Why do so many of you guys seem incapable of grasping the concept of evolution or natural selection? Seriously it's not that hard to understand.
> A gene exists that codes for a specific trait.
> A pool of alleles exist for said gene in a population, thus giving rise to variations of that trait within said population.
> Individuals that have a successful version of that trait for the environment that they inhabit will be more reproductively fit and thus they will have more offspring, thus their offspring will increase the ratio of their alleles in the gene pool of the population. On the other hand, non-reproductively fit alleles will reduce in number since they have less offspring until they are either selected for or go extinct.
> Random mutations, random rearrangements, etc... help create different versions of alleles, or genes, to be selected for or against in the population.
It's this general process that has driven almost all of evolution that has taken place in the world.