Mutation is when a change happens during replication of DNA. It is when the copy of the original DNA is not replicated completely correctly. There are multiple types of mutation including substitution, deletion, duplication, insertion, nonsense mutations, missense mutation, frameshift, and repeat expansion.
Another force of evolution, gene flow, also called migration is exactly that. When an individual moves or any of their genetic material they carry move to a different population. For example, if a bird flies to a different island where there is a different type of bird population that would be gene flow.
Natural selection