natural selection.

Natural selection is a normal process resulting in the evolution of organisms best adapted to the environment. The relation between Natural Selection and Mutations are very simple. Let's say a Giraffe has a small neck and his food is on a tree. It will cause him to keep reaching as far as he can, if the giraffe and its future generations continue to do this process, their necks will start to extend due to evolution which can affect the phenotypes of the Giraffes which will change the genotypes of the future generations so they will be able to have long necks.

 
Expatation is the shift in a function of a trait during evolution. Like, a trait can evolve because of one function, but subsequently it may come to serve another.

references:  www.unknownworlds.com/ns/
evolution.berkeley.edu/evolibrary/article/evo_25
en.wikipedia.org/wiki/Exaptation