Class 12 Notes

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

Priyanka Rawat

— 10 min read

Biology

Hey folks, here are the detailed notes for Chapter 7: Evolution, from the NCERT Class 12 Biology textbook especially made for your exam preparations.

Chapter 7: Evolution

1. Origin of Life

  • Universe Origin: The Big Bang theory explains the origin of the universe (approx. 20 billion years ago) as a singular huge explosion, leading to expansion, cooling, and formation of galaxies.
  • Earth Origin: Earth formed about 4.5 billion years ago. Early atmosphere likely contained water vapour (H₂O), methane (CH₄), carbon dioxide (CO₂), and ammonia (NH₃), but no free oxygen (O₂). It was a reducing atmosphere. UV rays broke H₂O into H₂ and O₂. Lighter H₂ escaped. Oxygen combined with NH₃ and CH₄ to form water, CO₂, etc. The ozone layer formed later. As Earth cooled, water vapour condensed into rain, forming oceans.
  • Theories on Origin of Life:
    • Theory of Special Creation: States that life was created by a supernatural power. Has three connotations: (i) All living organisms (species) were created as such. (ii) Diversity was always the same since creation. (iii) Earth is about 4000 years old. (Strongly challenged by scientific evidence).
    • Theory of Panspermia/Cosmozoic Theory: Proposed that units of life called 'spores' (panspermia) were transferred to different planets, including Earth, from outer space. Still a favoured idea for some astronomers.
    • Theory of Spontaneous Generation: Proposed that life originated from non-living matter (e.g., decaying straw, mud). Louis Pasteur disproved this theory experimentally using pre-sterilized, swan-necked flasks, demonstrating that life comes only from pre-existing life (Biogenesis).
    • Theory of Chemical Evolution (Oparin-Haldane Hypothesis): Proposed independently by A.I. Oparin (Russia) and J.B.S. Haldane (England). Stated that the first form of life could have come from pre-existing non-living organic molecules (e.g., RNA, proteins). Formation of life was preceded by chemical evolution, i.e., formation of diverse organic molecules from inorganic constituents under the conditions prevailing on early Earth (high temperature, volcanic storms, reducing atmosphere containing CH₄, NH₃, etc.). Energy sources were UV radiation and lightning.
  • Miller-Urey Experiment (1953):
    • Stanley Miller and Harold Urey experimentally supported the Oparin-Haldane hypothesis.
    • Created conditions similar to primitive Earth in a laboratory setup (closed flask with CH₄, H₂, NH₃, and water vapour at 800°C).
    • Electric discharges were passed to simulate lightning.
    • Observed formation of amino acids (glycine, alanine, aspartic acid).
    • Similar experiments by others resulted in the formation of sugars, nitrogen bases, pigments, and fats. Analysis of meteorite content also revealed similar compounds, indicating similar processes occurring elsewhere in space.
  • Formation of Macromolecules and First Cells:
    • Simple organic molecules (monomers like amino acids, nucleotides) polymerized to form complex macromolecules (proteins, nucleic acids).
    • These macromolecules perhaps combined to form the first non-cellular forms of life - giant molecules like RNA, protein, polysaccharides (capsules). These capsules could reproduce.
    • First cellular forms of life likely originated around 3 billion years ago. They were probably single-celled, anaerobic heterotrophs living in the aquatic environment.
    • The first self-replicating metabolic capsules are termed 'protobionts' or 'protocells'.
    • RNA World Hypothesis: RNA might have been the first genetic material and also acted as a catalyst (ribozyme). Life processes like metabolism, translation, splicing evolved around RNA. DNA, being more stable, later evolved as the primary genetic material.

2. Evidence for Evolution

  • Paleontological Evidence (Fossils):
    • Fossils are remains or impressions of hard parts of past organisms found in rocks (sedimentary rocks).
    • Study of fossils (paleontology) shows that life forms varied over geological time and certain forms are restricted to certain geological time spans.
    • Different-aged rock sediments contain fossils of different life forms, indicating evolutionary changes. Newer forms are found in upper strata, older forms in lower strata.
    • Fossils provide direct evidence of evolution and help trace evolutionary lineages (e.g., horse evolution, Archaeopteryx as a connecting link between reptiles and birds).
    • Geological Time Scale: Represents the sequence and duration of different eras, periods, and epochs in Earth's history, correlating them with the evolution of life forms.
  • Comparative Anatomy and Morphology: Shows similarities and differences among organisms today and those that existed years ago.
    • Homologous Structures: Organs having the same fundamental structure and origin but adapted for different functions. Indicates common ancestry and divergent evolution. Examples: Forelimbs of mammals (whale flippers, bat wings, cheetah legs, human hands), vertebrate hearts or brains, thorns of Bougainvillea and tendrils of Cucurbita.
    • Analogous Structures: Organs having different structures and origins but performing similar functions. Indicates convergent evolution (different structures evolving for the same function due to similar adaptive pressures). Examples: Wings of butterfly (insect) and birds, eyes of octopus (mollusc) and mammals, flippers of penguins (bird) and dolphins (mammal), sweet potato (root modification) and potato (stem modification).
    • Vestigial Organs: Organs that are rudimentary and non-functional in present-day organisms but were functional in their ancestors. Examples: Appendix in humans, nictitating membrane (plica semilunaris) in human eye, muscles of the ear pinna in humans, wisdom teeth, body hair, coccyx (tail bone).
  • Embryological Evidence (Based on observation of certain features during embryonic stage common to all vertebrates):
    • Proposed by Ernst Haeckel based on observations that embryos of different vertebrates pass through common stages.
    • Biogenetic Law/Recapitulation Theory: "Ontogeny recapitulates phylogeny" - meaning the developmental stages of an organism (ontogeny) repeat the evolutionary history of its ancestors (phylogeny).
    • Example: Embryos of all vertebrates (fish, amphibians, reptiles, birds, mammals) develop vestigial gill slits behind the head, although they become functional only in fish.
    • This proposal was disapproved by Karl Ernst von Baer who noted that embryos never pass through the adult stages of other animals.
  • Biogeographical Evidence: Study of the geographical distribution of species.
    • Species distribution is restricted to certain areas, suggesting a common origin and subsequent diversification.
    • 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: Different species of finches found on the Galapagos Islands evolved from a common mainland ancestor. They developed different beak shapes adapted to different food sources (seed-eating, insectivorous, cactus-eating).
      • Australian Marsupials: A variety of marsupials (pouched mammals) evolved from a common ancestral stock within Australia, each adapted to different niches (e.g., Marsupial mole, Tasmanian wolf, Kangaroo, Koala, Bandicoot). This is an example of adaptive radiation.
      • Placental Mammals in Australia: When more than one adaptive radiation appears to have occurred in an isolated geographical area (representing different habitats), it can be called convergent evolution. Example: Placental mammals show similarities to corresponding marsupials (e.g., Placental Wolf and Tasmanian Wolf-Marsupial).
  • Biochemical/Molecular Evidence:
    • Similarities in basic biomolecules (proteins, genes, DNA) and metabolic pathways among diverse organisms suggest common ancestry.
    • The closer the relationship between species, the more similar their DNA and proteins.
    • Universality of the genetic code across different organisms is strong evidence for common ancestry.

3. Theories of Biological Evolution

  • Lamarckism (Theory of Inheritance of Acquired Characters - Jean Baptiste de Lamarck):
    • Proposed that evolution occurred due to the use and disuse of organs and the inheritance of acquired characters.
    • Use of an organ leads to its development; disuse leads to its degeneration.
    • Characters acquired during an organism's lifetime are passed on to the next generation.
    • Example: Giraffes developed long necks by stretching to reach high leaves, and this trait was inherited.
    • Largely discredited as acquired characters are generally not heritable (changes in somatic cells are not passed on).
  • Darwinism (Theory of Natural Selection - Charles Darwin):
    • Based on observations during his voyage on H.M.S. Beagle. Influenced by Thomas Malthus's work on populations.
    • Alfred Russel Wallace, a naturalist working in the Malay Archipelago, had also come to similar conclusions around the same time.
    • Key Concepts:
      • Overproduction (Prodigality of Nature): Organisms produce more offspring than can possibly survive.
      • Struggle for Existence: Due to overproduction and limited resources (food, space), individuals compete (intraspecific, interspecific, environmental struggle).
      • Variations: Differences exist among individuals of the same species. Darwin recognized the importance of variation but couldn't explain its source. Variations can be favourable or unfavourable.
      • Survival of the Fittest (Natural Selection): Individuals with favourable variations (better adaptations) are more likely to survive, reproduce, and pass on these traits. Nature 'selects' the fittest individuals. Fitness according to Darwin refers ultimately and only to reproductive fitness.
      • Origin of Species: Over long periods, accumulation of favourable variations leads to the formation of new species.
    • Core Ideas: Branching Descent (evolution is tree-like, with species diverging from common ancestors) and Natural Selection (the mechanism driving evolution).
    • Examples of Natural Selection:
      • Industrial Melanism: Before industrialization in England, light-coloured peppered moths (Biston betularia) were abundant as they camouflaged against lichen-covered trees. Dark-coloured (melanic) moths were rare. After industrialization, pollution killed lichens and darkened tree bark. Melanic moths became better camouflaged and increased in number, while light-coloured moths became rare. This showed selection by predators based on camouflage. In areas without industrial pollution, the reverse occurred.
      • Antibiotic/Pesticide Resistance: Bacteria or pests exposed to antibiotics/pesticides develop resistance over time. Sensitive individuals die, while resistant ones survive and reproduce, leading to a resistant population. This is evolution by anthropogenic (human-caused) action.
  • Mutation Theory (Hugo de Vries):
    • Based on his work on evening primrose (Oenothera lamarckiana).
    • Proposed that evolution occurs due to sudden, large, inheritable changes called mutations (he called them 'saltations').
    • Believed mutations, not small variations (as Darwin suggested), were the cause of speciation.
    • Viewed evolution as a discontinuous process.
    • While mutations are indeed the ultimate source of variation, evolution is now understood to involve both mutation and natural selection acting on variations.

4. Mechanism of Evolution - Hardy-Weinberg Principle

  • Modern Synthetic Theory of Evolution: Integrates Darwinian natural selection with Mendelian genetics, population genetics, and other biological fields.
  • Population Genetics: Study of allele frequencies in populations. Evolution is defined as a change in the allele frequencies in a population over time.
  • Hardy-Weinberg Principle (Equilibrium):
    • States that allele frequencies (and genotype frequencies) in a population remain constant from generation to generation in the absence of other evolutionary influences. The gene pool remains constant. This is called genetic equilibrium.
    • Sum total of all allelic frequencies is 1.
    • Let 'p' be the frequency of allele 'A' and 'q' be the frequency of allele 'a'. Then p + q = 1.
    • The frequencies of genotypes AA, Aa, and aa are p², 2pq, and q² respectively.
    • The principle states: p² + 2pq + q² = 1 (Binomial expansion of (p+q)²).
    • This principle serves as a baseline to measure evolutionary change. If the measured frequencies in a population deviate from the expected equilibrium values, it indicates that evolution is occurring.
  • Factors Affecting Hardy-Weinberg Equilibrium (Factors causing evolutionary change):
    • Gene Flow (Gene Migration): Movement of alleles into or out of a population due to the migration of individuals or gametes. Can change allele frequencies in both the source and recipient populations. If migration happens multiple times, it leads to gene flow.
    • Genetic Drift: Random changes in allele frequencies, especially significant in small populations, purely by chance. Founder effect and bottleneck effect are examples.
      • Founder Effect: When a small group of individuals (founders) breaks off from a larger population to establish a new colony, they may carry alleles in different frequencies than the original population, simply by chance. The new population's gene pool will reflect that of the founders.
      • Bottleneck Effect: When a population undergoes a drastic reduction in size due to a random event (disaster), the surviving individuals may have different allele frequencies than the original population, leading to genetic drift.
    • Mutation: Spontaneous changes in DNA sequence. Creates new alleles, providing the raw material for variation. Mutations occur randomly, but their accumulation can change allele frequencies slowly.
    • Genetic Recombination: Shuffling of existing alleles during sexual reproduction (crossing over during meiosis, independent assortment of chromosomes). Creates new combinations of alleles.
    • Natural Selection: Differential survival and reproduction based on adaptations. Alleles conferring higher fitness increase in frequency over time. This is the most directed evolutionary force.
  • Types of Natural Selection:
    • Stabilizing Selection: Favours intermediate phenotypes and eliminates extreme variations. Reduces variation. Peak gets higher and narrower. Example: Average birth weight in humans.
    • Directional Selection: Favours one extreme phenotype over others. Shifts the population mean in one direction. Peak shifts in one direction. Example: Industrial melanism, pesticide resistance.
    • Disruptive Selection: Favours both extreme phenotypes over the intermediate phenotype. Can lead to the formation of two distinct subpopulations. Two peaks form in the distribution. Example: Beak sizes in certain African finches (adapted for soft or hard seeds, not intermediate).

5. A Brief Account of Evolution

  • Timeline: Life appeared ~500 million years after Earth's formation (~4 bya). Non-cellular forms ~3 bya. First cellular forms ~2 bya.
  • Evolutionary Sequence (approximate times):
    • Single-celled organisms -> Multicellular organisms
    • Invertebrates formed and active (~500 mya)
    • Jawless fish evolved (~350 mya)
    • Fish with stout fins moved onto land -> First amphibians (ancestors of modern frogs/salamanders)
    • Amphibians evolved into Reptiles (lay shelled eggs, do not need water for reproduction). Reptiles dominated Earth.
    • Land reptiles went back into water to evolve into fish-like reptiles (e.g., Ichthyosaurs) (~200 mya).
    • Giant reptiles (Dinosaurs) emerged, largest being Tyrannosaurus rex (~20 ft height). Dominated ~65 mya.
    • Dinosaurs suddenly disappeared (~65 mya - possibly climate change or asteroid impact).
    • Small reptile ancestors evolved into Mammals (viviparous, protected unborn young). Mammals diversified after dinosaur extinction.
    • Some mammals returned to water (whales, dolphins).
    • Birds evolved from reptiles (Archaeopteryx is a connecting link).
    • South America hosted mammals resembling horse, hippopotamus, bear, rabbit etc. Due to continental drift, it joined North America, and these animals were overridden by North American fauna.
  • Plant Evolution:
    • Single-celled -> Seaweeds -> Bryophytes (first land plants) -> Pteridophytes -> Gymnosperms -> Angiosperms (flowering plants).

6. Origin and Evolution of Man

  • Primate Ancestors: Dryopithecus and Ramapithecus (approx. 15 mya). Hairy, walked like gorillas/chimpanzees. Dryopithecus was more ape-like. Ramapithecus was more man-like. Fossils found in Ethiopia and Tanzania.
  • Australopithecines: (approx. 2 mya). Lived in East African grasslands. Evidence shows they hunted with stone weapons but essentially ate fruit. Height ~4 feet. Walked upright (bipedalism). Brain capacity ~400-600 cc. Often called 'ape-man'.
  • Homo habilis ("Handy Man"): (approx. 2 mya). Found in East Africa. First human-like being (hominid). Brain capacity ~650-800 cc. Probably did not eat meat. Used stone tools.
  • Homo erectus ("Upright Man"): (approx. 1.5 mya). Fossils found in Java ('Java Man'), Peking ('Peking Man'), etc. Brain capacity ~900 cc. Probably ate meat. Used more sophisticated tools, possibly used fire. Migrated to Asia and other parts.
  • Homo neanderthalensis (Neanderthal Man): (approx. 100,000 - 40,000 years ago). Found in East and Central Asia (Neander valley, Germany). Brain size ~1400 cc (comparable to modern humans). Used hides to protect their body, buried their dead. Co-existed with Homo sapiens for some time. Became extinct.
  • Homo sapiens (Modern Man):
    • Arose in Africa (approx. 75,000 - 10,000 years ago).
    • Called 'Cro-Magnon man' (early Homo sapiens). Brain capacity ~1400-1600 cc.
    • Migrated across continents and developed into distinct races.
    • Developed agriculture (~10,000 years ago) and human settlements started.
    • Pre-historic cave art developed about 18,000 years ago.
    • Rest is part of human history (growth, decline of civilizations).

Key Trends in Human Evolution:

  • Bipedal locomotion (walking upright)
  • Increase in brain size and intelligence
  • Flattening of the face (reduction in prognathism)
  • Reduction in jaw and teeth size
  • Development of tool use, language, art, and culture.

This covers the essential points from NCERT Chapter 7 for exam preparation. Focus on definitions, examples, key experiments (Miller-Urey), theories (Darwin, Lamarck, Oparin-Haldane), evidence types, Hardy-Weinberg principle and factors affecting it, and the sequence of human evolution with key ancestors and their characteristics. Good luck!

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