The environment cannot support unlimited growth of any one species of animal, not every living organism get to reproduce to their full potential. For example, there are some green beetles and some brown beetles, the green beetles get eaten by birds and/or reptiles etc. Which then allows the brown beetles to survive and breed with the other brown beetles, and their offspring will then be brown due to genes and genetic breeding. This is a reoccurring act, which eventually kills off all of the green beetles and leaves the brown beetles alive greatly increasing the population of the brown beetles. Another example, there are a group of rats, some rats bigger than others, the bigger rats are usually slower, easier to see and obviously have more meat. So predators such as foxes and various bird of prey species would kill them and eat them, leaving only the smaller rats. Due to the fall in the bigger rats the smaller rats will then breed and because size is a genetic trait.
http://evolution.berkeley.edu/evolibrary/article/evo_25

Darwin's ideas caused a lot of controversy, and this continues today, because they can be seen as conflicting with religious viewsabout the creation of the world and the creatures in it.

The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms. These simple life forms first developed more than 3 billion years ago (the Earth is about 4.5 billion years old). The timeline below shows some of the key events in the evolution of life on Earth, from the first bacteria to the first modern humans.