Natural selection is the concept that animals more suitable for their environment are more likely to survive. This will cause them to be more likely to reproduce and their offspring have that same ability, thereby changing the entire species by eventually killing off all the animals that do not have the necessary adaptations. Note: this does not come into play when the environmental puts little to no stress on the species. This is just a more exact way of saying survival of the fittest. Environmental factors can be any natural process that makes life more difficult. Ex. Poisonous plants, little water, excessively hot.
Study Darwin. He presumes that all species in order to be successful need to fill an environmental niche. Humans for example probably would not have survived w/o our opposing thumb and reasoning ability. generally as a species we are pretty weak.
I am only adding to what Kyle said (he gave a great answer)....Another part of natural selection comes to play when a species becomes over-populated, regardless of abundance of food and water, and that is disease. I see it where I live....we will have squirrels, way to many of them even though there is plenty of food & water. It seems to come as a cycle....a disease (and I cannot remember the name of it) will take hold and kill off quite a number, bringing things back into balance.