Evolution

This is the sixth chapter of NCERT's Class 12 Biology textbook (pages 110–126 of the 2026-27 reprint), the last of Unit VII, Genetics and Evolution. It has nine sections — origin of life (6.1), evolution of life forms (6.2), evidence (6.3), adaptive radiation (6.4), biological evolution (6.5), mechanism (6.6), the Hardy–Weinberg principle (6.7), a brief account of evolution (6.8) and the origin and evolution of man (6.9) — with Figures 6.1–6.11, a summary and 10 exercises. It is a short, narrative chapter; this page uses nine concept sections, one per NCERT section.

• Neighbouring chapters: Principles of Inheritance and Variation supplies variation, mutation and linkage; Molecular Basis of Inheritance supplies the RNA world, mutation at the DNA level and DNA polymorphism; the next unit's Human Health and Disease returns to drug resistance. The Class 11 geography chapters on the Earth's interior and on continents and oceans give continental drift.
• Prelims — science: UPSC has asked about Darwin's theory (1979), radiocarbon dating of fossils (1983), dinosaurs and early man (1985, 1989), living fossils (1987), homologous organs (1993), Miller-type experiments (1994), the operon/evolution match (1998), marsupials (2001, 2023), mutation and the origin of species (2002), snakes' lost limbs (2002), evolutionary sequences (2009, 2010), elements behind life's origin (2012), continental drift and glaciation (2014), Denisovans (2019) and drug resistance (2019).
• GS3 and GS2: antimicrobial resistance (Mains 2014) is evolution by selection in real time; pesticide resistance, biodiversity and conservation genetics build on this chapter. GS1: Bhimbetka rock art connects with prehistory.

• Origin: universe 13.8 bn yr (Big Bang); Earth 4.5 bn yr; life about 4 bn yr ago. Panspermia (life from space) vs spontaneous generation (disproved by Pasteur). Oparin–Haldane: chemical evolution in a reducing atmosphere (CH₄, NH₃). Miller 1953: sparks in CH₄, H₂, NH₃, H₂O → amino acids.
• Theory: Darwin (HMS Beagle) and Wallace (Malay Archipelago): variation + limited resources → natural selection; fitness = reproductive fitness. Two key concepts: branching descent and natural selection. Lamarck: use and disuse, inherited acquired characters (rejected).
• Evidence: fossils in dated rock layers; homologous organs (forelimbs of whale, bat, cheetah, human; Bougainvillea thorn and Cucurbita tendril) → divergent evolution; analogous organs (butterfly and bird wings, octopus and mammal eyes, penguin and dolphin flippers, sweet potato and potato) → convergent; biochemical similarity; artificial selection; industrial melanism (England, 1850s vs 1920); herbicide, pesticide and antibiotic resistance.
• Adaptive radiation: Darwin's finches (Galápagos), Australian marsupials; placental–marsupial look-alikes = convergence. de Vries (evening primrose): mutation, large and sudden — saltation; Darwin: small, gradual variation.
• Hardy–Weinberg: allele frequencies constant without disturbance; p² + 2pq + q² = 1. Five disturbing factors: gene flow, genetic drift, mutation, recombination, natural selection; founder effect. Selection: stabilising, directional, disruptive.
• History (NCERT): invertebrates 500 mya; lobefins → amphibians → reptiles; coelacanth found 1938; dinosaurs gone 65 mya. Man: Dryopithecus, Ramapithecus 15 mya; Australopithecines 2 mya; H. habilis 650–800 cc; H. erectus 1.5 mya, 900 cc (Java 1891); Neanderthal 1400 cc; cave art 18,000 yr (Bhimbetka); agriculture 10,000 yr.

Starlight we see left its source long ago, so looking at stars is looking back in time. The universe is about 13.8 billion years old; the Big Bang theory describes a single huge explosion, after which the universe expanded and cooled, hydrogen and helium formed, and gases condensed under gravity into galaxies. The Earth formed about 4.5 billion years ago in the solar system of the Milky Way. The early Earth had no atmosphere; water vapour, methane, carbon dioxide and ammonia released from the molten mass covered the surface; UV light split water, and as the planet cooled the vapour fell as rain and filled the oceans. Life appeared about 500 million years after the Earth formed — almost four billion years ago.

Miller's experiment (1953). S.L. Miller, an American scientist, recreated such conditions in the laboratory: an electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour. He found amino acids. Others, in similar experiments, found sugars, nitrogen bases, pigments and fats; meteorites contain similar compounds, suggesting such chemistry happens elsewhere in space. How the first self-replicating metabolic capsule arose is not known. NCERT suggests non-cellular forms (giant molecules — RNA, protein, polysaccharides) about 3 billion years ago and the first, single-celled, cellular forms perhaps not until about 2000 million years ago, all in water. This view — life arising slowly from non-living molecules (abiogenesis) — is accepted by the majority.

NCERT notes that conventional religious literature describes a theory of special creation with three connotations — that species were created as they are, that diversity has always been and will remain the same, and that the Earth is about 4000 years old — and that these ideas were strongly challenged in the nineteenth century by scientific observation. Charles Darwin, from observations on a round-the-world voyage of the sailing ship HMS Beagle, concluded that living forms share similarities with each other and with forms that lived millions of years ago; many have become extinct and new ones have arisen at different times — life has evolved gradually.

Any population has built-in variation. Characteristics that help some individuals survive better in the natural conditions (climate, food, physical factors) let them out-breed the less endowed. Darwin's fitness refers ultimately and only to reproductive fitness: the fitter leave more progeny and are thus ‘selected by nature’ — natural selection, the mechanism of evolution. Alfred Wallace, a naturalist working in the Malay Archipelago, reached similar conclusions at about the same time. All living forms share similarities and common ancestors, present at different periods (epochs, periods and eras) of the Earth's history; geological history matches biological history, and the Earth is billions, not thousands, of years old.

Palaeontological evidence. Fossils are remains of hard parts of life forms in rocks. Sediments are laid one over another through Earth's history, and rocks of different ages hold fossils of different life forms; some resemble modern organisms, others are extinct (e.g. dinosaurs). Life forms have varied over time, and certain forms are confined to certain geological spans. Fossils are dated by radioactive dating (NCERT's in-text question).

Embryological evidence was proposed by Ernst Haeckel (NCERT spells it ‘Heckel’): vertebrate embryos, including human, develop a row of vestigial gill slits behind the head, functional only in fish. NCERT says the proposal was disproved by Karl Ernst von Baer, who noted that embryos never pass through the adult stages of other animals.

Comparative anatomy and morphology. Whales, bats, cheetahs and humans have forelimbs that do different jobs but share the same bones — humerus, radius, ulna, carpals, metacarpals and phalanges. The same structure developed in different directions for different needs: divergent evolution, and the structures are homologous; homology indicates common ancestry. Vertebrate hearts and brains, and the thorn of Bougainvillea and tendril of Cucurbita, are other examples. Analogy is the opposite: wings of butterflies and birds look alike and do the same job but are not anatomically similar — convergent evolution. Other examples: the eyes of the octopus and of mammals, the flippers of penguins and dolphins, and sweet potato (a root) and potato (a stem). Similar habitats select similar adaptive features in different groups.

Biochemical evidence. Similar proteins and genes doing the same job in diverse organisms point to the same shared ancestry as structural similarities. Artificial selection: people have bred plants and animals for agriculture, horticulture, sport and security, creating breeds (e.g. of dogs) that differ yet remain one group — if humans did this in hundreds of years, nature could do it over millions.

Industrial melanism. In England, a moth collection of the 1850s, before industrialisation, had more white-winged than dark (melanised) moths; a collection from the same area in 1920, after industrialisation, had more dark moths — the proportion was reversed. Predators spot a moth against a contrasting background. Before industry, pale lichen covered the trees and white moths were camouflaged; afterwards, soot darkened the trunks and lichens — which do not grow in polluted air and so indicate pollution — disappeared, and dark moths survived. In rural, non-industrial areas melanic moths stayed few. No variant was completely wiped out.

Evolution by human action. Heavy use of herbicides and pesticides has selected resistant varieties; so have antibiotics against microbes and drugs against eukaryotic organisms or cells. Resistance appears in months or years, not centuries. Evolution is therefore not directed or deterministic; it is a stochastic process based on chance events in nature and chance mutations in organisms.

On the Galápagos Islands Darwin found many varieties of small black birds, later called Darwin's finches, on the same island. He conjectured that all evolved there: from an original seed-eating form, others arose with altered beaks, becoming insectivorous and vegetarian. The evolution of different species in a geographical area, starting from a point and radiating to other habitats, is adaptive radiation. Australian marsupials are another example: many different marsupials evolved from an ancestral stock within the island continent. When more than one adaptive radiation occurs in an isolated area, producing look-alikes in different groups, it can be called convergent evolution: Australian placental mammals also radiated, each resembling a marsupial (e.g. placental wolf and Tasmanian wolf).

Evolution by natural selection really began when cellular life forms with different metabolic capabilities appeared. The rate of appearance of new forms is linked to life span: bacteria dividing fast become millions in hours. If a colony (A) with built-in variation in using a food is moved to a changed medium, only part of it (B) survives and multiplies, and in time appears as a new kind — within days; in fish or fowl the same would take millions of years. B is ‘fitter’ than A in the new conditions; fitness is based on inherited characteristics, so selection needs a genetic basis. Branching descent and natural selection are the two key concepts of Darwinian theory.

Lamarck, a French naturalist before Darwin, held that evolution was driven by the use and disuse of organs and the inheritance of acquired characters — giraffes stretching for leaves passed on longer necks. Nobody accepts this any more. Thomas Malthus's work on populations probably influenced Darwin. NCERT also asks whether evolution is a process or the result of a process: in describing the world we treat it as a process; in describing life on Earth, as a consequence of natural selection.

What is the origin of variation, and how do species form? Darwin either ignored Mendel's inheritable ‘factors’ or kept silent about them. In the first decade of the twentieth century Hugo de Vries, working on the evening primrose, proposed that mutations — large differences arising suddenly — cause evolution, not Darwin's minor heritable variations. For de Vries mutations were random and directionless and caused speciation in a single large step, saltation; for Darwin evolution was gradual. Population genetics later brought clarity.

Natural selection can act on a character in three ways (Figure 6.8): stabilising — more individuals acquire the mean value; directional — more individuals acquire a value other than the mean; disruptive — more individuals acquire values at both ends of the distribution.

In a population one can measure how often each allele of a gene occurs. The Hardy–Weinberg principle states that these allele frequencies are stable and constant from generation to generation: the gene pool — all genes and their alleles in a population — stays constant, a genetic equilibrium. The frequencies of all alleles add up to 1. In a diploid, if p is the frequency of allele A and q that of allele a, the chance that A appears on both chromosomes is p × p = p² (genotype AA); likewise aa is q² and Aa is 2pq. Hence p² + 2pq + q² = 1, the binomial expansion of (p + q)². When measured frequencies differ from those expected, the difference shows the extent of evolutionary change: disturbance of Hardy–Weinberg equilibrium is evolution.

Five factors upset the equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection. When part of a population migrates to another place, gene frequencies change in both: new alleles are added to the new population and lost from the old. Repeated migration is gene flow. If the same kind of change happens by chance, it is genetic drift. Sometimes a small group's allele frequencies differ so much that it becomes a different species; the original drifted group are founders and this is the founder effect. Microbial experiments show that pre-existing advantageous mutations, when selected, give new phenotypes and, over generations, speciation. Variation from mutation, recombination, gene flow or drift changes allele frequencies; natural selection, coupled with reproductive success, makes the population look different.

NCERT's sequence: first cellular forms about 2000 mya; some cells released O₂, splitting water with captured light as in photosynthesis; single cells became multicellular; by 500 mya invertebrates were active; jawless fish about 350 mya; seaweeds and a few plants about 320 mya; plants invaded land first and were widespread when animals followed. Fish with stout, strong fins (lobefins) could move on land and back about 350 mya; a coelacanth, thought extinct, was caught off South Africa in 1938. Lobefins evolved into the first amphibians, ancestors of frogs and salamanders, and amphibians into reptiles, whose thick-shelled eggs do not dry up. For about 200 million years reptiles of many shapes dominated; giant ferns (pteridophytes) fell to form coal; some reptiles returned to water as fish-like Ichthyosaurs (about 200 mya). The land reptiles were the dinosaurs; the biggest, Tyrannosaurus rex, was about 20 feet tall. About 65 mya the dinosaurs suddenly disappeared — NCERT says we do not know why: perhaps climate change, perhaps most evolved into birds.

The first mammals were shrew-like and small; viviparous, protecting their young inside the mother, and better at sensing and avoiding danger. When reptiles declined, mammals took over. South America had mammals resembling horses, hippopotamuses, bears and rabbits, which were overridden by North American fauna when continental drift joined the two continents; the same drift let Australia's pouched mammals survive without competition. Whales, dolphins, seals and sea cows live wholly in water. Figures 6.9 and 6.10 sketch these histories on a geological scale.

About 15 mya, primates called Dryopithecus and Ramapithecus existed; they were hairy and walked like gorillas and chimpanzees — Ramapithecus more man-like, Dryopithecus more ape-like. Man-like bones from Ethiopia and Tanzania suggest that about 3–4 mya man-like primates no taller than 4 feet walked upright in eastern Africa. About 2 mya, Australopithecines probably lived in East African grasslands, hunting with stone weapons but eating mainly fruit. Some bones differed: the first human-like being, the hominid Homo habilis, with brain capacity 650–800 cc, probably not a meat-eater. Fossils found in Java in 1891 revealed Homo erectus (about 1.5 mya), with a brain of about 900 cc, who probably ate meat. Neanderthal man, brain 1400 cc, lived in the Near East and Central Asia between 1,00,000 and 40,000 years ago, used hides for protection and buried the dead. Homo sapiens arose in Africa and spread across continents; NCERT says modern H. sapiens arose during the ice age between 75,000 and 10,000 years ago. Prehistoric cave art developed about 18,000 years ago — seen at the Bhimbetka rock shelters, Raisen district, Madhya Pradesh. Agriculture began about 10,000 years ago, followed by settlements.

This chapter asks only two questions in its running text.

Several exercises are projects; the outlines give a starting frame.

Numbers

One-liners

• Pasteur showed that life comes only from pre-existing life.
• Oparin and Haldane proposed chemical evolution; Miller made amino acids in 1953.
• Darwin's fitness means reproductive fitness.
• Wallace reached natural selection independently in the Malay Archipelago.
• Homologous organs show divergent evolution and common ancestry.
• Analogous organs show convergent evolution.
• Industrial melanism is directional selection driven by predators and pollution.
• Drug and pesticide resistance is evolution by human action.
• Darwin's finches and Australian marsupials are adaptive radiations.
• Branching descent and natural selection are Darwin's two key concepts.
• Lamarck's use and disuse is no longer accepted.
• de Vries proposed mutation and saltation from work on evening primrose.
• Hardy–Weinberg equilibrium means allele frequencies do not change.
• Gene flow, drift, mutation, recombination and selection upset the equilibrium.
• Selection can be stabilising, directional or disruptive.
• Lobefins gave rise to amphibians; the coelacanth is a living lobefin.

Traps to avoid

• Sweet potato (root) and potato (stem) are analogous, not homologous.
• Butterfly and bird wings are analogous; bat and whale forelimbs are homologous.
• Miller used no oxygen; the atmosphere was reducing.
• NCERT's ‘800°C’ for Miller is a slip; the flask held boiling water.
• Heterozygote frequency is 2pq, not pq.
• Genetic drift is chance; gene flow is migration.
• Antibiotics select resistant bacteria; they do not create the resistance.
• Snakes' lost limbs by ‘disuse’ is a Lamarckian answer.
• Humans never coexisted with dinosaurs.
• Modern Homo sapiens is about 300,000 years old, not 75,000 (standard sources).
• Ramapithecus is now regarded as an ape (Sivapithecus), not a human ancestor.
• Adaptive radiation is divergence; marsupial–placental look-alikes are convergence.

Prelims — what UPSC asks

• Darwin, Lamarck and the evidence: survival of the fittest and struggle for existence (1979); radiocarbon dating for fossils (1983); fossils in sedimentary rocks (1980); poison glands of snakes homologous to salivary glands (1993); Ginkgo as a living fossil (1987); the evolution half of the theory-match — Darwin and de Vries (1998).
• Origin of life and history: simulated primitive Earth — methane → hydrogen cyanide → nitriles → amino acids (1994); carbon, hydrogen and nitrogen behind life's origin (2012); dinosaurs were reptiles (1989) and died out before man appeared (1985); evolutionary sequences salamander → python → kangaroo (2009) and shark → tortoise → otter (2010); marsupials are characteristic of Australia (2001) and not native to India (2023); continental drift and glacial cycles influenced evolution (2014); Denisovans were an early human species (2019); misuse of antibiotics, not human genes, drives multi-drug resistance (2019).
• Key notes: 2002 ‘origin of a new species’ keyed mutation (b) — disputed: mutation is the ultimate source of new variation (de Vries's view, which NCERT presents), but modern biology treats isolation as necessary for a new species and selection as the shaping force; pick (b) as the expected answer but know the debate. 2002 snakes' limbs keyed ‘adaptation to living in burrows’ (b) — disputed: the mechanism is natural selection (c); (b) names the selective setting; (a) and (d) are Lamarckian and wrong. 1994 is loosely worded: methane was a starting gas, not something that ‘appeared’; key (a) reads as the sequence from reactant to product. 1987 ‘living fossil’ and 2019 Denisovans go beyond NCERT's text — the weakest links kept.
• Likely next: homology vs analogy pairs, Hardy–Weinberg arithmetic (find q from q², carriers = 2pq), the five disturbing factors, types of selection, Miller's gases, adaptive radiation examples, human brain sizes, Bhimbetka, Deccan Traps and the dinosaur extinction, and AMR.

Mains — where it appears and how to answer

• GS3 2014 asked whether overuse and free availability of antibiotics contribute to drug-resistant diseases in India, and about monitoring and control. This chapter supplies the core: resistance is natural selection — the drug removes susceptible microbes and leaves resistant ones to multiply. Frame: mechanism (selection) → drivers (over-the-counter sales, incomplete courses, livestock use) → monitoring (surveillance networks, prescription rules) → One Health approach → way forward.
• Evolutionary thinking also helps in GS3 answers on pesticide resistance and integrated pest management, conservation of small populations (genetic drift, founder effects), and the evolution of new pathogen variants.
• Keep scientific claims dated and sourced; where NCERT's dates differ from current estimates, use the current estimate and note it.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

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