I have included a link below that will hopefully give you more details, so you can take my explanation and go and investigate further for more specifics.
Darwin's Theory of Evolution was based in part on Darwin's voyage to the Galapogas Islands. What is so amazing about these islands is that there are a number of islands which have obviously split off from one another over the years, resulting in species being left in very different environments. When Darwin went there, his study of the way that these species of animals and birds, which started off the same, but slowly evolved over the years to match their unique environments, gave him proof that animals and birds evolve as species to match their environments. I think the species that he focused on most was a kind of bird. Depending on where the bird was, it evolved differently over the years to help it survive. One bird on one island needed to develop a stronger beak to be able to feed, and so it did. Another bird needed to be smaller and quicker to escape enemies, and sure enough it did.
This of course develops Darwin's approach of the survival of the fittest, where he talks of how it is only the creatues that are best adapted to suit their environment that will survive. The gene pool gives an unlimited number of possibile variations, but only those that live (determined by the environment) will pass on their genes, ensuring that species evolve over the course of time to suit their environment.