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:
- variation - heritable differences
- overproduction - having many offspring raises the chances that some will survive, but it also leads to competition between offspring for resources
- adaptation - a variation can allow an individual to survive longer and produce offspring
- descent with modification - happens over time!
It is also important to remember that natural selection can act only on traits that already exist.