Evolution Class 12 Notes Biology Chapter 7

Chapter at a Glance

  • Evolution is the formation of newer types of organisms from the pre-existing ones through modification.


  • Origin of life is the process by which living organisms developed from inanimate matter, which is generally thought to have occurred on Earth between 3500 to 4000 millions years ago.
  • Earth originated 4600 million year ago.
  • Life appeared 3.7 billion years ago. This is indicated by the discovery of microfossils of cyanobacteria like organisms.
  • There was no atmosphere on early earth.
  • There are various theories which explain the phenomenon of origin of life. A few of them were only assumptions while others were based on scientific grounds. These theories are -Theory of special creation, Theory of spontaneous generation or Abiogenesis, Theory of biogenesis, and Oparin Haldane theory.
  • Oparin and Haldane proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g., RNA, protein etc.) and that formation of life was preceded by chemical evolution, i.e., formation of diverse organic molecules from inorganic constituents.
  • Oparin – Haldane theory was supported experimentally by Miller-Urey experiment in 1953 with the help of simulation experiments. Miller & Urey tried to create conditions existing in primitive atmosphere – in spark discharge
  • apparatus – a large flask containing mixture ofCH4, NH3, H2 and H20 with electric source and boiling of water for a week. They observed formation of amino acids.
  • The Miller-Urey experiment provided the first evidence that organic molecules needed for life could be formed from inorganic components.
  • Haldane coined the term ‘prebiotic soup’, and this became a powerful symbol of the Oparin-Haldane view of the origin of life.
  • Prebiotic soup is also called hot dilute soup.
  • Oparin postulated the spontaneous appearance of structures called coacervates.
  • The first prokaryotes were anaerobes and chemoheterotrophs.
  • Atmosphere becomes richer in oxygen due to photoautotrophs.
  • Darwin published his observations and conclusions regarding evolution in the book “Origin of Species” in 1859.
  • Darwin proposed the theory of evolution by natural selection. According to this theory, individuals that are better adapted to their environment survive and reproduce more successfully than less well adapted individuals.
  • Darwin travelled round the world in ship H.M.S. Beagle for 5 years and during journey, he noted a huge variety and remarkable similarity between organisms and their wonderful adaptation to environment and proposed the following factors:
  • Rapid multiplication
  • Limited environmental resources Struggle for existence
  • Variation
  • Natural selection Survival of fittest
  • Inheritance of useful variation
  • Formation of new species
  • Due to limited space and food, the following competition occurs for struggle for existence.
    • Intraspecific : Between individuals of same species.
    • Interspecific : Between individuals of different species.
    • Environmental : Natural calamites like flood, drought.
  • The mutation theory was put forward by Hugodevries to explain the mechanism of evolution.
  • Features of mutation theory are-
    • Mutation (Sudden, discontinuous, inheritable, genetic variation) form raw material for evolution.
    • It appears suddenly and produces effect immediately.
    • Mutants are different from parents.
    • Mutations are recurring, random, inheritable.
    • Collection of mutated forms leads to evolution.
  • The mutation observed by de Vries in Oenothera Lamarckiana was chromosomal number variant.
  • Genetic equilibrium (Hardy Weinberg law) states that “both gene frequency and genotype frequency” will remain constant from generation to generation in an infinitely large interbreeding population in which mating is random and no selection, migration or mutation occurs.
  • Genetic drift refers to chance elimination of gene(s) of certain traits when a section of population migrates or dies of natural calamity. It eliminates certain alleles and fixes other alleles.
  • Members of a population might enter a new population and interbreed there. They will add new allels to the local gene pool of the host population. This is called gene migration. (It may be in case of inter-specific fertile cross).
  • Types of selection
  • Stabilising selection : Intermediate characteristics are favoured.

Directional selection: One of the two possible extremes is favoured.

Disruptive selection : Natural selection moves characteristics toward both extremes, and intermediate phenotypes become rarest.

  • Paleontological evidences for evolution refer to the evidences from fossils.
  • Fossils are the naturally preserved remains or traces of organisms that lived in the geologic past. These organisms are preserved in the form of skeletal, foot prints, trails, casts and moulds, resin (amber) etc. They are generally found in sedimentary rocks.
  • Paleontology is the study of fossils.
  • Geological time scale is an essential tool for understanding the history of Earth and the evolution of life.
  • It indicates the sequence and duration of eras and periods with their dominant forms of life.
  • Divergent evolution and homologous organ : Whale, bats, cheetah and human (all animals) share similarities in pattern of bones of forelimb. Though they perform different function but they have same anatomical structure. Thus in these animals, same structure developed along different direction for adaptation to different needs. This is divergent evolution and structures are homologous.
  • Homology indicates common ancestory. Other examples are
    • Vertebrate hearts or brains
    • Thorn and tendrils of Bougainvillea and Cucurbita.
  • Analogous organs are the result of convergent or parallel evolution in separate lineage. Different structures evolve for same function and have similarity. Analogous organs are not anatomically similar enough though they perform similar functions. E.g.,
    • Wings of butterfly and birds
    • Eyes of octopus and mammals
    • Flippers of penguins and dolphins
    • Sweet potato (root modification) and potato (stem modification)
  • Missing link is the transitional forms between two groups of organisms occurring only in the fossil state. Eg. Archaeopteryx.
  • Vestigial organs are the useless & functionless degenerate structures which were large and functional in some other animals. Examples are vermiform appendix, coccyx, pinna muscle, wisdom tooth in man etc


Paleoanthropology is the scientific study of human evolution.

  • Humans, chimpanzees, gorillas, and the large apes of Africa share a common ancestor that lived between eight million and five million years ago.
  • Humans, or hominids, belong to the scientific order named Primates (that includes the monkeys, lemurs, and apes) of class mammalia.
  • Many experts believe that Australopithecus garhi or a similar species gave rise to the genus Homo.
  • Early hominids- members of the genus Homo-lived contemporaneously with australopithecines for perhaps a half million years.
  • The oldest fossil remains of a member of the genus Homo were discovered in Tanzania. It was named H habilis.
  • Homo erectus is the only other known extinct species of the genus Homo. H. erectus was replaced in tropical regions by Homo sapiens about 200,000 years ago.
  • Cro-Magnon man used to carry domesticated dogs while going for hunting. The most accepted line of descent in human evolution is –
  • Ramapithecus—> Homo habilis —->  Homo erectus——>      Homo sapiens.

The first hominid was known as Homo habilis

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