ICSE Class 10 Answered

1) Darwin put forth the Law of Natural Selection involving the struggle for existence and survival of the fittest. The Darwin’s theory is a unifying theory of life science and explains diversity of life.   

Overproduction of offspring

• All organisms have the capacity to reproduce at a very high rate.
• However, the organisms cannot survive by reproduction alone.
• Due to lack of food and space, the offspring soon begin to die.
• Some are eaten up by the predators, while some get destroyed due to adverse environmental conditions.

Struggle for existence

• Overproduction of organisms results in struggle for existence amongst the organisms.
• The struggle is to obtain food, space and mate.   

Natural selection

• Only those organisms who are fit for the changing environment have the right to survive.
• The organisms which are unfit are eliminated and ultimately die.
• The fit species reproduce. The essential variations in the characteristics are passed on from one generation to the next.
• These variations make them fit to survive in the constantly changing environment.  

Survival of the fittest

• In the struggle for existence, the organisms that develop new favourable characteristics will survive in the long run. This idea is called the “Survival of the fittest”.
• The organisms who survive will transmit the favourable characters to their offspring.
• These characters get accumulated and give rise to new species. 

2) The best example is industrial melanism showing the effect of industrial pollution on the moth population on a nearby tree.

Before industrialisation, light-coloured lichen used to grow on trees and moth predators could not spot white moths easily, while dark moths were lesser in the moth population. However, due to industrial pollution, lichens could not grow on trees and it became difficult for predators to spot dark moths on the dark background of the stem bark, and thus, the population of white moths became less than that of dark moths.

This process is termed industrial melanism. Before industrialisation, white moths were better adapted towards nature, but after industrialisation, dark-coloured moths were more fit towards the changed environmental conditions.