Evolution
NCERT Line-by-Line Breakdown for NEET 2026
Unit 2: Genetics and EvolutionEvolutionary Biology is the study of history of life forms on earth. From the Big Bang to the first cellular form of life, and finally to complex biodiversity, this chapter unravels the mystery of our origins.
1. Origin of Life
A. Theories
- Big Bang Theory: Explains origin of Universe (20 billion years ago). Earth formed 4.5 billion years ago.
- Panspermia: Life came from outer space (Spores).
- Spontaneous Generation: Life from decaying matter (Disproved by Louis Pasteur).
- Chemical Evolution (Oparin & Haldane): Life arose from non-living organic molecules (RNA, Protein, etc.).
B. Urey & Miller Experiment (1953)
Created conditions similar to primitive earth in a flask:
• Gases: CH₄, H₂, NH₃ and Water vapor.
• Temp: 800°C with Electric Discharge.
• Result: Formation of Amino acids.
2. Evidences for Evolution
A. Paleontological Evidence
Study of Fossils (remains of hard parts of life-forms found in rocks).
B. Comparative Anatomy & Morphology
| Type | Definition | Evolution Type | Examples |
|---|---|---|---|
| Homologous Organs | Same origin/structure, different function. | Divergent | • Forelimbs of Whale, Bat, Cheetah, Human. • Thorns of Bougainvillea & Tendrils of Cucurbita. |
| Analogous Organs | Different origin, same function. | Convergent | • Wings of Butterfly & Birds. • Eye of Octopus & Mammals. • Sweet Potato (Root) & Potato (Stem). |
C. Industrial Melanism (Natural Selection in Action)
In England (1850s): White-winged moths were more.
After Industrialization (1920s): Dark-winged (Melanised) moths became more due to soot covering trees (Camouflage).
Note: No variant was completely wiped out.
3. Theories of Biological Evolution
A. Lamarckism
Use and Disuse of organs. Example: Giraffes stretching necks to eat tall leaves passed acquired character to progeny. (Disproved).
B. Darwinism
Theory of Natural Selection. Based on observations during sea voyage in H.M.S. Beagle.
- Branching Descent and Natural Selection are the two key concepts.
- “Survival of the Fittest” (Fitness = Reproductive fitness).
C. Mutation Theory (Hugo de Vries)
Worked on Evening Primrose.
- Mutation: Sudden, large, heritable change.
- Mutations are random and directionless (Darwinian variations are small and directional).
- Saltation: Single step large mutation.
4. Hardy-Weinberg Principle
Allele frequencies in a population are stable and constant from generation to generation (Genetic Equilibrium).
Where: p² = Homozygous Dominant (AA), 2pq = Heterozygous (Aa), q² = Homozygous Recessive (aa).
Factors affecting Equilibrium: (1) Gene Migration (Gene flow), (2) Genetic Drift (Founder Effect), (3) Mutation, (4) Genetic Recombination, (5) Natural Selection.
5. Evolution of Man
| Stage | Time (mya) | Brain Capacity | Features |
|---|---|---|---|
| Dryopithecus | 15 mya | – | Ape-like. Hairy. Walked like gorillas. |
| Ramapithecus | 15 mya | – | Man-like. |
| Australopithecines | 2 mya | – | Lived in East African grasslands. Hunted with stones. |
| Homo habilis | – | 650-800 cc | First human-like being. Did not eat meat. |
| Homo erectus | 1.5 mya | 900 cc | Ate meat. |
| Neanderthal | 100k – 40k yrs | 1400 cc | Used hides. Buried dead. |
| Homo sapiens | 75k – 10k yrs | – | Cave art (18k yrs ago). Agriculture (10k yrs ago). |
📝 Rapid Fire MCQs
Q1. Analogous organs arise due to:
- A) Divergent evolution
- B) Artificial selection
- C) Genetic drift
- D) Convergent evolution
Click to check Answer
Answer: D) Convergent evolution.
Q2. Which of the following used hides to protect their body and buried their dead?
- A) Homo habilis
- B) Neanderthal man
- C) Homo erectus
- D) Dryopithecus
Click to check Answer
Answer: B) Neanderthal man.