I found this interesting video on YouTube that illustrates how evolution works:
"Evolution is the process of change in all forms of life over generations, and evolutionary biology is the study of how evolution occurs. An organism inherits features (called traits) from its parents through genes. Changes (called mutations) in these genes can produce a new trait in the offspring of an organism. If a new trait makes these offspring better suited to their environment, they will be more successful at surviving and reproducing. This process is called natural selection, and it causes useful traits to become more common. Over many generations, a population can acquire so many new traits that it becomes a new species."