Class 10 Science


The change in inherited traits in biological population over subsequent generations is called evolution. Scientists have proven that life evolved in the form of simple unicellular organisms on this earth, and all the organisms which are present today have evolved from a common ancestor. The idea of evolution is based on the premise of a common ancestry.

There is a huge variety of living beings around us. But this was not the case when life began on earth. Life began on earth in the form of simple unicellular living beings. With passage of time, living beings changed in order to adapt to changing environment. These changes resulted in increasing degree of complexity in living beings. That is how, complex living beings like mammals and flowering plants evolved from simple unicellular organisms.

To understand how evolution takes place, let us take some imaginary examples.

example of evolution

Situation 1: A group of red ants is living in a bush. As hunting birds can easily spot red ants in the green background, they enjoy feasting on ants. Because of some error in DNA replication, some blue ants come into origin. Hunting birds cannot spot blue ants against a green background. As a result, blue ants survive and red ants become extinct over a period of time. The origin of blue ants happened by chance but it gave survival benefit to the ants. Finally, blue ants could survive and proliferated in the surrounding.

example of evolution

Situation 2: In the same group; some blue ants came into origin. Ants of both colours were almost equal in population. One day, an elephant came and trampled the bushes. All the red ants perished in the accident; leaving only the blue ants. This resulted in extinction of red ants but blue ants could continue their race. The survival of blue ants was because of an accident and the accident was the cause of natural selection.

example of evolution

Situation 3: A group of red ants was living in a bush. Due to draught like conditions, availability of food became a problem for the ants. All the ants became weak and underweight. Subsequent generations comprised of smaller ants and the trend continued for a few generations. Situations changed and plenty of food became available. Ants once again developed to their normal size. In this case, the change in size was a change in phenotype and hence was not inheritable. The change in size could not produce variation and evolution in the species.

Darwin's Theory of Evolution:

Charles Darwin wrote his famous book Origin of Species. He threw new insights on 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 portion of eggs from the spawn develops into tadpoles. Once again, many tadpoles are eaten up by predators; leaving a few which develop into adults. It is evident, that a large number of eggs is needed to ensure that at least some of them develop into adults.

Natural Selection

Different individuals of a particular species have different traits. Those with more suitable traits are selected by the 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.

Molecular Origin of Life:

Stanley L. Miller and Harold C. Urey, conducted the Miller-Urey experiment in 1953 to demonstrate how the life would have originated on the earth. They created an environment in laboratory which mimicked the environment of earth as it was during the time of origin of life. Water, methane, ammonia and hydrogen were used in that experiment. The liquid was heated to initiate evaporation and electrodes were used to create electric discharge. At the end of two weeks, some organic molecules were formed in the setup. Some amino acids and sugar were also formed. This proved the hypothesis of J. B. S. Haldane that life originated from inorganic raw materials.


The process of origin of a new species is called speciation. A species is a group of organisms in which most of the characters are similar and members of a species are able to breed among themselves. Speciation can happen if two groups of the same species are somehow prevented from interbreeding for several generations. This can happen because of geographical segregation or because of some genetic changes.