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

Alright students, today we will tackle Chapter 7: Evolution. This is a cornerstone chapter in biology, explaining the history of life on Earth and the mechanisms driving its diversity. Understanding this well is crucial not just for your board exams but also for many competitive government exams with a biology component. Let's get straight into the detailed notes.

Chapter 7: Evolution - Detailed Notes for Government Exam Preparation

1. Origin of Life

• The Universe and Earth: The Big Bang theory explains the origin of the universe (~20 billion years ago). Earth formed ~4.5 billion years ago (bya). Early Earth had a reducing atmosphere (no free O2), containing methane (CH4), ammonia (NH3), water vapor (H2O), and hydrogen (H2). High temperatures, volcanic eruptions, and UV radiation were prevalent.
• Theories of Origin of Life:
  • Theory of Special Creation: Life was 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 Panspermia/Cosmozoic Theory: Life came to Earth from outer space as spores or seeds ('panspermia').
  • Theory of Chemical Evolution (Oparin-Haldane Hypothesis): Proposed independently by A.I. Oparin (Russia) and J.B.S. Haldane (England). Stated that life originated from pre-existing non-living organic molecules (e.g., RNA, protein) through a process of chemical evolution in the primitive Earth's oceans ('primordial soup'). Conditions required: Reducing atmosphere, energy sources (UV radiation, lightning).
• Miller-Urey Experiment (1953): Stanley Miller and Harold Urey experimentally supported the Oparin-Haldane hypothesis. They created conditions simulating primitive Earth in a closed flask (CH4, H2, NH3, water vapor at 800°C, electric discharge). They observed the formation of amino acids (like glycine, alanine, aspartic acid), proving that organic molecules could be synthesized abiogenically.
• Formation of Protobionts: Formation of complex organic molecules (RNA, proteins, polysaccharides) -> Aggregation into membrane-bound structures called protobionts (e.g., coacervates, microspheres). These were not true cells but exhibited some life-like properties (simple metabolism, growth, division). RNA is thought to have been the first genetic material ('RNA World' hypothesis).

2. Evolution of Life Forms - Evidences

• Paleontological Evidence (Fossils):
  • Fossils are remains or impressions of past organisms preserved in rocks (usually sedimentary).
  • Study of fossils is Paleontology.
  • Show that life forms varied over time and certain forms are restricted to certain geological time spans.
  • Allow reconstruction of evolutionary lineages (e.g., evolution of the horse).
  • Missing links: Fossil organisms showing characteristics of two different groups (e.g., Archaeopteryx - features of both reptiles and birds).
  • Fossil Dating: Relative dating (position in rock strata) and absolute dating (radiometric dating, e.g., Carbon-14 dating).
• Comparative Anatomy and Morphology:
  • Homologous Structures: Organs having the same fundamental structure and origin but adapted for different functions. Indicate common ancestry and divergent evolution. Examples: Forelimbs of whales, bats, cheetahs, and humans; Thorns of Bougainvillea and tendrils of Cucurbita; Vertebrate hearts or brains.
  • Analogous Structures: Organs having different structures and origins but performing similar functions. Indicate adaptation to similar environments/needs and convergent evolution. Examples: Wings of butterflies and birds; Eye of the octopus and mammals; Flippers of Penguins and Dolphins; Sweet potato (root modification) and potato (stem modification).
• Embryological Evidence (Based on observation of embryos):
  • Proposed by Ernst Haeckel – Biogenetic Law or Recapitulation Theory: "Ontogeny recapitulates phylogeny" (embryonic development repeats evolutionary history). This is an oversimplification.
  • Karl Ernst von Baer noted that embryos never pass through the adult stages of other animals. Embryonic similarities (e.g., gill slits in vertebrate embryos, including humans) suggest common ancestry. These are generally lost before adulthood in terrestrial vertebrates.
• Biogeographical Evidence:
  • Study of the geographical distribution of plants and animals.
  • Different species evolving in isolated geographical areas indicate common ancestry followed by adaptation to local conditions.
  • Adaptive Radiation: The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats). Examples: Darwin's Finches on the Galapagos Islands (beak shape adapted to different food sources); Australian Marsupials (evolved into diverse forms from a common ancestor).
  • Convergent Evolution in Biogeography: Placental mammals in other continents show convergence with Australian marsupials (e.g., Placental Wolf and Tasmanian Wolf - Marsupial).
• Biochemical and Molecular Evidence:
  • Similarities in basic biomolecules (DNA, RNA, ATP, enzymes) and metabolic processes across diverse organisms point to a common ancestor.
  • Universality of the genetic code.
  • Sequence similarity in DNA and proteins reflects evolutionary relatedness (more similarity = closer relationship).

3. Theories of Biological Evolution

• Lamarckism (Theory of Inheritance of Acquired Characters): Proposed by Jean Baptiste de Lamarck.
  • Main points: Use and disuse of organs (organs used more develop better, unused ones degenerate); Inheritance of acquired characters (characters acquired during an organism's lifetime are passed to offspring).
  • Example cited: Long neck of giraffe (stretched neck to reach leaves, passed on).
  • Largely discredited (acquired characters are generally not heritable as they don't change germ cells/DNA). August Weismann's experiment (cutting tails of mice) disproved it.
• Darwinism (Theory of Natural Selection): Proposed by Charles Darwin (and Alfred Russel Wallace independently). Based on observations during his voyage on H.M.S. Beagle. Book: "On the Origin of Species by Means of Natural Selection".
  • Key concepts:
    • Overproduction (Prodigality): Organisms produce more offspring than can possibly survive.
    • Variation: Differences exist among individuals of a species. Darwin couldn't explain the source of variation (now known to be mutation and recombination).
    • Struggle for Existence: Competition for limited resources (food, space, mates), predation, disease.
    • Survival of the Fittest (Natural Selection): Individuals with variations better suited (more 'fit') to their environment survive and reproduce more successfully, passing on advantageous traits. Fitness here refers to reproductive fitness.
  • Examples supporting Natural Selection:
    • Industrial Melanism: Peppered moths (Biston betularia) in England. Before industrialization, light-colored moths were camouflaged against lichen-covered trees; dark moths were rare. After industrialization, pollution killed lichens and darkened bark; dark moths became better camouflaged and increased in frequency. Reverted when pollution controlled.
    • Drug/Pesticide Resistance: Evolution of resistance in bacteria (to antibiotics) and insects (to pesticides like DDT). Resistant individuals survive exposure and reproduce, increasing resistance in the population.
• Mutation Theory: Proposed by Hugo de Vries based on his work on Evening Primrose (Oenothera lamarckiana).
  • Believed evolution occurred due to sudden, large, discontinuous variations called mutations (which he termed 'saltations').
  • Considered mutations, not small Darwinian variations, as the raw material for evolution.
  • Modern view: Mutations are indeed the ultimate source of variation, but natural selection usually acts on smaller variations over long periods (though large mutations can sometimes be significant).

4. Mechanism of Evolution - The Modern Synthetic Theory

• Combines Darwinian selection with modern genetics. Population genetics is key.
• Hardy-Weinberg Principle (Equilibrium):
  • States that in a large, randomly mating population, the allele frequencies and genotype frequencies will remain constant from generation to generation if other evolutionary influences are not operating.
  • Mathematical expression: p² + 2pq + q² = 1 (Genotype frequency) and p + q = 1 (Allele frequency). Where 'p' is the frequency of the dominant allele (A) and 'q' is the frequency of the recessive allele (a). p² = frequency of AA, 2pq = frequency of Aa, q² = frequency of aa.
  • Conditions required for equilibrium (i.e., no evolution):
    1. No Mutation
    2. Random Mating
    3. No Gene Flow (no migration)
    4. No Genetic Drift (large population size)
    5. No Natural Selection
  • Significance: Provides a baseline (null hypothesis) to measure evolutionary change. If frequencies deviate, evolution is occurring.
• Factors Affecting Hardy-Weinberg Equilibrium (Causes of Evolution):
  • Gene Flow (Gene Migration): Movement of alleles between populations through migration of individuals or gametes. Can introduce new alleles or change existing allele frequencies.
  • Genetic Drift: Random changes in allele frequencies, especially significant in small populations (due to chance events).
    • Founder Effect: A small group ('founders') breaks off from a larger population to establish a new one. The new population's gene pool may have different allele frequencies than the source population, purely by chance.
    • Bottleneck Effect: A drastic reduction in population size (due to natural disaster, disease, etc.) leads to a random subset of survivors whose allele frequencies may differ from the original population.
  • Mutation: The ultimate source of new alleles and genetic variation. Occur randomly. While mutation rate is low, over long periods, it's crucial.
  • Genetic Recombination: Shuffling of existing alleles during sexual reproduction (crossing over, independent assortment) creates new combinations of traits, increasing variation.
  • Natural Selection: Differential survival and reproduction based on fitness in a given environment. Leads to adaptations.
    • Stabilizing Selection: Favors intermediate phenotypes, selects against extremes. Reduces variation. Example: Human birth weight.
    • Directional Selection: Favors one extreme phenotype, shifts the population mean. Occurs during environmental change. Example: Industrial melanism, pesticide resistance.
    • Disruptive Selection: Favors both extreme phenotypes, selects against the intermediate. Can lead to divergence and potentially speciation. Example: Beak sizes in certain finches (adapted to large or small seeds, not medium).

5. A Brief Account of Evolution (Timeline)

• ~4 bya: Origin of life (first non-cellular forms, RNA world).
• ~3 bya: First cellular life forms (likely anaerobic prokaryotes).
• ~2 bya: Oxygen released by photosynthetic cyanobacteria, changing atmosphere to oxidizing.
• ~1.5 bya: First eukaryotic cells formed.
• ~500 mya (Paleozoic Era): Invertebrates active, first jawless fish, invasion of land by plants, then arthropods. Fish with stout fins moved onto land -> first amphibians.
• ~350 mya: Jawed fishes, sea weeds abundant. Amphibians evolved into reptiles.
• ~320 mya: Amphibians abundant. Reptiles lay shelled eggs (adaptation to land). Ferns grow large (form coal deposits).
• ~200 mya (Mesozoic Era - Age of Reptiles): Reptiles dominate. Dinosaurs appear and diversify. First mammals (shrew-like) and birds (Archaeopteryx) appear. Gymnosperms dominant plants.
• ~65 mya: Mass extinction event (likely asteroid impact), wiping out dinosaurs. Mammals begin to diversify and radiate. Angiosperms (flowering plants) become dominant. (Start of Cenozoic Era - Age of Mammals).
• Quaternary Period (within Cenozoic): Evolution of humans.

6. Origin and Evolution of Man

• Order: Primates. Family: Hominidae.
• Key Trends: Bipedal locomotion, increased brain size (cranial capacity), tool use, language, culture.
• Key Hominids:
  • Dryopithecus: Ape-like, more arboreal, ancestor to apes and humans. (Miocene)
  • Ramapithecus: More man-like, walked erect? Fossils include teeth and jaw fragments. (Miocene/Pliocene)
  • Australopithecus: (~4-2 mya, Africa). Walked upright (bipedal - confirmed by Laetoli footprints). Small brain (~400-500 cc). Ape-like face. Hunted with stone weapons but essentially fruit eaters. A. africanus, A. afarensis ('Lucy'). Considered connecting link between apes and humans.
  • Homo habilis ('Handy Man'): (~2.5-1.5 mya, Africa). First definite hominid tool maker (stone tools). Larger brain (~650-800 cc). Did not eat meat.
  • Homo erectus ('Upright Man'): (~1.8 mya - 100,000 ya, Africa, Asia, Europe). Larger brain (~900 cc, up to 1100 cc). Used more sophisticated tools, discovered fire. Probably ate meat. Examples: Java Man, Peking Man.
  • Homo neanderthalensis (Neanderthal Man): (~100,000 - 40,000 ya, Europe, Asia). Brain size large (~1400 cc, slightly larger than modern humans). Stocky build, prominent brow ridges. Used hides for clothing, buried their dead (evidence of ritual/culture?). Co-existed with early Homo sapiens. Not considered direct ancestor of modern humans.
  • Homo sapiens (Modern Human): Arose in Africa (~75,000-100,000 ya). Spread across continents.
    • Homo sapiens fossilis (Cro-Magnon Man): Early H. sapiens. Brain capacity ~1600 cc (later reduced slightly). Developed art (cave paintings ~18,000 ya).
    • Modern Man (Homo sapiens sapiens): Started agriculture (~10,000 ya), settlements. Further development of language, art, culture. Brain size ~1350-1400 cc.

Multiple Choice Questions (MCQs)

1. The Miller-Urey experiment demonstrated:
   a) Life originated spontaneously from non-living matter.
   b) The formation of protobionts from organic molecules.
   c) Abiogenic synthesis of amino acids under simulated primitive Earth conditions.
   d) The Panspermia theory is correct.

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

3. Darwin's finches on the Galapagos Islands are a classic example of:
   a) Convergent evolution
   b) Adaptive radiation
   c) Genetic drift
   d) Stabilizing selection

4. According to the Hardy-Weinberg principle, which factor would NOT disrupt the genetic equilibrium of a population?
   a) Gene flow
   b) Random mating
   c) Genetic drift
   d) Mutation

5. The theory of inheritance of acquired characters was proposed by:
   a) Charles Darwin
   b) Hugo de Vries
   c) Gregor Mendel
   d) Jean Baptiste de Lamarck

6. The phenomenon of Industrial Melanism, observed in Biston betularia, demonstrates:
   a) Genetic drift
   b) Disruptive selection
   c) Natural selection (Directional)
   d) Founder effect

7. Archaeopteryx is considered a missing link because it shows characteristics of both:
   a) Reptiles and Mammals
   b) Birds and Mammals
   c) Amphibians and Reptiles
   d) Reptiles and Birds

8. Which hominid species was the first definite tool maker and is known as 'Handy Man'?
   a) Australopithecus
   b) Homo habilis
   c) Homo erectus
   d) Homo neanderthalensis

9. The random change in allele frequency in a small population due to chance events is termed:
   a) Gene flow
   b) Natural selection
   c) Genetic drift
   d) Mutation pressure

10. The concept of 'Saltation' (single-step large mutations causing speciation) was central to the evolutionary theory proposed by:
    a) Charles Darwin
    b) Alfred Wallace
    c) Hugo de Vries
    d) Ernst Haeckel

Answer Key for MCQs:

1. c
2. b
3. b
4. b
5. d
6. c
7. d
8. b
9. c
10. c

Study these notes thoroughly. Focus on definitions, examples, key experiments, and the scientists associated with different theories. Remember the conditions for Hardy-Weinberg equilibrium and the factors that disrupt it. The human evolution sequence and key characteristics are also frequently tested. Good luck with your preparation!