Principle #1 Genetic Variation In the Giraffe population there are some Giraffe's with long necks and the others have short necks. Both of these mutations can be passed on to offspring and the generations to come.
Principle #2 Overproduction & the Struggle for Existence The Giraffes who have short necks can not reach the leaves on the tree and die from starvation. Giraffes with long necks are able to reach the food and keep producing offspring. In this case the size of a giraffes neck is the selective pressure
Principle #3 Differential Survival & Reproduction Giraffes who are able to reach their food will be able to produce offspring because they are healthy due to the fact that they can eat while the giraffes with the small necks can die from starvation and not increase their population
Principle #4 Changes in Gene Pool As a result of the giraffes with sort necks not being able to reach thier food they can not produce offspring and in a few years only tall neck giraffes population will increase