gene, allele, mutation, variation
• What is a fossil (Class 10 'Heredity' level)
• Basic idea of the adaptations of organisms to their environment

1. Simple meaning of evolution

   Evolution (विकास) = gradual change in organisms over a long period of time, leading to the formation of new types (species). It does not happen in a year or two but over millions of years. All present-day organisms have arisen through gradual change from older organisms.

2. How did life begin?

   Earth was formed about 4.5 billion years ago. Oparin and Haldane proposed that organic molecules first formed from simple chemicals (such as CH₄, NH₃, water) — this is called chemical evolution. Miller tried this in the lab and showed that amino acids could be formed.

3. Where does the evidence come from?

   Fossils (जीवाश्म) — remains of ancient organisms buried in rocks — tell us which organisms existed earlier. Comparing the organs of different organisms (e.g., the human arm and the bat's wing) also shows that they had common ancestors.

4. Natural selection — 'the fit move ahead'

   According to Darwin, every population has variations. Individuals that survive better in their environment and produce more offspring pass on their traits, which increase over successive generations. 'Fitness' here does not mean strength; it means reproductive fitness.

5. A real example: peppered moth

   In England, before industrialisation, white moths were more numerous. Pollution turned the tree trunks dark, so white moths became easily visible to birds and were eaten, while dark moths survived. Hence dark moths later became more numerous — a direct example of natural selection.

6. Human evolution

   Humans also came about through evolution. From Australopithecines, through Homo habilis, Homo erectus and Neanderthal, to Homo sapiens — the brain (brain capacity) gradually increased. Homo sapiens arose in Africa and spread from there across the world.

Quick check ✔

Basics clear? Now move on to Learn in Detail 👆

As per NCERT 2026-27 — every point you need for the board exam.

1. Origin of life

   The universe is ~13.8 billion years old (Big Bang), the earth ~4.5 billion years. Life appeared ~4 billion years ago. Oparin & Haldane: the first life arose from non-living organic molecules (chemical evolution). Spontaneous generation was disproved by Louis Pasteur.

2. Miller's experiment (1953)

   S.L. Miller passed an electric discharge through CH₄, H₂, NH₃ and water vapour at 800°C → amino acids were formed. This supports chemical evolution.

3. Paleontological evidence

   Fossils in rock layers — organisms of different ages; some extinct (e.g., dinosaurs). Life-forms changed over time.

4. Homologous organs (divergent evolution)

   Same structure, different function: forelimbs of whale, bat, cheetah and human; heart, brain. Plants: thorn of Bougainvillea and tendril of Cucurbita.

5. Analogous organs (convergent evolution)

   Different structure, same function: wings of butterfly and bird; eye of octopus and mammal; flippers of penguin and dolphin; sweet potato (root) and potato (stem).

6. Natural selection – examples

   Natural selection in peppered moths (England, effect of industrial pollution on moth colour): in the 1850s, before industrialisation, white-winged moths were more numerous; after industrialisation (1920) dark-winged moths became more numerous. Herbicide/pesticide resistance, antibiotic resistance — evolution due to anthropogenic action.

7. Adaptive radiation

   New forms evolving by radiating from one point into different habitats. Darwin's finches (Galapagos), Australian marsupials. Convergent evolution between placental mammals and marsupials.

8. Darwin's theory

   Branching descent + natural selection. Fitness = reproductive fitness. Alfred Wallace also reached the same conclusion. Lamarck: use and disuse (giraffe's neck) — no longer accepted.

9. Mutation theory

   Hugo de Vries (evening primrose): change through mutation — sudden, large steps (saltation). Darwin's variations were small and directional.

10. Hardy–Weinberg principle

    Allele frequencies in a population remain constant from generation to generation: p² + 2pq + q² = 1. Factors that change them: gene migration / gene flow, genetic drift (founder effect), mutation, genetic recombination, natural selection.

11. Types of natural selection

    Stabilising (more individuals at the mean value), directional (towards one extreme), disruptive (towards both extremes).

12. Brief history

    Jawless fish ~350 mya; lobefins (coelacanth) → first amphibians; reptiles → dinosaurs; dinosaurs disappeared ~65 mya; mammals expanded.

13. Human evolution

    Dryopithecus, Ramapithecus (~15 mya) → Australopithecines (~2 mya, East Africa) → Homo habilis (first human-like, 650–800 cc, did not eat meat) → Homo erectus (~1.5 mya, 900 cc, ate meat) → Neanderthal (1400 cc, buried their dead) → Homo sapiens (arose in Africa). Agriculture began ~10,000 years ago.

The concept in depth — the why and the how. Everything stays within the NCERT syllabus.

1. Homology vs analogy: which direction of evolution?

   Homologous organs (forelimbs of whale, bat, cheetah, human; thorn of Bougainvillea and tendril of Cucurbita) — same structure/origin, different function → divergent evolution and common ancestry. Analogous organs (wings of butterfly and bird; eyes of octopus and mammals; flippers of penguin and dolphin; sweet potato (root) and potato (stem)) — different structure, same function → convergent evolution, due to similar habitat.

2. Moths and pollution — selection, not mutation

   Dark moths were not 'created' after industrialisation; both types of moths already existed. Pollution killed the lichens and tree trunks turned dark, so the environment 'selected' the dark moths. Lichens are indicators of pollution. Similarly, the use of antibiotics/pesticides causes resistant microbes or insects to increase through selection — this is evolution by anthropogenic (human) action, and it links to antibiotics in Chapter 8.

3. Adaptive radiation and convergence together

   Many forms evolving from one ancestor in one area for different habitats = adaptive radiation (Darwin's finches; Australian marsupials). Similar forms in Australian marsupials and placental mammals (e.g. marsupial wolf and placental wolf) — two separate adaptive radiations looking alike — is an example of convergent evolution.

4. Hardy–Weinberg: the 'nothing has changed' equation

   In a population, allele frequencies (p, q) remain constant from generation to generation if there is no disturbing factor: p² + 2pq + q² = 1. Five factors disturb the equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection. If the measured value differs from the equilibrium, it means evolution is taking place. The founder effect is a form of drift in which the allele frequency of a small new population changes.

5. Three patterns of selection

   Stabilising: middle (mean) individuals are favoured → variation decreases. Directional: shift towards one extreme. Disruptive: both extremes are favoured, middle ones decrease → two peaks form. Identify the type from the shape of the curve in the graph.

6. Darwin vs de Vries

   Darwin: small, directional variations and gradual change. Hugo de Vries, working on evening primrose, proposed that large, sudden variations — mutations — cause evolution; mutations are random and directionless. He called a single-step large mutation saltation. According to the present understanding, variation (mutation + recombination) and natural selection work together.

Common confusions — cleared up

1. Homologous vs Analogous organs

   Homologous: same origin/structure, different function (divergent). Analogous: different origin/structure, same function (convergent).

2. Lamarck vs Darwin

   Lamarck: acquired characters are inherited through use and disuse (giraffe's neck). Darwin: variations already exist; natural selection picks the fit ones from among them.

3. Genetic drift vs Gene flow

   Gene flow = movement of alleles into/out of a population (migration). Genetic drift = change in allele frequency by chance, especially in small populations.

4. Adaptive radiation vs Convergent evolution

   Adaptive radiation: many different forms arise from one ancestor (divergence). Convergent: organisms from different ancestors come to look similar.

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