How does natural selection and evolution change species?

Mutations are believed by many evolutionists to provide a means for variablity in some of the characteristics of a species. For example, extra fingers and toes. However, since all mutations are DNA-destructive and result in a loss of information, increasing complexity from mutations is believed by many to be impossible. This is especially true if one subscribes to Information Theory, which states that information only comes from greater pre-existing information. Extra fingers only demonstrate an error in creating the correct number of digits, not additional information.

If a certain variation provides an advantage for an individual (or inter-breeding group) they these individuals will survive and breed where all others will die. This is natural selection (not evolution). The offspring of the survivors will all carry the traits of their parents. Therefore, the fittest will tend to thrive, but not change into a different species, at least not through this mechanism.