Well, first, let me ask you these questions:
Question One: Heredity exists, right? (this is a rhetorical question)
Question Two: Natural selection (survival of the fittest) occurs, right?
Question Three: Mutations happen, right?
Question Four: If natural selection occurs, would it not weed out the harmful mutations if mutations happen?
Question Five: If harmful mutations are weeded out, this would leave only harmless and even helpful mutations, right?
Question Six: If heredity exists, would it not allow these helpful mutations to be inherited?
Question Seven: If these helpful mutations are inherited, would it not be possible that they'd change the appearance of a species over time, perhaps even making a new species?
If you answered all of these questions "yes", that's evolution for you, and scientists HAVE observed it in nature.