Evolution
Welcome to HSLC Guru! This complete English-medium study guide is crafted for ASSEB Class 12 Biology students preparing for the HS Final Examination. Chapter 7 — Evolution explores the grand story of life on Earth, from the origin of the first cell in a primitive ocean to the emergence of modern humans. You will learn about the chemical evolution of life, the major theories that explain biological change, the powerful evidences that confirm evolution, the mechanisms that drive it, and how new species are formed. Every concept here follows the ASSEB syllabus and includes summary, question-answers of all marks, MCQs, fill-in-the-blanks, true/false, and a glossary to help you score top marks.
Chapter Summary
Origin of Life: The universe is believed to have originated about 13.7 billion years ago through the Big Bang, a massive explosion that created matter, energy, space, and time. The Earth was formed about 4.5 billion years ago and was initially hot and lifeless. According to the chemical theory of origin of life proposed independently by A. I. Oparin (1924) and J. B. S. Haldane (1929), life arose on Earth through a sequence of chemical reactions in the primitive ocean, often called the “hot dilute soup” or primordial soup. The primitive atmosphere was reducing in nature, containing methane (CH4), ammonia (NH3), water vapour (H2O), and hydrogen (H2), with no free oxygen. Energy from ultraviolet rays, lightning, and volcanic heat caused these gases to combine and form simple organic molecules — amino acids, sugars, and nitrogen bases — which gradually assembled into complex molecules and eventually the first protocell or coacervate.
Miller–Urey Experiment (1953): Stanley L. Miller and Harold C. Urey experimentally proved Oparin–Haldane’s theory by simulating the conditions of primitive Earth in the laboratory. Inside a closed apparatus, they circulated a mixture of CH4, NH3, H2, and water vapour and exposed it to electric sparks (representing lightning) at 800 °C for one week. After analysis, they found amino acids such as glycine, alanine, and aspartic acid, along with sugars and nitrogenous bases. This proved that organic compounds essential for life can form from inorganic gases under primitive Earth conditions. The theory of biogenesis (“life from pre-existing life”) replaced the older theory of abiogenesis (“life from non-living matter”), which had been disproved by Pasteur’s swan-neck flask experiment.
Evolution of Life through Geological Eras: The geological time scale divides Earth’s history into eras and periods. The Precambrian era (4.5 billion to 570 million years ago) saw the origin of prokaryotes, eukaryotes, and the first multicellular organisms. The Palaeozoic era (570–245 mya) is the age of invertebrates, fishes, amphibians, and the first reptiles, with major events like the Cambrian explosion. The Mesozoic era (245–65 mya) is the “Age of Reptiles,” dominated by dinosaurs; it also saw the rise of the first birds (Archaeopteryx) and mammals. The Cenozoic era (65 mya to present) is the “Age of Mammals” and witnessed the rise of flowering plants and the evolution of humans.
Theories and Mechanisms of Evolution: Lamarck (1809) proposed inheritance of acquired characters and “use and disuse of organs”; Darwin (1859) gave the theory of natural selection in his book On the Origin of Species, based on overproduction, struggle for existence, variation, survival of the fittest, and inheritance of useful variations. The Modern Synthetic Theory (Neo-Darwinism) integrates Mendelian genetics with Darwin’s natural selection and recognises mutation, recombination, gene flow, genetic drift, natural selection, and reproductive isolation as the main factors of evolution. The Hardy–Weinberg principle states that gene and genotype frequencies in a large, randomly mating population remain constant across generations in the absence of mutation, migration, selection, and drift. Speciation may be allopatric (due to geographical isolation) or sympatric (within the same area through reproductive isolation). Adaptive radiation, beautifully illustrated by Darwin’s finches of the Galápagos Islands, shows how a single ancestor evolves into many forms suited to different niches. Human evolution proceeds through Australopithecus → Homo habilis → Homo erectus → Homo sapiens neanderthalensis → Homo sapiens sapiens (modern humans).
Question and Answers
1-Mark Questions
Q1. Who proposed the chemical theory of origin of life?
Answer: A. I. Oparin (1924) and J. B. S. Haldane (1929).
Q2. Name the scientists who experimentally proved Oparin–Haldane’s theory.
Answer: Stanley L. Miller and Harold C. Urey (1953).
Q3. What is a fossil?
Answer: A fossil is the preserved remains, impression, or trace of an organism that lived in the geological past, found in sedimentary rocks.
Q4. Which fossil is considered the connecting link between reptiles and birds?
Answer: Archaeopteryx, found in the Jurassic period rocks of Germany.
Q5. Define vestigial organs with one example.
Answer: Vestigial organs are reduced, non-functional organs that were functional in ancestors. Example: vermiform appendix in humans.
Q6. Who wrote the book On the Origin of Species?
Answer: Charles Darwin, published in 1859.
Q7. What is genetic drift?
Answer: Genetic drift is the random change in allele frequencies in a small population due to chance events, not due to selection.
Q8. Define adaptive radiation.
Answer: Adaptive radiation is the evolutionary process by which a single ancestral species diversifies into many different forms adapted to varied ecological niches, e.g., Darwin’s finches.
Q9. Name the immediate ancestor of modern humans.
Answer: Homo sapiens neanderthalensis (Neanderthal man).
Q10. Who proposed the theory of inheritance of acquired characters?
Answer: Jean Baptiste de Lamarck (1809).
Q11. What is meant by the term “biogenesis”?
Answer: Biogenesis is the theory that life arises only from pre-existing life, proposed by Louis Pasteur and Francesco Redi.
Q12. Name the era known as the “Age of Mammals.”
Answer: The Cenozoic era.
2 to 3 Marks Questions
Q1. Differentiate between homologous and analogous organs with examples.
Answer: Homologous organs have the same basic structure and origin but perform different functions, indicating divergent evolution; example — forelimbs of human, whale, bat, and cheetah. Analogous organs have different structure and origin but perform the same function, showing convergent evolution; example — wings of insects and wings of birds. Homology suggests common ancestry, while analogy suggests adaptation to similar environments.
Q2. State the postulates of Darwin’s theory of natural selection.
Answer: The main postulates are: (i) Overproduction — organisms produce more offspring than can survive; (ii) Struggle for existence — competition for limited resources; (iii) Variation — individuals show heritable variations; (iv) Survival of the fittest — individuals with favourable variations survive better; (v) Natural selection — these favourable traits accumulate over generations; (vi) Origin of new species — gradual change leads to formation of new species.
Q3. Briefly describe the Miller–Urey experiment.
Answer: In 1953, Miller and Urey created a closed glass apparatus simulating primitive Earth’s atmosphere with methane, ammonia, hydrogen, and water vapour. They passed electric sparks at 800 °C through the mixture for a week to mimic lightning. After analysis they detected amino acids (glycine, alanine), sugars, and nitrogenous bases, proving that organic molecules of life could form from inorganic gases under primitive Earth conditions.
Q4. What is the Hardy–Weinberg principle? Write its conditions.
Answer: The Hardy–Weinberg principle states that in a large, randomly mating population, the gene and genotype frequencies remain constant from generation to generation. The expression is p² + 2pq + q² = 1. Conditions: (i) large population, (ii) random mating, (iii) no mutation, (iv) no migration (gene flow), (v) no natural selection, (vi) no genetic drift.
Q5. Differentiate between allopatric and sympatric speciation.
Answer: Allopatric speciation occurs when populations of a species become geographically isolated (by mountains, rivers, oceans) and evolve independently into separate species. Sympatric speciation occurs without geographical separation, when populations within the same area diverge due to reproductive isolation, polyploidy, or ecological factors. Allopatric is more common in animals; sympatric is common in plants.
Q6. Explain the significance of Darwin’s finches.
Answer: On the Galápagos Islands, Darwin observed 13 species of finches that had evolved from a common ancestor reaching the islands from the mainland. Their beaks varied according to feeding habits — seed eaters, insect eaters, fruit eaters, and tool users. This is a classic example of adaptive radiation, where a single species diversified into many forms occupying different ecological niches, providing direct evidence for evolution by natural selection.
5 to 7 Marks Questions
Q1. Discuss in detail the Oparin–Haldane theory of origin of life. How was it proved by Miller and Urey?
Answer: The Oparin–Haldane theory, also called the chemical theory of origin of life, proposes that life arose on primitive Earth through a long sequence of chemical reactions. The major steps are:
(i) Primitive atmosphere: The early Earth had a reducing atmosphere with methane, ammonia, water vapour, and hydrogen, but no free oxygen. (ii) Energy sources: Ultraviolet radiation, lightning, volcanic heat, and cosmic rays supplied energy. (iii) Formation of simple organic molecules: The gases combined to form aldehydes, hydrocarbons, and simple organic compounds, which dissolved in primitive oceans, forming a “hot dilute soup.” (iv) Formation of complex molecules: Simple molecules polymerised into amino acids, nucleotides, sugars, and fatty acids, then into proteins, nucleic acids, and lipids. (v) Formation of protocells (coacervates): Macromolecules aggregated into colloidal droplets surrounded by a membrane-like boundary. (vi) Origin of first cell: Eventually, self-replicating systems with RNA/DNA appeared as the first true cells.
Miller–Urey experiment (1953): Miller and Urey designed a closed apparatus with two flasks connected by tubes. The lower flask contained boiling water (representing the ocean) and the upper flask contained CH4, NH3, H2, and water vapour. Electric sparks at 60,000 volts and 800 °C simulated lightning. Water condensed and circulated for one week. On analysing the collected liquid, they found glycine, alanine, aspartic acid, sugars, and bases — proving that organic compounds essential to life can form abiotically from inorganic gases. This experimentally validated the Oparin–Haldane theory and established the foundation of modern theories of chemical evolution.
Q2. Describe the various evidences of organic evolution.
Answer: Evidences of evolution come from several branches of biology:
(i) Palaeontological evidence: Fossils preserved in sedimentary rocks show stages of evolution. Archaeopteryx is a connecting link between reptiles and birds. The horse evolution series (Eohippus → Mesohippus → Merychippus → Pliohippus → Equus) shows gradual changes in size, leg structure, and teeth. (ii) Morphological and anatomical evidence: Homologous organs (forelimbs of vertebrates) indicate divergent evolution and common ancestry. Analogous organs (wings of insect and bird) indicate convergent evolution. Vestigial organs (appendix, wisdom teeth, nictitating membrane in humans) are remnants of ancestral structures. (iii) Embryological evidence: Early embryos of fish, amphibians, reptiles, birds, and mammals show striking similarities — gill slits, tail, and notochord — supporting common ancestry (Karl Ernst von Baer). (iv) Biogeographical evidence: Distribution of organisms across geographical regions, e.g., Darwin’s finches in the Galápagos and marsupials in Australia, demonstrates how species evolve to fit local environments. (v) Molecular evidence: Similarities in DNA, RNA, proteins, and the genetic code among all organisms reveal a common origin. Cytochrome-c, haemoglobin, and ribosomal RNA sequence comparisons help build phylogenetic trees.
Q3. Explain the Modern Synthetic Theory of evolution.
Answer: The Modern Synthetic Theory, developed by Dobzhansky, Mayr, Huxley, Stebbins, and Simpson during 1930–1950, integrates Darwin’s natural selection with Mendelian genetics, population genetics, and concepts of speciation. The main factors are:
(i) Mutation: Sudden heritable change in genetic material; the ultimate source of all variation. (ii) Recombination: Reshuffling of genes during meiosis and fertilisation creates new combinations. (iii) Gene flow (migration): Movement of individuals between populations changes allele frequencies. (iv) Genetic drift: Random fluctuation of allele frequencies, especially in small populations; includes founder effect and bottleneck effect. (v) Natural selection: Differential reproduction of fitter individuals leads to gradual change. (vi) Reproductive isolation: Prevents interbreeding between diverged populations and leads to formation of new species.
Together, these factors act on the gene pool of populations and produce evolutionary change over time. This theory thus explains evolution at both micro (within species) and macro (above species) levels.
Q4. Describe the major stages of human evolution.
Answer: Human evolution is the gradual development of modern humans from ape-like ancestors over millions of years. Major stages are:
(i) Dryopithecus and Ramapithecus (15–10 mya): Common ancestors of apes and humans; Dryopithecus was more ape-like, Ramapithecus was more man-like. (ii) Australopithecus (4–2 mya): Discovered in Africa; walked upright (bipedal), brain capacity ~450–500 cc, used stones as weapons. (iii) Homo habilis (2.5–1.5 mya): The “handy man,” brain capacity ~650–800 cc, made simple stone tools. (iv) Homo erectus (1.8 mya–300,000 ya): The “upright man,” brain ~900–1100 cc, used fire, made advanced tools, included Java and Peking man. (v) Homo sapiens neanderthalensis (1,50,000–40,000 ya): Lived in caves, brain ~1400 cc, buried their dead, used skins as clothes. (vi) Homo sapiens sapiens (Cro-Magnon, 40,000 ya–present): Modern humans; brain ~1350–1500 cc, developed language, art, agriculture, and civilisation.
The line of evolution: Dryopithecus → Australopithecus → Homo habilis → Homo erectus → Homo sapiens neanderthalensis → Homo sapiens sapiens.
Q5. Compare Lamarckism and Darwinism. Why is Darwinism more accepted today?
Answer: Lamarckism (1809): Proposed by Jean Baptiste de Lamarck. Main points — (i) internal vital force drives evolution; (ii) effect of environment creates new needs; (iii) use and disuse of organs leads to development or reduction; (iv) inheritance of acquired characters to offspring. Example: long neck of giraffe due to constant stretching to reach high leaves.
Darwinism (1859): Proposed by Charles Darwin. Main points — (i) overproduction; (ii) struggle for existence; (iii) variations among individuals; (iv) natural selection of fitter individuals; (v) survival of the fittest; (vi) inheritance of useful variations leads to new species.
Why Darwinism is accepted: Lamarck’s theory failed because August Weismann’s experiment (cutting off rat tails for 22 generations) proved that acquired characters are not inherited. Only changes in germ cells (genetic) are passed on. Darwinism, supported by genetics, fossil records, and modern molecular biology, explains evolution scientifically. The Modern Synthetic Theory is essentially Darwinism updated with genetics.
Multiple Choice Questions (MCQ)
Q1. The chemical theory of origin of life was proposed by:
(a) Darwin and Wallace
(b) Oparin and Haldane
(c) Miller and Urey
(d) Lamarck and Mendel
Answer: (b) Oparin and Haldane.
Q2. The Miller–Urey experiment was conducted in:
(a) 1924
(b) 1859
(c) 1953
(d) 1809
Answer: (c) 1953.
Q3. Archaeopteryx is a connecting link between:
(a) Fish and amphibians
(b) Amphibians and reptiles
(c) Reptiles and birds
(d) Reptiles and mammals
Answer: (c) Reptiles and birds.
Q4. Forelimbs of humans, whales, and bats are examples of:
(a) Analogous organs
(b) Vestigial organs
(c) Homologous organs
(d) Atavistic organs
Answer: (c) Homologous organs.
Q5. Wings of insects and wings of birds are:
(a) Homologous
(b) Analogous
(c) Vestigial
(d) Atavistic
Answer: (b) Analogous.
Q6. Darwin’s finches are an example of:
(a) Convergent evolution
(b) Adaptive radiation
(c) Genetic drift
(d) Natural selection only
Answer: (b) Adaptive radiation.
Q7. Hardy–Weinberg principle gives the equation:
(a) p + q = 1
(b) p² + 2pq + q² = 1
(c) p² + q² = 1
(d) 2p + q = 1
Answer: (b) p² + 2pq + q² = 1.
Q8. The first man-like ancestor was:
(a) Homo sapiens
(b) Australopithecus
(c) Homo erectus
(d) Homo habilis
Answer: (b) Australopithecus.
Q9. The Mesozoic era is known as:
(a) Age of fishes
(b) Age of mammals
(c) Age of reptiles
(d) Age of amphibians
Answer: (c) Age of reptiles.
Q10. Lamarckism is also called the theory of:
(a) Natural selection
(b) Inheritance of acquired characters
(c) Mutation
(d) Genetic drift
Answer: (b) Inheritance of acquired characters.
Fill in the Blanks
Q1. The book On the Origin of Species was written by ________.
Answer: Charles Darwin.
Q2. The primitive atmosphere of Earth was ________ in nature.
Answer: Reducing.
Q3. The vermiform appendix in humans is a ________ organ.
Answer: Vestigial.
Q4. Random change in allele frequency in small populations is called ________.
Answer: Genetic drift.
Q5. The “handy man” stage of human evolution is ________.
Answer: Homo habilis.
Q6. The connecting link between reptiles and birds is ________.
Answer: Archaeopteryx.
True / False
Q1. Free oxygen was abundant in the primitive atmosphere of Earth.
Answer: False.
Q2. Homologous organs indicate divergent evolution.
Answer: True.
Q3. Sympatric speciation requires geographical isolation.
Answer: False.
Q4. Darwin’s finches are a famous example of adaptive radiation.
Answer: True.
Q5. The Cenozoic era is known as the Age of Mammals.
Answer: True.
Q6. Lamarck’s theory of inheritance of acquired characters is fully accepted today.
Answer: False.
Glossary
| Term | Meaning |
|---|---|
| Evolution | Gradual change in inherited characters of populations over generations |
| Big Bang | Massive explosion ~13.7 billion years ago that originated the universe |
| Coacervate | Colloidal aggregate considered the precursor of the first cell |
| Biogenesis | Theory that life arises from pre-existing life |
| Abiogenesis | Theory that life arose from non-living matter |
| Fossil | Preserved remains or impressions of ancient organisms |
| Homologous organs | Same origin, different functions; show divergent evolution |
| Analogous organs | Different origin, same function; show convergent evolution |
| Vestigial organ | Reduced, non-functional remnant organ |
| Natural selection | Differential survival and reproduction of fitter individuals |
| Genetic drift | Random change in allele frequency in small populations |
| Gene flow | Transfer of alleles between populations through migration |
| Hardy–Weinberg principle | Allele frequencies remain constant in an ideal population |
| Speciation | Formation of new species from an existing one |
| Adaptive radiation | Diversification of a species into many forms in different niches |
| Archaeopteryx | Connecting link between reptiles and birds |
| Australopithecus | First bipedal man-like ancestor |
| Homo sapiens | Modern human species |