Class 12 Biology Notes
Complete, exam-ready notes on evolution: how life originated from inorganic molecules, the evidences that support evolutionary theory, Darwin's natural selection, the modern synthetic theory, Hardy-Weinberg equilibrium, speciation and the story of human evolution. Essential reading for CBSE Class 12 and NEET Biology.
Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali
Allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary influences, described by p² + 2pq + q² = 1.
In the reducing atmosphere of early Earth (CH₄, NH₃, H₂O, H₂ — no free O₂), simple inorganic molecules formed organic monomers (amino acids, sugars, nucleotides) through energy from lightning and UV radiation. These monomers polymerised in warm seas to form primitive life. This abiogenic origin of life from non-living matter is the Oparin-Haldane hypothesis.
Stanley Miller and Harold Urey simulated early Earth conditions in a closed apparatus: water vapour, CH₄, NH₃ and H₂ were subjected to electric discharge (simulating lightning). After a week, amino acids (glycine, alanine, aspartic acid) and other organic compounds were detected in the trap. This experiment demonstrated that organic molecules could arise from inorganic precursors under prebiotic conditions.
Exam favourite
NEET frequently asks the exact gases used in the Miller-Urey experiment and the type of energy source (electric discharge / spark discharge). Remember: CH₄, NH₃, H₂, H₂O vapour — a reducing atmosphere without O₂.
Multiple independent lines of evidence converge to support the theory of organic evolution.
Organs with similar basic structure and embryonic origin but different functions — e.g. the forelimbs of humans, whales, bats and cheetah. They indicate divergent evolution from a common ancestor.
Organs with similar function but different structure and origin — e.g. wings of insects and birds, or the flippers of penguins and dolphins. They indicate convergent evolution (similar environments selecting for similar solutions).
Variation exists in every population. Individuals with traits better suited to their environment survive and reproduce more — 'survival of the fittest.' Over generations, favourable alleles accumulate, and the population adapts. Darwin proposed branching descent with modification from common ancestors.
What Darwin did NOT know
Darwin had no knowledge of Mendelian genetics, mutations or DNA. His theory could not explain the source of variation or how traits are inherited. The modern synthetic theory later integrated Mendelian genetics with natural selection.
Key observations by Darwin during the Beagle voyage included: overproduction of offspring, struggle for existence, limited food and space, and competition leading to differential survival.
The rapid diversification of a single ancestral lineage into multiple species occupying different ecological niches. Classic examples include Darwin's finches in the Galápagos Islands (beak shape diversified for different food sources) and Australian marsupials (a single ancestral stock radiated into kangaroos, koalas, wombats, Tasmanian devils and more).
Combines Darwinian natural selection with Mendelian genetics, mutation theory and population genetics. Evolution is defined as a change in allele frequencies in a population over generations. Key contributors include Hugo de Vries (mutation theory), Hardy and Weinberg (equilibrium), Fisher, Haldane and Wright (population genetics).
In an ideal population (random mating, no mutation, no migration, no selection, large size), allele frequencies (p and q) and genotype frequencies (p², 2pq, q²) remain constant across generations. Any deviation means evolution is occurring.
Five agents break Hardy-Weinberg equilibrium and drive evolution: (1) Gene flow — migration of alleles between populations; (2) Genetic drift — random change in allele frequencies, especially in small populations; the founder effect occurs when a few individuals colonise a new area; (3) Mutation — introduces new alleles; (4) Natural selection — differential survival and reproduction favour certain genotypes; (5) Non-random mating — changes genotype frequencies.
NEET numerical
If the frequency of the homozygous recessive genotype (q²) in a population is 0.09, then q = 0.3, p = 0.7. Carrier frequency (2pq) = 2 × 0.7 × 0.3 = 0.42 or 42%. Practice these calculations — they appear every year.
The formation of one or more new species from an existing species. Reproductive isolation is essential — populations that can no longer interbreed to produce fertile offspring have speciated.
Human evolution traces the lineage from a common ancestor shared with great apes (orangutan, gorilla, chimpanzee) to modern Homo sapiens. The fossil record shows a trend of increasing brain size, bipedalism, tool use and complex social behaviour.
Remember the sequence
Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens. NEET loves asking the correct chronological order and distinguishing features.
Example: In a population of 100 individuals, 36 show the recessive phenotype. Using the Hardy-Weinberg principle, calculate the allele frequencies and the number of carriers.
Solution: Recessive genotype q² = 36/100 = 0.36, so q = 0.6 and p = 1 − 0.6 = 0.4. Carrier frequency 2pq = 2 × 0.4 × 0.6 = 0.48. Number of carriers = 0.48 × 100 = 48 individuals. Homozygous dominant = p² × 100 = 0.16 × 100 = 16 individuals.
Example: Wings of butterflies and wings of birds are analogous organs. Explain why this is evidence for convergent evolution rather than divergent evolution.
Solution: Butterfly wings and bird wings serve the same function (flight) but have completely different structural origins — butterfly wings are chitinous membranes, while bird wings are modified forelimbs with bones, muscles and feathers. Since the common ancestor of insects and vertebrates did not have wings, both lineages independently evolved flight in response to similar selective pressures. This is convergent evolution, not divergent evolution from a winged ancestor.
Revision
Memorise these before attempting numericals — most exam questions hinge on one of them.
Hardy-Weinberg genotype eq.
Allele frequency sum
Carrier frequency
Recessive freq from q^2
Homozygous dominant freq
Diploid chromosome number
Brain capacity trend
Exam tips
Where this topic appears in CBSE, JEE Main and NEET papers.
FAQ
It showed that amino acids and other organic molecules can form spontaneously from inorganic precursors (CH₄, NH₃, H₂, H₂O) under simulated early-Earth conditions with electric discharge, supporting the Oparin-Haldane chemical evolution hypothesis.
Homologous organs share a similar basic structure and embryonic origin but have different functions (e.g. human arm and bat wing), indicating divergent evolution. Analogous organs share the same function but differ in structure and origin (e.g. insect wing and bird wing), indicating convergent evolution.
Genetic drift is a random, chance-based change in allele frequencies that is more pronounced in small populations (including the founder effect). Natural selection is a non-random process where alleles conferring higher survival and reproduction increase in frequency.
It provides a mathematical baseline — if allele and genotype frequencies in a population deviate from the expected p² + 2pq + q² = 1 equilibrium, evolution is occurring. It is also used to calculate carrier frequencies for genetic diseases.
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