Explain four observable facts on which "Theory of Natural Selection" is based.

Charles Darwin's theory of Natural Selection is based on certain observable facts which are as follows:

1. Overproduction — Overproduction is a driving force in natural selection, as it can lead to adaptation and variations in a species. Darwin argued that all species overproduce, since they have more offspring than can realistically reach reproductive age, based on the resources available. All organisms have the capacity to reproduce at a very high rate. However, organisms cannot survive by reproduction alone. Due to lack of food and space, offspring soon begin to die. Some are eaten by predators, while some get destroyed due to adverse environmental conditions.
2. Struggle for existence — According to Darwin, individuals multiply in a geometric ratio whereas space and food remain almost constant. Overproduction of organisms results in a struggle for existence among organisms. The struggle is to obtain food, space and mate.
3. Variation — Progeny of the same parents are not exactly alike. Such differences are known as variations. The variations may be harmful or advantageous.
4. Survival of fittest — In the struggle for existence, organisms that develop new favourable characteristics will survive in the long run. This idea is called 'Survival of the fittest'. Organisms which survive will transmit favourable characters to their offspring. These characters get accumulated and give rise to new species.