Class 12 Biology Notes Chapter 7 (Evolution) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 7, 'Evolution'. This is a crucial chapter, not just for your board exams but also for various competitive government exams where Biology is a component. We'll break down the key concepts from the NCERT Exemplar perspective, focusing on what's essential.

Chapter 7: Evolution - Key Concepts for Competitive Exams

1. Origin of Life

• The Universe and Earth: Big Bang theory explains the origin of the universe (~20 billion years ago). Earth formed ~4.5 billion years ago. Early Earth had a reducing atmosphere (no free O₂), high temperatures, volcanic storms, lightning.
• Theories on Origin of Life:
  • Theory of Special Creation: Life created by a supernatural power. (Not scientific)
  • Theory of Spontaneous Generation (Abiogenesis): Life arose from non-living matter spontaneously. Disproved by Louis Pasteur's swan-neck flask experiment.
  • Theory of Biogenesis: Life arises only from pre-existing life. (Proven by Pasteur)
  • Theory of Panspermia/Cosmozoic Theory: Life came to Earth from outer space as spores or seeds.
  • Oparin-Haldane Hypothesis (Chemical Evolution/Abiogenesis): This is the most accepted scientific theory. Proposed independently by A.I. Oparin (Russia) and J.B.S. Haldane (England).
    • States that life originated abiogenetically (from inorganic matter) but only after the origin of the first life forms did biogenesis become the norm.
    • Steps:
      1. Formation of simple inorganic molecules (NH₃, CH₄, H₂O, H₂) in the reducing atmosphere.
      2. Formation of simple organic monomers (amino acids, simple sugars, nitrogenous bases, fatty acids) using energy from lightning, UV radiation, volcanic activity. Conditions: High temperature, reducing atmosphere, water bodies ('hot dilute soup' or 'primordial soup' - term by Haldane).
      3. Formation of complex organic polymers (proteins, polysaccharides, nucleic acids, lipids) through polymerization.
      4. Formation of protobionts/protocells: Aggregates of organic molecules capable of simple metabolism and self-replication, enclosed within a membrane-like structure (e.g., Coacervates - Oparin, Microspheres - Sidney Fox). These were not true cells.
      5. Origin of first true cells (anaerobic heterotrophs, later chemoautotrophs, then photoautotrophs releasing O₂).
• Miller-Urey Experiment (1953): Provided experimental evidence for chemical evolution.
  • Setup: Simulated early Earth conditions in a closed flask system – CH₄, NH₃, H₂, H₂O vapor at 800°C, electric discharge (simulating lightning).
  • Result: Formation of amino acids (like glycine, alanine, aspartic acid) and other organic molecules after a week.
  • Significance: Showed that organic monomers could indeed form abiogenetically under primitive Earth conditions.

2. Evidences for Evolution

• Paleontological Evidence (Fossils):
  • Fossils are remains or impressions of past organisms preserved in rocks (usually sedimentary).
  • Show that life forms varied over time and certain forms are restricted to certain geological time spans.
  • Study of fossils: Paleontology.
  • Methods of dating fossils: Radiometric dating (e.g., Carbon-14 dating for recent fossils, Potassium-Argon dating for older ones).
  • Missing links: Fossil organisms showing characteristics of two different groups (e.g., Archaeopteryx - link between reptiles and birds).
  • Evolutionary lineages: Fossils reveal evolutionary history of organisms (e.g., Horse evolution).
• Comparative Anatomy and Morphology:
  • Homologous Organs: Organs having the same fundamental structure and origin but adapted for different functions. Indicate Divergent Evolution (common ancestry).
    • Examples: Forelimbs of whales, bats, cheetahs, and humans; Thorns of Bougainvillea and tendrils of Cucurbita (both modified stems); Vertebrate hearts or brains.
  • Analogous Organs: Organs having different structures and origins but performing similar functions. Indicate Convergent Evolution (adaptation to similar environments/needs, not common ancestry).
    • Examples: Wings of butterfly (insect) and birds; Eye of octopus (mollusc) and mammals; Flippers of penguins (bird) and dolphins (mammal); Sweet potato (root modification) and potato (stem modification) for storage.
  • Vestigial Organs: Organs that are reduced and non-functional in an organism but were functional in their ancestors.
    • Examples: Appendix, nictitating membrane (plica semilunaris), coccyx (tail bone), wisdom teeth, body hair in humans; Pelvic girdle in pythons.
  • Atavism (Reversion): Sudden reappearance of ancestral characters (e.g., humans born with a small tail).
• Embryological Evidence (Based on study of embryos):
  • Ernst Haeckel's Biogenetic Law: "Ontogeny recapitulates Phylogeny" - meaning the developmental stages of an individual embryo (ontogeny) repeat the evolutionary history of its ancestors (phylogeny). This law is now considered an oversimplification and largely discredited in its original form.
  • Karl Ernst von Baer's Laws: Noted that embryos never pass through the adult stages of other animals. General characters appear earlier in development than specialized characters. Embryos of related groups resemble each other more closely than their adults do.
  • Example: Presence of gill slits and notochord in vertebrate embryos (including humans), which are lost or modified later.
• Biogeographical Evidence (Distribution of species):
  • Adaptive Radiation: The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas (habitats). Occurs when a species colonizes a new area with diverse, unoccupied ecological niches.
    • Examples: Darwin's Finches on the Galapagos Islands (varied beak shapes adapted to different food sources); Australian Marsupials (diverse forms like Kangaroo, Tasmanian wolf, Marsupial mole evolved from a common marsupial ancestor). Adaptive radiation often leads to homologous structures.
  • Convergent Evolution in Biogeography: Unrelated organisms in similar environments in different geographical regions evolve similar adaptations (e.g., Placental mammals in North America and corresponding Marsupial mammals in Australia - Placental Wolf and Tasmanian Wolf). Leads to analogous structures.
• Biochemical/Molecular Evidence:
  • Remarkable similarity in basic biomolecules (DNA, RNA, ATP, enzymes) and metabolic pathways across diverse organisms.
  • Universality of the genetic code.
  • Similarity in proteins (e.g., Cytochrome c) and gene sequences reflects evolutionary relatedness. The closer the relationship, the greater the similarity.

3. Theories of Biological Evolution

• Lamarckism (Theory of Inheritance of Acquired Characters): Proposed by Jean Baptiste de Lamarck.
  • Postulates:
    1. Internal vital force causes organisms to increase in size.
    2. New needs lead to the formation of new organs.
    3. Use and disuse of organs: Organs used more develop better; unused organs degenerate.
    4. Inheritance of acquired characters: Changes acquired during an organism's lifetime are passed on to offspring.
  • Examples used by Lamarck: Long neck of giraffe (stretching to reach leaves), loss of limbs in snakes.
  • Criticism: Acquired characters (somatic changes) are generally not heritable as they don't affect germ cells (DNA). August Weismann's experiment (cutting tails of mice for generations) disproved it.
• Darwinism (Theory of Natural Selection): Proposed by Charles Darwin (influenced by Thomas Malthus's essay on population). Alfred Russel Wallace independently reached similar conclusions.
  • Based on observations during his voyage on H.M.S. Beagle.
  • Key Concepts:
    1. Overproduction (Prodigality): Organisms produce more offspring than can possibly survive.
    2. Struggle for Existence: Competition for limited resources (food, space, mates), predation, disease. (Intraspecific, Interspecific, Environmental struggle).
    3. Variation: Differences exist among individuals of a species. Darwin recognized variation but couldn't explain its source (now known to be mutation and recombination).
    4. Survival of the Fittest (Natural Selection): Individuals with variations better suited (more advantageous) to their environment survive, reproduce more successfully, and pass on these favourable variations to their offspring. Fitness here refers to reproductive fitness. Nature 'selects' the best-adapted individuals.
    5. Origin of New Species: Accumulation of favourable variations over long periods leads to the formation of new species.
  • Examples: Industrial melanism in peppered moths (Biston betularia) in England (light vs. dark forms selected based on pollution levels affecting tree bark colour); Antibiotic/pesticide resistance in bacteria/insects.
• Mutation Theory: Proposed by Hugo de Vries based on his work on evening primrose (Oenothera lamarckiana).
  • Believed that evolution occurs through sudden, large, discontinuous variations called mutations (saltations), not small, gradual Darwinian variations.
  • He considered mutations as the raw material for evolution.
  • While mutations are the ultimate source of variation, Darwinian gradual variations and natural selection are now considered the primary driving forces of evolution, with mutation providing the raw material.

4. Mechanism of Evolution & Hardy-Weinberg Principle

• Modern Synthetic Theory of Evolution: Combines Darwinian natural selection with genetics, population dynamics, and other fields. Considers the population as the unit of evolution.
• Gene Pool: The sum total of all genes and their alleles present in a population.
• Allele Frequency: The proportion of a specific allele in the gene pool.
• Hardy-Weinberg Principle (Genetic Equilibrium):
  • States that allele frequencies and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.
  • Represents a hypothetical non-evolving population (a baseline).
  • Conditions for Equilibrium:
    1. No Mutation
    2. Random Mating
    3. No Gene Flow (No migration)
    4. Large Population Size (No Genetic Drift)
    5. No Natural Selection
  • Equation: For a gene with two alleles, A (dominant) and a (recessive):
    • Let frequency of allele A = p
    • Let frequency of allele a = q
    • Then, p + q = 1
    • Frequency of genotype AA = p²
    • Frequency of genotype Aa = 2pq
    • Frequency of genotype aa = q²
    • Then, p² + 2pq + q² = 1 (Binomial expansion of (p+q)²)
  • Significance: If the observed genotype frequencies deviate from the expected frequencies calculated using the equation, it indicates that evolution is occurring. The principle helps measure the rate of evolutionary change.
• Factors Affecting Hardy-Weinberg Equilibrium (Causes of Evolution):
  • Gene Flow (Gene Migration): Movement of alleles between populations through migration and interbreeding. Can introduce new alleles or change existing allele frequencies.
  • Genetic Drift: Random changes in allele frequencies, especially significant in small populations, purely by chance.
    • Founder Effect: A small group ('founders') breaks off from a larger population to start a new one. The new population's gene pool may have different allele frequencies than the original population, simply because the founders were not genetically representative of the source population.
    • Bottleneck Effect: A drastic reduction in population size due to a random event (disaster like flood, fire, earthquake). The surviving population may have different allele frequencies than the original population, by chance.
  • Mutation: The ultimate source of new alleles (variations). Changes the gene pool slowly but provides the raw material for selection.
  • Genetic Recombination: Shuffling of existing alleles during sexual reproduction (crossing over, independent assortment). Creates new combinations of alleles.
  • Natural Selection: Differential survival and reproduction based on adaptations. Leads to changes in allele frequencies favouring advantageous traits.
    • Types of Natural Selection:
      1. Stabilizing Selection: Favours average phenotypes, eliminates extremes. Reduces variation. (e.g., human birth weight). Peak gets higher and narrower.
      2. Directional Selection: Favours one extreme phenotype. Shifts the population mean in one direction. (e.g., industrial melanism, pesticide resistance). Peak shifts in one direction.
      3. Disruptive Selection: Favours both extreme phenotypes, selects against the average. Can lead to the formation of two distinct phenotypes. (e.g., different beak sizes in finches specializing on different seed types). Two peaks form.
• Speciation: Formation of new species. Requires reproductive isolation.
  • Allopatric Speciation: Occurs when populations are geographically separated, preventing gene flow. They evolve independently and accumulate differences, eventually becoming reproductively isolated. (Most common mode).
  • Sympatric Speciation: Occurs within the same geographical area, without physical barriers. Can happen due to polyploidy (especially in plants) or disruptive selection leading to ecological or behavioural isolation.

5. A Brief Account of Evolution

• Origin of First Cellular Life: ~3 billion years ago (non-cellular aggregates like RNA capsules ~3 bya, first cells ~2 bya). Likely anaerobic heterotrophs, then chemoautotrophs, then photoautotrophs (like cyanobacteria) releasing O₂ which changed the atmosphere from reducing to oxidizing.
• Timeline Highlights: (Approximate, focus on sequence)
  • ~500 mya: Invertebrates active.
  • ~450 mya: First land organisms (plants).
  • ~350 mya: Jawless fish evolved, amphibians appeared. Fish with stout fins could move on land (ancestors of amphibians - Lobefins like Coelacanth).
  • ~320 mya: Seaweeds and few plants existed.
  • ~200 mya (Triassic): Reptiles dominate, first mammals (shrew-like) appear. Dinosaurs originated.
  • ~150 mya (Jurassic): Dinosaurs peak diversity. First birds (Archaeopteryx) appear.
  • ~65 mya (Cretaceous-Tertiary boundary): Mass extinction event, dinosaurs disappear. Mammals begin to diversify and radiate.
  • Cenozoic Era: Age of Mammals.

6. Origin and Evolution of Man

• Primate Ancestry: Common ancestor with apes.
• Key Trends in Human Evolution: Increasing brain size (cranial capacity), bipedal locomotion, reduction in jaw size and teeth, development of tool use, language, and culture.
• Major Stages (Fossil evidence):
  • Dryopithecus & Ramapithecus: (Miocene, ~15 mya). Dryopithecus was more ape-like. Ramapithecus was more man-like, walked erect (?). Fossils found in Africa and Asia.
  • Australopithecus: (Pliocene, ~2-4 mya). Found in East Africa (e.g., Lucy). Probably hunted with stone weapons, ate fruit. Bipedal locomotion. Cranial capacity ~400-500 cc. Considered connecting link between apes and man. Height ~4 ft.
  • Homo habilis ('Handy Man'): (Pleistocene, ~2 mya). Found in East Africa. First hominid tool-maker. Did not eat meat. Cranial capacity ~650-800 cc.
  • Homo erectus ('Upright Man'): (Pleistocene, ~1.5 mya). Fossils like Java Man (Pithecanthropus erectus), Peking Man. Probably ate meat. Used fire. Migrated to Asia and Europe. Cranial capacity ~900 cc (up to 1100 cc).
  • Homo neanderthalensis (Neanderthal Man): (Late Pleistocene, ~100,000 - 40,000 years ago). Found in East and Central Asia, Europe. Buried their dead, used hides for protection. Cranial capacity ~1400 cc (larger than modern humans on average). Co-existed with early Homo sapiens. Not direct ancestors of modern humans, but interbreeding occurred.
  • Homo sapiens (Modern Man): Arose in Africa ~75,000-10,000 years ago (Ice Age). Spread across continents. Developed cave art (~18,000 years ago). Started agriculture (~10,000 years ago) and human settlements. Cranial capacity ~1300-1600 cc (average ~1350-1400 cc). Homo sapiens sapiens is the subspecies for modern humans.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the concepts we've just discussed. Try to answer them yourself first.

1. The Miller-Urey experiment provided evidence for:
   a) Theory of Biogenesis
   b) Oparin-Haldane hypothesis of chemical evolution
   c) Theory of Special Creation
   d) Panspermia theory

2. Which of the following pairs represents analogous organs?
   a) Forelimbs of whale and bat
   b) Thorns of Bougainvillea and tendrils of Cucurbita
   c) Wings of butterfly and wings of bird
   d) Heart of fish and heart of crocodile

3. The phenomenon of 'Industrial Melanism' demonstrates:
   a) Genetic Drift
   b) Natural Selection
   c) Mutation
   d) Reproductive Isolation

4. The concept of 'Saltation' (sudden, large mutations causing evolution) was proposed by:
   a) Charles Darwin
   b) Jean Baptiste de Lamarck
   c) Hugo de Vries
   d) Alfred Russel Wallace

5. Which condition is essential for the Hardy-Weinberg equilibrium to be maintained in a population?
   a) Small population size
   b) Presence of natural selection
   c) Random mating
   d) Frequent mutations

6. Adaptive radiation of Australian marsupials is an example of:
   a) Convergent evolution
   b) Divergent evolution
   c) Stabilizing selection
   d) Genetic drift

7. Archaeopteryx is considered a connecting link between:
   a) Amphibians and Reptiles
   b) Reptiles and Birds
   c) Birds and Mammals
   d) Fish and Amphibians

8. According to the Hardy-Weinberg principle, if the frequency of the dominant allele 'A' is 0.6, what will be the frequency of the homozygous recessive genotype 'aa'?
   a) 0.36
   b) 0.16
   c) 0.48
   d) 0.40

9. Which hominid was the first to use fire and had a cranial capacity of around 900 cc?
   a) Australopithecus
   b) Homo habilis
   c) Homo erectus
   d) Homo neanderthalensis

10. The ultimate source of all genetic variation is:
    a) Natural selection
    b) Genetic drift
    c) Recombination
    d) Mutation

Answer Key:

1. b
2. c
3. b
4. c
5. c
6. b
7. b
8. b (If p=0.6, then q=1-0.6=0.4. Frequency of aa = q² = (0.4)² = 0.16)
9. c
10. d

Revise these notes thoroughly. Pay close attention to the examples, scientists' names, and the conditions required for different evolutionary processes. Understanding the Hardy-Weinberg principle and the factors affecting it is particularly important for numerical problems that sometimes appear in exams. Good luck with your preparation!