Natural Selection is one of the main concepts found within the theory of evolution. It was discovered by Charles Darwin and Alfred Russel Wallace though Darwin championed the idea in his book "On the Origin of Species".
Natural selection can be defined as the process by which random evolutionary changes are selected for by nature in a consistent, orderly, non-random way.
When coupled with descent with modification, Natural Selection can cause a population to evolve for fitness within a given environment over multiple generations.
Natural Selection is an observable fact. By carefully observing populations of living things with short life cycles you can actually watch it happen.
Want to learn more? Check out our notes for this video. Included are links to three examples of natural selection witnessed by researchers. There are many more as well.