We have actually seen, with our own eyes, the emergence of new biological species from pre-existing species. If I were you, Edgar, I'd do some research before posting here. But the result of their findings about the species must not be evolution but interrelation alone. Interrelated Species is a direct explanation of two species that are similar in many different aspects. Like apes and humans are interrelated.
Evolution explains WHY apes and humans look similar.
It gives a specific mechanism- inheritance of common traits from a common ancestor. When we say natural, we mean to say that it is obvious and can be seen in our everyday life. Einstein's theory of relativity is correct, but relativity isn't at all obvious. Also, what is obvious to you may not be obvious to me. We have actually seen, with our own eyes, the emergence of new biological species from pre-existing species.