Fitness is based on certain characteristics which are inherited and the ability to adapt to the changing environment. It is the end result of adaptation because a fit individual survives and unfit individuals are eliminated from the population.

Individuals continuously compete with each other in a population for food, space and light. The one which is better adapted and naturally selected by nature survives and reproduces. 

For example, industrial evidence : It is a case of natural selection. In England, it was observed before industrialization that white-winged moth were more than dark-winged moths. But the situation became reversed after industrialization. it was found that predators would spot and pick a moth against a contrasting background. During pre-industrialization, the tree trunks were covered with white lichens and on white-background dark coloured moth could be picked up. 

During post industrialization, the tree trunks were covered by dark, dust, coal particles and became dark, on which white moth could easily be picked up. Thus, it was found that industrial melanism supports evolution by natural selection.