CBSE Class 12 Biology: Evolution — Complete Notes 2026

Origin of Life

• Chemical evolution: Oparin-Haldane hypothesis — primitive Earth's reducing atmosphere; Miller-Urey experiment (1953)
• Miller-Urey: H₂, CH₄, NH₃, H₂O vapour in flask with electric sparks → amino acids, purines, sugars formed
• First cells: coacervates and microspheres (proto-cells); eventually leading to prokaryotic cells ~3.5 billion years ago

Darwin's Theory of Natural Selection

• Variations exist in population; heritable; some variants more fit for environment → survive and reproduce more
• Fitness: relative contribution to next generation; adaptive traits increase fitness in specific environment
• Evidence: fossil record, comparative anatomy (homologous organs), molecular biology (cytochrome c similarities)

Hardy-Weinberg Principle

• In absence of evolutionary forces, allele and genotype frequencies remain constant across generations
• H-W equation: p² + 2pq + q² = 1 where p = frequency of dominant allele, q = frequency of recessive allele
• Forces disturbing H-W equilibrium: mutation, natural selection, genetic drift, gene flow, non-random mating

Types of Natural Selection

• Directional: mean shifts toward one extreme (industrial melanism — dark Biston betularia favoured on soot-covered trees)
• Stabilising: intermediate phenotype favoured; birth weight in human babies (extremes more risk)
• Disruptive: two extreme phenotypes favoured; leads to divergence and potentially two species

Speciation and Adaptive Radiation

• Speciation: new species from existing; allopatric (geographic isolation), sympatric (within same area)
• Adaptive radiation: single ancestor → multiple species filling different niches; Darwin's finches (14 species from one finch)
• Convergent evolution: unrelated species evolve similar features in similar environment; wings of bat and bird

Brief History of Life on Earth

• Prokaryotes: 3.5 billion years ago; eukaryotes: 1.5 billion years ago; multicellular: 600 million years ago
• Cambrian explosion (~540 Mya): rapid diversification; most animal phyla appeared
• Humans: Homo habilis (2.5 Mya), Homo erectus (1.8 Mya), Homo sapiens (~0.35 Mya); out of Africa migration ~70,000 years ago

CBSE Board Focus

• Evolution: 4–6 marks; Hardy-Weinberg principle, industrial melanism, natural selection types
• Define: microevolution, macroevolution, adaptive radiation, convergent evolution
• H-W numerical: given q (recessive allele frequency), find carrier frequency (2pq); common 3-mark question