There is variation naturally present between members of a species. These variations can prove advantagous, thus making an individual in a species better adapted for survival. For example, giraffes with a longer neck have the ability to reach higher up on trees and therefore acquire more food than those with a shorter neck. These advatgous traits are often the result of a genetic mutation, those with these traits are more likely to survive and will therefore pass on their favourable genes to their off spring. This process where by the individuals within a species with the most beneficial genetic traits survive and those without them do not is known as survival of the fittest and was theorised by Charles Darwin.