• Charles Darwin wrote his famous book “Origin of Species.
  • He threw new insights on the evolution of species.

Some salient points of Darwin’s theory are as follows:

  • Organisms have unlimited capacity to reproduce
  • Organisms can reproduce offspring at a fast pace.
  • This is necessary for survival, because a higher number of offspring ensures that at least some of them could survive.
  • Each organism has to struggle for its day to day survival.
  • For example; a frog lays thousands of eggs at one go.
  • The spawn is released in water and it is left to fend for itself.
  • Most of the eggs are either washed away or are eaten by predators.
  • However, Some portions of eggs from the spawn develop into tadpoles.
  • Once again, many tadpoles are eaten by predators; leaving a few. which develop into adults.

Natural Selection:

  • Different individuals of a particular species have different traits.
  • Those with more suitable traits are selected by nature.
  • Each organism needs a particular trait for finding food and finding a mate.
  • Those with better traits are finally able to pass on their traits to the next generation.

Survival of the Fittest:

  • Those organisms which are the fittest are able to survive, while others perish.
  • That is how many species become extinct and some Species continue to evolve over a period of time.


  • The modern system of classification is based on evolutionary relationships.
  • Due to this, this is also known as phylogenetic classification.
  • The kingdom is the highest taxa, while the species is the lowest taxa.
  • Members of a species have a higher number of common characters, than members of a kingdom.
  • For example; all human beings belong to the species Homo sapiens.
  • Human beings can interbreed; irrespective of their race or skin colour.
  • All human beings come under the class mammalia; as do the monkeys, elephants and cows.
  • Apparently, each species of the class mammalian is quite different yet they have certain common characters; like hairs on the body and mammary glands in females.
  • Similarly, all animals are eukaryotes and the cell wall is absent in their cells.
  • The degree of similarity or dissimilarity shows that all animals have evolved from a common ancestor.

Some of the important sources which provide evidences for evolution are :


  • Organs which have common design but serve different functions in different animals are called homologous organs.
  • For example; the forelimbs of all tetrapods are composed of humerus, radio-ulna, tarsals and metatarsals.
  • Yet, the forelimbs of frogs are adapted to a jumping movement, those of birds are used for flying and those of humans are used for handling tools.
  • This shows that frogs, birds and humans have evolved from a common ancestor.


  • Organs which have different designs but serve a common function in different animals are called analogous organs.
  • Wings of birds and wings of bats are good examples of a pair of analogous organs.
  • Wings of birds are composed of all the bones of the forelimb and are covered with feathers.
  • Wings of bats are mainly composed of the digital bones and a thin membrane covering the structure.
  • Yet wings in both the organisms are used for flying.

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