Evolution NCERT

• Evolutionary Biology


  The study of history of life forms on earth
• Origin of life


  1. Formation of diverse organic molecules from inorganic constituents

  2. First non-cellular forms of life

  3. First cellular forms of life
• The Big Bang theory attempts to explain the origin of the universe
• Life appeared 500 million years after the formation of earth, i.e., almost four billion years back
• Panspermia


  The idea that life came from outside the earth
• Louis Pasteur demonstrated that life comes only from pre-existing life
• Chemical evolution


  Formation of diverse organic molecules from inorganic constituents
• The first non-cellular forms of life could have originated 3 billion years back
• The first cellular form of life did not possibly originate till about 2000 million years ago
• Theory of special creation


  All living organisms were created as such, diversity was always the same, and earth is about 4000 years old
• Natural selection


  Mechanism of evolution where those characteristics which enable some to survive better in natural conditions outbreed others
• Evidences for evolution


  • Fossils
  • Comparative anatomy and morphology
  • Biochemical similarities
  • Artificial selection by humans
• Homology


  Similarities in anatomical structure indicating common ancestry
• Analogy


  Similarities in function but not in anatomical structure
• Adaptive radiation


  Evolution of different species in a given geographical area starting from a point and radiating to other areas of geography
• Adaptive radiation is exemplified by Darwin's finches and Australian marsupials
• Biological evolution


  Evolution by natural selection, with the rate linked to the life cycle or lifespan of the organism
• Selection


  The process by which certain traits become either more or less common in a population over successive generations
• Selection would have started when cellular forms of life with differences in metabolic capability originated on earth
• Darwinian theory of evolution


  Natural selection is the essence
• Rate of appearance of new forms


  Linked to the life cycle or the life span
• Microbes


  • Divide fast, ability to multiply and become millions of individuals within hours
• Bacterial colony growth


  1. Built-in variation in ability to utilise a feed component

  2. Change in medium composition brings out only the part of the population that can survive

  3. Variant population outgrows others and appears as new species within days. For the same thing to happen in a fish or fowl, it would take millions of years as life spans of these animals are in years
• Fitness


  Based on characteristics which are inherited, has a genetic basis
• Adaptive ability


  Inherited, has a genetic basis
• Fitness


  • The end result of the ability to adapt and get selected by nature
• Darwinian Theory of Evolution

  • Branching descent and natural selection are the two key concepts
• Lamarck had said that evolution of life forms had occurred driven by use and disuse of organs
• Nobody believes Lamarck's conjecture about the evolution of giraffe necks any more
• Evolution


  Can be viewed as either a process or the result of a process
• The world we see, inanimate and animate, is only the success stories of evolution
• The work of Thomas Malthus on populations may have influenced Darwin
• Theoretically population size will grow exponentially if everybody reproduced maximally, but in reality population sizes are limited due to competition for resources
• Darwin's insight


  Variations that are heritable and make resource utilisation better for few (adapted to habitat better) will enable only those to reproduce and leave more progeny, leading to change in population characteristics and new forms appearing over generations
• Mutation


  Large difference arising suddenly in a population, random and directionless, causes evolution according to de Vries
• Darwinian variations


  Small and directional, evolution is gradual according to Darwin
• Hardy-Weinberg principle


  Allele frequencies in a population are stable and constant from generation to generation, the gene pool remains in genetic equilibrium
• Factors affecting Hardy-Weinberg equilibrium

  • Gene migration or gene flow
  • Genetic drift
  • Mutation
  • Genetic recombination
  • Natural selection
• Microbial experiments show that pre-existing advantageous mutations when selected will result in observation of new phenotypes over few generations, leading to speciation
• Natural selection can lead to


  • Stabilisation
  • Directional change
  • Disruption