Evolution
'Nothing in biology makes sense EXCEPT in the light of evolution.' — Theodosius Dobzhansky
1. Chapter Overview
This chapter explores THE ORIGIN OF LIFE and the MECHANISMS of evolutionary change. Topics include: the ORIGIN OF LIFE (Oparin-Haldane theory, Miller-Urey experiment), EVIDENCE FOR EVOLUTION (fossils, comparative anatomy, embryology, molecular biology), LAMARCKISM (inheritance of acquired characters), DARWIN'S THEORY of Natural Selection, the HARDY-WEINBERG PRINCIPLE (conditions for genetic equilibrium, factors that disrupt it), ADAPTIVE RADIATION (Darwin's finches, marsupials), and HUMAN EVOLUTION.
2. Origin of Life
- Early Earth (~4.5 billion years ago) : Reducing atmosphere (CH₄, NH₃, H₂O, H₂) — NO free O₂. High temperature, lightning, UV radiation.
Oparin-Haldane Theory (1920s)
- 'Life arose from NON-LIVING matter through a series of CHEMICAL EVOLUTION steps — inorganic molecules → organic molecules → protocells → primitive life.'
Miller-Urey Experiment (1953)
- Setup: Simulated early Earth conditions — CH₄, NH₃, H₂O vapour, H₂ in a closed system with ELECTRIC SPARKS (simulating lightning).
- Result: After a week, 15% of carbon formed SIMPLE ORGANIC COMPOUNDS — including AMINO ACIDS (glycine, alanine, aspartic acid).
- 'Miller-Urey showed that the BUILDING BLOCKS OF LIFE could form spontaneously under early Earth conditions.'
3. Evidences for Evolution
| Type of Evidence | Description | Example |
|---|---|---|
| Palaeontological (Fossils) | Remains of extinct organisms in rock layers | Transitional fossils: Archaeopteryx (reptile → bird). Horse evolution series |
| Comparative anatomy | HOMOLOGOUS ORGANS: Same structure, DIFFERENT function — indicate COMMON ANCESTRY | Forelimbs of humans, whales, bats, horses |
| Analogous organs | DIFFERENT structure, SAME function — CONVERGENT evolution | Wings of birds and insects, eyes of vertebrates and octopus |
| Embryology | Similarities in EMBRYONIC development suggest common ancestry | Vertebrate embryos (gill slits, tail — at early stages) |
| Molecular biology | DNA and protein SEQUENCE similarities indicate relatedness | Humans and chimpanzees share ~99% DNA identity |
| Vestigial organs | Non-functional remnants of organs that were functional in ancestors | Human appendix, wisdom teeth, tailbone (coccyx) |
4. Theories of Evolution
Lamarckism (1809)
- Principles: (1) Use and disuse of organs. (2) Inheritance of ACQUIRED CHARACTERS.
- Example: Giraffes STRETCHED their necks to reach high leaves → that stretching was INHERITED by offspring.
- Criticism: Acquired characters are NOT inherited (e.g., a person who loses a finger does NOT have children missing that finger). 'Lamarck was WRONG about the mechanism — but RIGHT that evolution occurs.'
Darwin's Natural Selection (1859)
- Key observations: (1) Overproduction of offspring. (2) Variation among individuals. (3) Limited resources lead to STRUGGLE FOR EXISTENCE. (4) Individuals with FAVOURABLE variations SURVIVE and REPRODUCE more (SURVIVAL OF THE FITTEST).
- Result: Over generations, the population becomes BETTER ADAPTED to its environment.
Modern Synthetic Theory (Neo-Darwinism)
- Combines: Natural Selection + MUTATION + Genetic Drift + Gene Flow + Recombination.
- 'The MODERN SYNTHESIS integrates Darwinian selection with MENDELIAN GENETICS.'
5. Hardy-Weinberg Principle
Statement
- 'The ALLELE FREQUENCIES in a population remain CONSTANT from generation to generation — in the ABSENCE of evolutionary influences.'
- Equation: p² + 2pq + q² = 1. Where p = frequency of dominant allele, q = frequency of recessive allele.
Conditions for Hardy-Weinberg Equilibrium
- LARGE population size (no genetic drift).
- RANDOM MATING (no sexual selection).
- NO MUTATION.
- NO GENE FLOW (no migration).
- NO NATURAL SELECTION.
Factors That Disrupt Equilibrium (Cause Evolution)
- Genetic drift: Random changes in allele frequency — especially significant in SMALL populations.
- Founder effect: A SMALL GROUP colonises a new area — limited genetic diversity.
- Gene flow: Migration of individuals between populations (gene flow).
- Natural selection: Favours alleles that increase fitness.
- Mutation: Introduces NEW genetic variation.
Worked Example 1
Problem: In a population of 1000 individuals, 360 have recessive phenotype (q²). Find p, q, and number of carriers (heterozygotes). Solution: q² = 360/1000 = 0.36. q = 0.6. p = 1 − 0.6 = 0.4. Heterozygotes (2pq) = 2×0.4×0.6 = 0.48 = 480 individuals.
6. Adaptive Radiation
- 'The EVOLUTION OF DIFFERENT species from a COMMON ANCESTOR in DIFFERENT environments — resulting in diverse adaptations.'
- Darwin's finches (Galapagos) : ONE ancestral finch species RADIATED into 14 species with different beak shapes adapted to different food sources.
- Australian marsupials: Adaptive radiation produced marsupial equivalents of placental mammals (e.g., Tasmanian wolf = marsupial wolf, sugar glider = marsupial flying squirrel).
7. Human Evolution
- Timeline: Hominins diverged from chimpanzee lineage ~6-7 million years ago.
- Key stages:
- Australopithecus (~4 mya): BIPEDAL, small brain. 'Lucy' — the famous fossil.
- Homo habilis (~2.5 mya): First to use SIMPLE STONE TOOLS.
- Homo erectus (~1.8 mya): Larger brain, CONTROL of FIRE.
- Homo neanderthalensis (~400,000-40,000 years ago): Lived in Europe and Asia.
- Homo sapiens (~300,000 years ago to present): MODERN humans — migrated from AFRICA (~60,000 years ago) to colonise the world.
- 'The OUT OF AFRICA hypothesis suggests that ALL modern humans descended from a SMALL POPULATION that lived in EAST AFRICA about 200,000 years ago.'
8. Comparison Table: Lamarckism vs Darwinism
| Aspect | Lamarckism | Darwinism |
|---|---|---|
| Source of variation | RESPONSE to environment | RANDOM variations (by chance) |
| Mechanism | Use and DISUSE → INHERITANCE of acquired characters | NATURAL SELECTION — differential survival and reproduction |
| Role of environment | CREATES need for change | SELECTS among existing variations |
| Giraffe neck | STRETCHING to reach leaves — inherited | LONGER necks existed by chance — those with longer necks survived BETTER |
| Modern status | REJECTED (acquired characters NOT inherited) | ACCEPTED (with modifications — Modern Synthesis) |
9. Common Mistakes
- Evolution does NOT mean 'progress': Evolution is about ADAPTATION to local environments — not a ladder of progress. A bacterium is JUST AS EVOLVED as a human.
- Natural selection does NOT create new traits: It selects among EXISTING variations produced by MUTATION.
- 'Survival of the fittest' does NOT mean strongest: FITNESS = ability to SURVIVE and REPRODUCE in a given environment — not physical strength.
- Humans evolved from apes? NO: Humans and apes share a COMMON ANCESTOR that lived ~6-7 million years ago. Modern apes and humans evolved SEPARATELY from that ancestor.
10. CBSE Exam Focus
- Origin of life — Oparin-Haldane, Miller-Urey experiment
- Evidences for evolution — homologous vs analogous organs, fossils, embryology, molecular evidence
- Darwin's theory — natural selection, overproduction, variation
- Hardy-Weinberg principle — equation, conditions for equilibrium, factors that change gene frequency
- Adaptive radiation — Darwin's finches, marsupials
- Human evolution — key stages, out of Africa hypothesis
11. Self-Test
Q1: What is the difference between homologous and analogous organs? Give an example of each. A1: Homologous: SAME STRUCTURE, DIFFERENT FUNCTION — indicate COMMON ANCESTRY (e.g., human arm, bat wing, whale flipper). Analogous: DIFFERENT STRUCTURE, SAME FUNCTION — indicate CONVERGENT EVOLUTION (e.g., bird wing and insect wing).
Q2: State the Hardy-Weinberg principle. What are the conditions required for genetic equilibrium? A2: The Hardy-Weinberg principle states that ALLELE FREQUENCIES remain CONSTANT across generations in the ABSENCE of evolutionary forces. Conditions: (1) Large population, (2) Random mating, (3) No mutation, (4) No gene flow, (5) No natural selection.
Q3: In a population, 1% of individuals have a recessive disorder. Calculate the frequency of carriers. A3: q² = 0.01 ⇒ q = 0.1. p = 0.9. Carriers (2pq) = 2×0.9×0.1 = 0.18 = 18%.
Q4: What is adaptive radiation? Explain with reference to Darwin's finches. A4: Adaptive radiation is the evolution of MULTIPLE SPECIES from a COMMON ANCESTOR in response to DIFFERENT ENVIRONMENTS. Darwin's finches in the Galapagos evolved from ONE ancestral species into 14 species with DIFFERENT BEAK SHAPES adapted to eating seeds, insects, cactus, etc.
Q5: What evidence suggests that humans originated in Africa? A5: (1) The OLDEST hominin fossils (~300,000 years) have been found in AFRICA. (2) GENETIC diversity is HIGHEST in African populations (indicating the longest history). (3) DNA analysis of Mitochondrial Eve suggests a single African ancestor ~200,000 years ago.
12. Conclusion
Evolution is the UNIFYING THEME of biology:
- MECHANISM: 'Natural selection ACTS on genetic variation — differential survival and reproduction over generations.'
- EVIDENCE: 'Fossils, anatomy, embryology, and DNA — ALL point to common descent.'
- POPULATION GENETICS: 'Hardy-Weinberg gives us a MATHEMATICAL baseline — deviations MEASURE the rate of evolution.'
- 'Evolution is NOT a controversial theory among biologists — it is as well-established as GRAVITY or GERM THEORY of disease.'
