Natural selection states that the organisms that are able to survive and reproduce with the changing environmental conditions are selected by nature while the ones that cannot survive are eliminated.
The genes responsible for the survival of an organism are passed on to successive generations.
First, most characteristics of organisms are inherited, or passed from parent to offspring, although how traits were inherited was unknown.
Second, more offspring are produced than are able to survive. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers.
For example, it is believed that the giraffes had shorter necks. Their feeding habits led to the elongation of their necks and the genes from such animals were transferred to the future generations.
It took millions of years for the evolution of the giraffe we see today.
Natural selection leads to greater adaptation of the population to its local environment; it is the only mechanism known for adaptive evolution.
Darwin put forward the concept of evolution during his journey to the Galapagos Islands.
He noted that all living species change both their physical and anatomical structure over a long period of time for better adaptations to the developing environment.
The difference is by natural process and the species which do not get adjust will find it difficult to survive.
This was the proposed concept of natural selection and Darwin called it ‘Survival of the fittest'.