Natural selection is an important mechanism of evolution. It 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 the 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 eg., 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.