Natural selection is the result of the interactions between genetic variations in a population and the environment. The environment determines which variations are more favorable. More favorable traits are thereby passed on to the population as a whole.
Genetic variation occurs mainly through DNA mutation , gene flow (movement of genes from one population to another) and sexual reproduction. Due to the fact that environments are unstable, populations that are genetically variable will be able to adapt to changing situations better than those that do not contain genetic variation.