Factors in evolution

How do competition and variation play a role in natural selection and can you give an example?

competetive- is having genes that give animals certain traits that make them stronger and more like to mate. therefore they will outcompete the weaker once and lead to evolution.
variation if there is nor variation natural selection cant happen because there is not much variety to select from. therefore variation gives certain memebers a species some genes that make them stronger and more likely to mate and therefore they lead to evolution.... however it is important that there is always some variation present otherwise it can lead to serious heridity problems and extinction of species.

Competition is a kind of struggle. It may be passive or active. In any case, there is a selection pressure, to select those that are better equipped.Those that are selected are better in terms of genetic endowment, which means favorable variation(mutation). If such mutations keep on being selected, there will come a time when something in the gene pool will occur to keep it protected by making a reproductive barrier.This becomes the turning point of the creation or splitting of one species into two.

it has all the stuff u wanted.
http://wiki.answers.com/Q/What_are_five_factors_that_influence_evolution