Principle #1: Genetic Variation In the giraffe population of southern Africa, adult giraffes necks differ from each other. They all have long necks, but some necks are shorter than others while some are longer. There is a reasonable amount of both of these giraffes.
Principle #2: Overproduction and The Struggle for Existence Giraffes usually have one baby, in some rare cases two. But the struggle would be the giraffes with shorter necks not being able to reach the trees, or get enough to eat from them. This would lead to starvation for those giraffes.
Principle #3: Differential Survival and Reproduction The giraffes with longer necks are more likely to eat from the trees with no problem, and get enough food for themselves. This makes them have an advantage over the ones with shorter necks, and have a better chance at reproducing since they survive the longest.
Principle #4: Changes in The Gene Pool As generations go by, there will be an increase of giraffes with necks long enough that can easily reach to trees, thus benefiting the species. The amount of ones with smaller necks will decrease as time goes on.