Ironneck your example of the giraffe was completely wrong, that is not how evolution works.
suckmymuscle, take into account that many mutations are neutral, they have no effect on survivability or reproduction. Neutral, and even harmful mutations, can piggyback ride on the success of other mutations. Mutations occur in each en every generation of each and every plant/animal and fungus, meaning a gazillion-trillion-billion-shitload mutations each year.
A mutation is beneficial when it increases reproduction chances directly (example, having stronger/more sperm) or indirectly (being stronger than other rivals, so able to be top dog and getting more 'tail'. By having greater survivability, chances of reproduction are increased also; according to Darwin "dead animals don't reproduce too well".
We can glean a number of basic examples form this:
-bird that lays more eggs than others of its species, reproductive advantage, this bird's dna is spread wider through the population.
-alpha-gorilla stronger than other gorillas, more intercourse, more siblings.
-lizard is better able to withstand the severe conditions of it's habitat, he lives longer and manages to spawn more siblings.
Take into account that there's many characteristics working at once: the stronger alpha male may have bad sperm: despite his increased reproduction chances it may not lead to an increased spread of his dna.
What is beneficial for reproduction does not have to be good for survivability, and vice versa.
As much as benefit, luck is a huge factor. Creationists paint a picture of nature following the ideal path from one-celled organism to homo sapiens. That is a very flawed idea, along the course of history 90% of species if not more died out. Over-specialization, bad luck or pushed off by competition, most of nature's mutations find a very early grave. What is left is a group of mutations that is proven over time, for instance sexual reproduction: it significantly increases mutations in dna, increasing adaptiveness and ultimately survivability and reproduction chances.