Darwin proposed natural selection as a mechanism for evolution

The process humans use to change a species by selectively breeding for specific traits is called artificial selection. Darwin noticed a lot of variation in domestic plants and animals. 

Watch the Crash Course video, Natural Selection

After watching the video, read these three pages on the Berkley, Evolution 101 site: 

The four main principles of the theory of natural selection are:

It is also important to remember that natural selection can act only on traits that already exist.