Natural selection is a mechanism of evolution. Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success. This process causes species to change and diverge over time. For example, evolving long necks have enabled giraffes to feed on leaves that others can't reach, giving them a competitive advantage. Thanks to a better food source, those with longer necks were able to survive to reproduce and so pass on the characteristic to the succeeding generation. Those with shorter necks and access to less food would be less likely to survive to pass on their genes.

Source: National History Museum