Evolution

• Evolution


  An orderly change from one form to another
• Evolutionary Biology


  The study of evolutionary history of life forms
• Origin of Life


  • Big Bang Theory
  • Earth formed 4.5 billion years ago
  • No atmosphere on early earth
  • Water vapour, CH4, CO2 & NH3 released
  • UV rays broke up water
  • Oxygen combined with NH3 & CH4
  • Ozone layer formed
  • Oceans formed
  • Life appeared 4 billion years ago
• Theories of Origin of Life

  • Theory of spontaneous generation (Abiogenesis)
  • Biogenesis
  • Cosmic theory (Theory of Panspermia)
  • Theory of special creation
  • Theory of chemical evolution
• Urey-Miller experiment


  1. Created condition like primitive earth

  2. Electric discharge in closed flask with CH4, NH3, H2 and water vapour at 800o C

  3. Amino acids, sugars, nitrogen bases, pigment and fats formed
• First non-cellular forms of life originated 3 billion years ago
• Evidences for Evolution


  • Paleontological evidences
  • Morphological & Anatomical evidences
  • Adaptive radiation (Biogeographical evidences)
  • Biochemical evidences
  • Embryological evidences
  • Evidences for evolution by natural selection
• Homologous organs


  Organs having fundamentally similar structure and origin but different functions
• Analogous organs


  Organs having similar function but different structure & origin
• Divergent evolution


  Evolution by which related species become less similar to survive and adapt in different environmental condition
• Convergent evolution


  Evolution by which unrelated species become more similar to survive and adapt in similar environmental condition
• Adaptive radiation


  • Darwin's finches in Galapagos Islands
  • Australian marsupials
  • Placental mammals in Australia
• Industrial melanism is an example of natural selection by anthropogenic action
• Lamarckism (Theory of Inheritance of Acquired characters)


  Evolution of life forms occurred by the inheritance of acquired characters
• Darwinism (Theory of Natural selection)


  • All organisms are modified descendants of previous life forms
  • Natural selection - Organisms with beneficial variations survive and reproduce
• Darwin ignored about origin of variation
• A variant population (B) outgrows the others and appears as new species, i.e. B is better than A under new condition. Thus, nature selects for fitness.
• Facts of natural selection

  • Heritable minor variations: It is either beneficial or harmful to the organisms
  • Overproduction: Population size grows exponentially due to maximum reproduction (E.g. bacterial population)
  • Limited natural resources: Resources are not increased in accordance with the population size
  • Struggle for existence: It is the competition among organisms for resources so that population size is limited
  • Survival of the fittest: In struggle for existence, organisms with beneficial variations can utilize resources better. Hence, they survive and reproduce. This is called Survival of the fittest. It leads to a change in population characteristics and new forms appear
• Darwin ignored about origin of variation and mechanism of evolution or speciation.
• Mutation Theory of evolution

  Evolution takes place through mutation and not by minor variation
• Darwinian variation


  Minor, slow and directional. It results in gradual evolution.
• Mutational variation


  Sudden, random & directionless. Here, speciation is by saltation (single step, large mutation).
• Mutation is the origin of variation for evolution.
• Hardy-Weinberg Principle


  Allele frequencies in a population are stable and is constant from generation to generation in the absence of disturbing factors. The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium (Hardy-Weinberg equilibrium).
• Factors affecting Hardy-Weinberg equilibrium

  • Gene migration: Gene flow from one population to another
  • Genetic drift: The gene flow by chance causing change in frequency
  • Mutation: It results in formation of new phenotypes
  • Genetic recombination: Reshuffling of gene combinations during crossing over resulting in genetic variation
  • Natural selection: Stabilizing selection, Directional selection, Disruptive selection
• Geological time scale


  • Proterozoic era (2500 - 541 million yrs ago)
  • Palaeozoic era (540 - 252 mya)
  • Mesozoic era (252 - 66 mya)
  • Cenozoic era (66 - 0 mya)
• Proterozoic era


  • First cellular forms of life appeared
  • Some of the cells had the ability to release O2 as the light reaction in photosynthesis
  • Single celled organisms became multicellular organisms
• Palaeozoic era


  • Invertebrates were formed
  • First land organisms (plants) appeared
  • Arthropods invaded the land
  • Jawless fishes were evolved
  • Lobefins (stout & strong finned fishes) could move on land and go back to water. They evolved to first amphibians
  • Amphibians evolved to reptiles
  • Giant ferns (Pteridophytes) were present but they all fell to form coal deposits slowly
• Mesozoic era


  • Age of reptiles and gymnosperms
  • Some of the land reptiles went back into water to evolve into fish-like reptiles (E.g. Ichthyosaurs)
  • The land reptiles were dinosaurs (Tyrannosaurus rex, Triceratops, Stegosaurus, Brachiosaurus etc.)
  • Toothed birds were emerged
• Cenozoic era


  • Age of Mammals & Angiosperms
  • Dinosaurs suddenly disappeared
  • First mammals were shrew-like
  • In South America, there were mammals resembling horse, hippopotamus, bear, rabbit etc. Due to continental drift, when South America joined North America, these animals were overridden by North American fauna
  • Due to continental drift, Australian marsupials survived because of lack of competition from any other mammals
• Origin and evolution of man

  • Dryopithecus & Ramapithecus (15 mya)
  • Man-like primates walked up right in eastern Africa (3-4 mya)
  • Australopithecus (2 mya)
  • Homo habilis (First human-like being, 2 mya)
  • Homo erectus (1.5 mya)
  • Homo neanderthalensis (1 lakh - 40,000 yrs ago)
  • Homo sapiens (Modern man, 75,000 - 10,000 yrs ago)
• The theory of evolution is the unifying principle that explains how all living things are related.