Biology

• Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are produced
• Since we are talking about living organisms, evolution is known as 'organic evolution'
• All the plants and animals which we see today around us have evolved from some or the ancestors that lived on this earth long, long ago
• Development of 'pterosaur' bird

  • An ancient flying reptile which lived on the earth about 150 million years ago
  • Began life as a big lizard which could just crawl on land
  • Over millions of years, small folds of skin developed between its feet which enabled it to glide from tree to tree
  • Over many, many generations, spread over millions of years, the folds of skin, and the bones and muscles supporting them grew to form wings which could make it fly
• The development of 'pterosaur' from a big lizard is an example of evolution
• It is considered that they have evolved from the common ancestor
• The more characteristics or features two species have in common, the more closely they will be related
• Important sources which provide evidences for evolution

  • Homologous organs
  • Analogous organs
  • Fossils
• Homologous organs


  Organs which have the same basic structure (or same basic design) but different functions
• Homologous organs


  • Forelimbs of a man, a lizard (reptile), a frog (amphibian), a bird and a bat (mammal)
• Analogous organs


  Organs which have different basic structure (or different basic design) but have similar appearance and perform similar functions
• Analogous organs


  • Wings of an insect and a bird
• Fossils


  The remains or impressions of dead animals or plants that lived in the remote past
• Fossil bird called Archaeopteryx


  • Looks like a bird but has many other features which are found in reptiles, such as feathered wings like those of birds but teeth and tail like those of reptiles
• Fossils are formed when organisms die, their bodies will decompose by the action of micro-organisms in the presence of oxygen, moisture etc.
• Fossils are also obtained by digging into the earth
• Darwin's Theory of Evolution


  The theory of evolution proposed by Charles Robert Darwin in his famous book 'The Origin of Species', also known as 'The Theory of Natural Selection'
• Darwin's Theory of Natural Selection

  • The best adapted organisms are selected by nature to pass on their characteristics (or traits) to the next generation
  • Applies to plants as well as animals
• Darwin's Theory of Evolution (Natural Selection)

  1. Within any population, there is natural variation

  2. Even though all species produce a large number of offsprings, populations remain fairly constant naturally

  3. Struggle between members of the same species and different species for food, space and mate

  4. Struggle for survival within populations eliminates the unfit individuals

  5. Fit individuals possessing favourable variations survive and reproduce (survival of fittest)

  6. Individuals having favourable variations pass on these variations to their progeny from generation to generation

  7. These variations when accumulated over a long period of time, lead to the origin of a new species
• Though Darwin's theory was widely accepted, it was criticised on the ground that it could not explain 'how the variations arise'
• With the progress in genetics, the source of variations was explained to be the 'genes'
• The most accepted theory of evolution is the Synthetic Theory of Evolution in which the origin of species is based on the interaction of 'genetic variation' and 'natural selection'
• Sometimes a species may completely die out and become extinct
• Once a species is extinct, its genes are lost forever and it cannot re-emerge at all
• The French biologist Lamarck proposed, in 1809, a hypothesis to account for the mechanism of evolution, based on two conditions: the use and disuse of parts, and the inheritance of acquired characteristics
• According to Lamarckism, the long neck and legs of the modern giraffe were the result of generations of short-necked and short-legged ancestors feeding on leaves at progressively higher levels of trees
• In November 1859, Darwin published the Origin of Species by Means of Natural Selection, proposing that natural selection is the mechanism by which new species arise from pre-existing species
• The 'struggle for existence' described by Darwin was popularised by the coining of the terms such as 'survival of the fittest'
• The theory of evolution, as proposed by Darwin and Wallace, has been modified in the light of modern evidence from genetics, molecular biology, palaeontology, ecology, and ethology (the study of behaviour) and is known as neo- Darwinism (neo or new)
• Neo-Darwinism may be defined as the theory of organic evolution by the natural selection of inherited characteristics
• Most scientists believe that human beings and apes, such as chimpanzees and gorillas, share a common ancestor
• The ancestors of human beings probably began evolving separately from the ancestors of apes, between about 10 million and 5 million years ago
• This evolutionary split marks the beginning of the development of hominids
• Hominids are members of the scientific family made up of human beings and early humanlike ancestors
• Most anthropologists believe the first hominids were humanlike creatures called australopithecines
• The australopithecines first appeared more than 4 million years ago in Africa
• Types of Australopithecines

  • A. ramidus
  • A. afarensis
  • A. africanus
  • A. robusius
  • A. boisei
• The most complete australopithecine fossil scientists have found is a partial skeleton of a female A. afarensis, nicknamed "Lucy"
• Lucy was probably more than 110 centimetres tall and weighed about 30 kilograms
• By about 2.5 million years ago, A. africanus replaced A. afarensis