How might alterations in DNA structure be harmful to…

How might alterations in DNA structure be harmful to a species? How might such alterations be beneficial? What type of genetic change is most important for evolution?

Additional Requirements 

Level of Detail: Only answer needed 
Other Requirements: Lots of environment factors, such as ultraviolet light from the Sun, chemical pollutants, X-rays, cosmic rays from outer space, may produce an error in a gene. Such errors are called mutations. Just as in misspelling a word in a paragraph, may change the meaning of the paragraph, inserting the wrong amino acid at a certain spot in a protein may change the function of that protein. However, certain mutations must have been beneficial in the past, or most organisms would not be as well adapted as they are to their present environments.