Evolution – Class 12 Biology Chapter 6 Notes & NCERT PDF

Class 12BiologyChapter 6NCERT book: Biology - Class 12

Notes and a simple summary of Chapter 6 of the NCERT Class 12 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 6: Evolution

Introduction

Evolutionary biology is the study of the history of life forms on earth. To understand how flora and fauna changed over millions of years, we must see life against the evolution of the earth, the stars and the universe itself. This chapter moves from the origin of life, through Darwin's theory and its evidence, to the mechanism of evolution, the Hardy-Weinberg principle, a brief history of life and finally the origin and evolution of man.

Key Concepts

6.1 Origin of Life

The universe is about 13.8 billion years old. The Big Bang theory explains its origin as a single huge explosion; the universe expanded and cooled, hydrogen and helium formed, and gases condensed under gravity into galaxies.

Earth formed about 4.5 billion years ago. Water vapour, methane, CO₂ and ammonia from the molten mass covered the surface; UV split water, H₂ escaped, and the ozone layer formed. As earth cooled, rain filled depressions to form oceans.

Life appeared about 500 million years after earth formed, i.e., almost four billion years back.

Panspermia: early Greek thinkers believed units of life called spores were transferred to planets including earth.

Spontaneous generation (life from decaying matter) was disproved by Louis Pasteur: in pre-sterilised flasks no life arose from killed yeast, while flasks open to air produced organisms. Life comes only from pre-existing life.

Oparin (Russia) and Haldane (England) proposed that life arose from pre-existing non-living organic molecules, preceded by chemical evolution under high temperature, volcanic storms and a reducing atmosphere (CH₄, NH₃).

S.L. Miller (1953) created electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800°C and observed amino acids. Others obtained sugars, nitrogen bases, pigments and fats; meteorites contain similar compounds.

First non-cellular life (giant molecules such as RNA, proteins, polysaccharides) may have arisen about 3 billion years back; the first cellular forms, probably single cells in water, about 2000 million years ago.

6.2 Evolution of Life Forms – A Theory

Special creation had three ideas: all species were created as such, diversity has always been the same, and earth is about 4000 years old.

Charles Darwin, after his voyage on H.M.S. Beagle, concluded that living forms share similarities with each other and with extinct forms, and that there has been gradual evolution.

Populations carry built-in variation. Those better fit leave more progeny; fitness means reproductive fitness. Darwin called this natural selection.

Alfred Wallace, working in the Malay Archipelago, reached similar conclusions. The earth is billions, not thousands, of years old.

6.3 Evidences for Evolution

Paleontological evidence: fossils are remains of hard parts in rocks. Different sedimentary layers carry different life forms, so life forms varied over time (e.g., dinosaurs, now extinct). Ages are found by radioactive dating.

Embryological evidence: Ernst Haeckel noted features common to all vertebrate embryos, such as a row of vestigial gill slits, functional only in fish. Karl Ernst von Baer disproved this, noting that embryos never pass through adult stages of other animals.

Comparative anatomy – homology: forelimbs of whales, bats, cheetah and humans all have humerus, radius, ulna, carpals, metacarpals and phalanges but perform different functions. Same structure, different functions = divergent evolution; such organs are homologous and indicate common ancestry. Other examples: vertebrate hearts and brains; thorn of Bougainvillea and tendril of Cucurbita.

Analogy: wings of butterfly and bird look alike and perform the same function but are anatomically different. Different structures, same function = convergent evolution. Examples: eye of octopus and mammals; flippers of penguins and dolphins; sweet potato (root) and potato (stem).

Biochemical similarities in proteins and genes also point to common ancestry.

Artificial selection: humans bred distinct breeds of dogs within hundreds of years; nature could do far more over millions of years.

Industrial melanism: in England, collections of the 1850s had more white-winged moths; by 1920, after industrialisation, dark-winged (melanised) moths were more common. Soot darkened tree trunks and killed the white lichens, so predators picked off white moths. In rural areas melanic moths stayed rare. No variant is completely wiped out.

Anthropogenic evolution: excess herbicides, pesticides, antibiotics and drugs select resistant varieties in months or years. Evolution is a stochastic process based on chance events and chance mutations, not a directed one.

6.4 Adaptive Radiation

Evolution of different species in a geographical area starting from one point and radiating to other habitats. Darwin's finches on the Galapagos Islands evolved from seed-eating ancestors into insectivorous and vegetarian forms with altered beaks. Australian marsupials radiated from one ancestral stock. When more than one adaptive radiation occurs in an isolated area, it is convergent evolution: Australian placental mammals resemble corresponding marsupials (placental wolf and Tasmanian wolf).

6.5 Biological Evolution

Natural selection began once cellular life with different metabolic capability existed. Fast-dividing microbes show it within days. Fitness is inherited, so selection needs a genetic basis. Branching descent and natural selection are the two key concepts of Darwinism. Lamarck earlier proposed evolution through use and disuse of organs (the giraffe's neck); nobody accepts this now. Darwin may have been influenced by Thomas Malthus. Limited resources and heritable variation mean competition; the better adapted leave more progeny.

6.6 Mechanism of Evolution

Hugo de Vries, working on evening primrose, proposed mutation: large differences arising suddenly. He called single-step large mutation saltation. Mutations are random and directionless, while Darwin's variations are small and directional; Darwin saw evolution as gradual.

6.7 Hardy-Weinberg Principle

Allele frequencies in a population are stable and constant from generation to generation; the gene pool stays constant (genetic equilibrium). The sum of allele frequencies is 1.

If p and q are the frequencies of alleles A and a: AA = p², aa = q², Aa = 2pq, so p² + 2pq + q² = 1, the binomial expansion of (p + q)².

A measured deviation from expected values indicates evolutionary change.

Five factors disturb equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection. When change happens by chance it is genetic drift; if the drifted population becomes a new species, the original drifted population are founders (founder effect).

Natural selection can be stabilising (more individuals take the mean value), directional (more take a value away from the mean) or disruptive (more take peripheral values at both ends).

6.8 A Brief Account of Evolution

About 2000 mya the first cells appeared; some released O₂. Invertebrates formed by 500 mya; jawless fish around 350 mya; sea weeds and a few plants around 320 mya. Plants invaded land first. Lobefins with stout fins (a Coelacanth was caught in South Africa in 1938) evolved into the first amphibians, which gave rise to reptiles with thick-shelled eggs. Giant ferns fell to form coal. Fish-like reptiles (Ichthyosaurs) arose about 200 mya; dinosaurs such as Tyrannosaurus rex (about 20 feet tall) vanished about 65 mya. The first mammals were shrew-like and viviparous. Continental drift let Australian pouched mammals survive without competition.

6.9 Origin and Evolution of Man

About 15 mya: Dryopithecus (more ape-like) and Ramapithecus (more man-like).

About 3–4 mya: man-like primates walked upright in eastern Africa, not taller than 4 feet.

About 2 mya: Australopithecines in East African grasslands hunted with stone weapons but mainly ate fruit.

**Homo habilis:** first human-like being, brain 650–800 cc, probably did not eat meat.

*Homo erectus* (Java fossils, 1891), about 1.5 mya, brain about 900 cc, probably ate meat.

Neanderthal man: 1400 cc, near east and central Asia, 1,00,000–40,000 years back; used hides and buried their dead.

*Homo sapiens* arose in Africa; modern Homo sapiens arose during the ice age, 75,000–10,000 years ago. Cave art developed about 18,000 years ago (Bhimbetka, Raisen district, Madhya Pradesh); agriculture came around 10,000 years back.

Diagrams to Know

Miller's experiment: flask with CH₄, NH₃, H₂ and water vapour, electrodes giving sparks, condenser, and a trap collecting amino acids.

Homologous forelimbs: the same bone series in whale, bat, cheetah and human.

Darwin's finches: beak shapes for seeds, insects and plants.

Natural selection curves: stabilising (peak narrows at mean), directional (peak shifts), disruptive (two peaks).

Key-Facts Sheet

Universe 13.8 billion years; earth 4.5 billion; life about 4 billion years ago.

Brain sizes: 650–800 cc, 900 cc, 1400 cc (habilis, erectus, Neanderthal).

Board Exam Focus

Explain antibiotic resistance in bacteria using Darwinian selection (Exercise 1).

Describe one example of adaptive radiation; can human evolution be called adaptive radiation?

Differentiate homologous and analogous organs with examples.

Solve Hardy-Weinberg questions using p² + 2pq + q² = 1.

Trace human evolution by brain size, posture and diet.

Key Terms

Evolution (विकास)

Natural selection (प्राकृतिक वरण)

Homologous organs (समजात अंग)

Analogous organs (समवृत्ति अंग)

Adaptive radiation (अनुकूली विकिरण)

Divergent evolution (अपसारी विकास)

Convergent evolution (अभिसारी विकास)

Fossil (जीवाश्म)

Genetic drift (आनुवंशिक विचलन)

Gene flow (जीन प्रवाह)

Founder effect (संस्थापक प्रभाव)

Saltation (साल्टेशन / एकल चरण परिवर्तन)

Gene pool (जीन कोश)

Common Mistakes

Calling bird and butterfly wings homologous; they are analogous.

Thinking fitness means physical strength; for Darwin it is reproductive fitness.

Saying industrial pollution caused moths to turn dark; it only selected existing dark variants.

Writing Hardy-Weinberg as p² + pq + q² = 1; the heterozygote term is 2pq.

Mixing up Lamarck (use and disuse) with de Vries (mutation, saltation).

💡 Key Learning Points

✓Life arose after chemical evolution (Oparin–Haldane, Miller's 1953 experiment) on an earth about 4.5 billion years old; Pasteur disproved spontaneous generation.
✓Darwin's theory rests on branching descent and natural selection, where fitness means reproductive fitness; Wallace reached similar conclusions.
✓Evidence for evolution comes from fossils, homologous (divergent) and analogous (convergent) structures, biochemistry, artificial selection, industrial melanism and resistance to pesticides and antibiotics.
✓Hardy-Weinberg equilibrium (p² + 2pq + q² = 1) is disturbed by gene flow, genetic drift, mutation, recombination and natural selection; selection can be stabilising, directional or disruptive.
✓Human evolution runs from Dryopithecus and Ramapithecus through Australopithecines, Homo habilis, Homo erectus and Neanderthals to Homo sapiens, with increasing brain size.

👨‍🏫 Teaching Tips

→Draw a vertical timeline on the board from the Big Bang (13.8 billion years) to agriculture (10,000 years) and let students place events on it.
→Show forelimb bones of whale, bat, cheetah and human in the same colours to make homology visible at a glance.
→Run a 'peppered moth' activity: scatter white and dark paper moths on light and dark sheets and let students 'predate' for 30 seconds.
→Solve two Hardy-Weinberg numericals together, starting from the frequency of recessive phenotype (q²).
→Contrast Lamarck, Darwin and de Vries in a three-column comparison written as a list on the board, ending with the modern synthesis.
→Sketch the three natural-selection curves and ask students to name a real example for each.
→Arrange hominid skull pictures in order and ask students to justify the order using brain size and diet.

📋 Assessment Questions

Can the student describe Miller's experiment and explain what it supports?
Can the student differentiate homologous and analogous organs with two examples each?
Can the student explain industrial melanism and antibiotic resistance as natural selection?
Can the student state the Hardy-Weinberg principle and list the five factors disturbing it?
Can the student sequence the stages of human evolution with brain capacities?

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