Evolution

2.1 Core concepts

• The universe is ~13.8 billion years old; the Big Bang theory explains its origin via a singular explosion, after which expansion cooled it and hydrogen and helium condensed under gravity into galaxies (NCERT §6.1, p. 111).
• Earth formed ~4.5 billion years back; the early atmosphere had water vapour, methane (CH₄), CO₂ and ammonia (NH₃) — no free oxygen. UV radiation split water; lighter H₂ escaped; oxygen combined with NH₃ and CH₄ to form water and CO₂; the ozone layer formed and rains filled oceans (NCERT §6.1, p. 111).
• Life appeared ~4 billion years ago, i.e., about 500 million years after earth's formation (NCERT §6.1, p. 111).
• Panspermia (early Greek idea, still favoured by some astronomers) holds that spores of life travelled from outer space to earth; spontaneous generation (life from decaying matter) was disproved by Louis Pasteur, who showed life arises only from pre-existing life (NCERT §6.1, p. 111).
• Oparin (Russia) and Haldane (England) proposed chemical evolution — first life came from pre-existing non-living organic molecules (RNA, protein); preceded by abiotic formation of organic from inorganic matter under reducing atmosphere (CH₄, NH₃, high temperature, volcanic storms) (NCERT §6.1, pp. 111-112).
• S.L. Miller (1953) simulated early-earth conditions in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800°C with electric discharge; observed formation of amino acids (and in similar experiments, sugars, nitrogen bases, pigments, fats). Meteorite analysis confirmed similar compounds form in space (NCERT §6.1, p. 111, Figure 6.1).
• First non-cellular life ~3 billion years back (giant RNA, protein, polysaccharide molecules); first cellular life ~2000 million years ago — single cells in water (NCERT §6.1, pp. 112; §6.8, p. 121).
• Theory of special creation (challenged in the 19th century) claimed species were created as-is, diversity is unchanging, and earth is ~4000 years old — all three points were rejected (NCERT §6.2, p. 112).
• Charles Darwin (HMS Beagle voyage) concluded existing life forms share similarities with past forms; gradual evolution operates via natural selection — heritable variations that improve reproductive fitness are preserved. Alfred Wallace (Malay Archipelago) reached similar conclusions independently (NCERT §6.2, pp. 112-113).
• Paleontological evidence: fossils in different sedimentary layers show life forms varied over time, with certain forms restricted to certain geological time-spans; ages are determined by radioactive dating (NCERT §6.3, p. 113).
• Embryological evidence proposed by Ernst Heckel (vertebrate embryos show vestigial gill slits, etc.) was disapproved by Karl Ernst von Baer, who showed embryos never pass through the adult stages of other animals (NCERT §6.3, p. 113).
• Homologous organs (whale flipper, bat wing, cheetah forelimb, human arm — all with humerus, radius, ulna, carpals, metacarpals, phalanges) — same structure, different function — result of divergent evolution, indicate common ancestry. Plant example: thorn of Bougainvillea and tendril of Cucurbita (NCERT §6.3, pp. 114-115, Figure 6.3).
• Analogous organs — different structure, same function — result of convergent evolution. Examples: wings of butterfly vs birds; eye of octopus vs mammals; flippers of penguins vs dolphins; sweet potato (root) vs potato (stem tuber) (NCERT §6.3, p. 115).
• Biochemical evidence — similarities in proteins and genes across diverse organisms indicate shared ancestry (NCERT §6.3, p. 115).
• Artificial selection — man's breeding of dogs, crops etc. in hundreds of years argues that nature could do the same over millions (NCERT §6.3, p. 115).
• Industrial melanism (England, peppered moths) — before industrialisation (1850s) white-winged moths dominated on lichen-covered trees; post-industrialisation (1920) dark/melanised moths dominated on soot-blackened trunks. In unpolluted rural areas melanic count remained low. Demonstrates natural selection; no variant is completely wiped out (NCERT §6.3, pp. 115-116, Figure 6.4).
• Anthropogenic evolution — pesticide/herbicide/antibiotic resistance arises in months-to-years, showing evolution is a stochastic (chance-based) not directed process (NCERT §6.3, p. 116).
• Adaptive radiation — evolution of different species in a geographical area from a point and radiating to other habitats. Darwin's finches of Galapagos (varied beaks: seed-eating → insectivorous → vegetarian) and Australian marsupials (multiple forms from common ancestral stock) are the classic examples (NCERT §6.4, pp. 116-117, Figures 6.5, 6.6).
• Convergent evolution at the radiation level: placental mammals (e.g., placental wolf) and Australian marsupials (Tasmanian wolf-marsupial) — two adaptive radiations in isolated areas producing similar forms (NCERT §6.4, pp. 117-118, Figure 6.7).
• Branching descent and natural selection are the two key concepts of Darwinian theory (NCERT §6.5, p. 118).
• Lamarck's theory (use and disuse of organs, inheritance of acquired characters — e.g., giraffe neck elongation) — no longer believed (NCERT §6.5, pp. 118-119).
• Darwin was influenced by Thomas Malthus on populations: natural resources are limited, populations would grow exponentially if unchecked, hence competition; heritable variations favouring resource use leave more progeny → new forms arise (NCERT §6.5, p. 119).
• Hugo de Vries (working on evening primrose) proposed mutation theory — large, sudden, random, directionless changes cause saltation (single-step large mutation), in contrast to Darwin's small directional gradual variations (NCERT §6.6, p. 119).
• Hardy-Weinberg principle: allele frequencies in a population are stable and constant across generations — "genetic equilibrium." For two alleles A (p) and a (q): p² (AA) + 2pq (Aa) + q² (aa) = 1; this is a binomial expansion of (p+q)². Deviation from expected indicates evolutionary change (NCERT §6.7, pp. 120-121).
• Five factors that disturb Hardy-Weinberg equilibrium: (i) gene migration / gene flow, (ii) genetic drift, (iii) mutation, (iv) genetic recombination, (v) natural selection. Founder effect — when drift-altered allele frequencies make the founders of a new population a new species (NCERT §6.7, p. 121).
• Three types of natural selection (Figure 6.8): stabilising (mean character favoured — peak narrows), directional (peak shifts toward one extreme), disruptive (two peaks at extremes) (NCERT §6.7, p. 121).
• Geological/evolutionary timeline: first cellular life ~2000 mya; invertebrates by 500 mya; jawless fish ~350 mya; sea-weeds and plants ~320 mya; plants invaded land first; lobefins (Coelacanth-type fish with stout fins) gave rise to first amphibians ~350 mya; amphibians → reptiles (thick-shelled eggs); dinosaurs dominated land; Tyrannosaurus rex was ~20 feet tall; some reptiles re-entered water (Ichthyosaurs, ~200 mya); dinosaurs disappeared ~65 mya (NCERT §6.8, pp. 121-124, Figures 6.9, 6.10).
• First mammals were shrew-like, viviparous, intelligent; took over after reptiles declined. Continental drift explains why North American fauna overran South American mammals, and why Australian pouched mammals (marsupials) survived in isolation (NCERT §6.8, p. 124).
• Human evolution (§6.9): Dryopithecus and Ramapithecus ~15 mya — Ramapithecus more man-like, Dryopithecus more ape-like. Hominid fossils from Ethiopia/Tanzania show ~3–4 mya upright primates (<4 feet). Australopithecines ~2 mya in East African grasslands; hunted with stone weapons but ate fruit. Homo habilis — first hominid, brain 650–800 cc, did not eat meat. Homo erectus — fossils Java, 1891, ~1.5 mya, brain ~900 cc, ate meat. Neanderthal man — brain 1400 cc, 1,00,000–40,000 years back, used hides, buried dead. Homo sapiens arose in Africa; modern Homo sapiens during ice age (75,000–10,000 years ago). Pre-historic cave art ~18,000 years ago (e.g., Bhimbetka, Raisen, MP). Agriculture ~10,000 years back (NCERT §6.9, pp. 124-125, Figure 6.11).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 6.1 — Miller's experiment apparatus (p. 112): closed flask with electrodes, gases CH₄, NH₃, H₂O, H₂, spark discharge, condenser with water in/out, boiling water flask, U-shaped trap for liquid water containing organic compounds. Conditions: 800°C, electric discharge.
• Figure 6.2 — Dinosaur family tree (p. 114): Triceratops, Tyrannosaurus, Pteranodon, Crocodilian, Archaeopteryx, Stegosaurus, Brachiosaurus — modern crocodiles and birds are living counterparts.
• Figure 6.3 — Homologous organs (p. 115): (a) plants — thorn of Bougainvillea vs tendril of Cucurbita; (b) animals — forelimbs of man, cheetah, whale, bat all share humerus + radius + ulna + carpals + metacarpals + phalanges.
• Figure 6.4 — Industrial melanism (p. 116): white-winged and dark-winged (melanised) moths on (a) unpolluted lichen-covered trunk (white survives) vs (b) polluted soot-covered trunk (dark survives).
• Figure 6.5 — Darwin's finches (p. 117): four beak varieties from Galapagos — adaptive radiation from seed-eating ancestor to insectivorous/vegetarian forms.
• Figure 6.6 — Australian marsupial radiation (p. 117): Tasmanian wolf, sugar glider, marsupial mole, koala, bandicoot, wombat, kangaroo, marsupial rat, banded anteater, tiger cat — all from one ancestral stock.
• Figure 6.7 — Convergent evolution (p. 118): pairs of placental mammals vs Australian marsupials — mole/marsupial mole, anteater/numbat, mouse/marsupial mouse, lemur/spotted cuscus, flying squirrel/flying phalanger, bobcat/Tasmanian tiger cat, wolf/Tasmanian wolf.
• Figure 6.8 — Types of natural selection (p. 120): (a) stabilising (peak higher and narrower), (b) directional (peak shifts in one direction), (c) disruptive (two peaks form).
• Figure 6.9 — Plant evolution timeline (p. 122): Paleozoic → Mesozoic → Cenozoic; from chlorophyte ancestors → tracheophyte ancestors → Rhynia-type plants → Psilophyton → seed ferns → progymnosperms → conifers/cycads/ginkgos/gnetales → angiosperms (monocots, dicots).
• Figure 6.10 — Vertebrate evolutionary tree (p. 123): early reptiles (~350 mya) → sauropsids and synapsids → therapsids, thecodonts, pelycosaurs, dinosaurs (all extinct) → modern turtles, lizards, snakes, tuataras, crocodiles, birds, mammals.
• Figure 6.11 — Skull comparison (p. 125): adult modern human, baby chimpanzee, adult chimpanzee — baby chimp skull is more like adult human skull than adult chimp skull.

2.4 Common confusions / NTA trap points

• Homology vs analogy: homology = same structure, different function, common ancestor, divergent evolution; analogy = different structure, similar function, convergent evolution. Distractors often swap "divergent" and "convergent."
• Hardy-Weinberg algebra: if dominant allele frequency p = 0.6, then heterozygotes are 2pq (not p² or q²). Remember q = 1 − p; recessive homozygotes are q² (often the easiest unknown to extract from a stated frequency).
• Brain capacities of hominids: Homo habilis 650–800 cc, Homo erectus ~900 cc, Neanderthal 1400 cc — students confuse the order. Homo erectus ate meat; Homo habilis did NOT.
• Heckel vs von Baer: Heckel proposed the embryological evidence; von Baer disapproved it by showing embryos do not pass through adult stages of other animals.
• Miller's experiment gases: CH₄, H₂, NH₃ and water vapour at 800°C — distractors often add O₂ (early atmosphere had no free oxygen) or remove ammonia.
• Lamarck vs Darwin vs de Vries: Lamarck = use/disuse + inheritance of acquired characters (giraffe neck); Darwin = small, gradual, directional, heritable variations + natural selection; de Vries = saltation by large, sudden, random, directionless mutations.
• Five Hardy-Weinberg factors: gene flow/migration, genetic drift, mutation, recombination, natural selection — students often forget recombination.
• Convergent at two scales: (i) analogous organs in unrelated species, and (ii) entire radiations producing similar forms in isolated continents (placental wolf vs Tasmanian wolf) — both are convergent.
• Sweet potato vs potato is analogous (root vs stem); **thorn of Bougainvillea and tendril of *Cucurbita*** is homologous (both modified shoots).