Mutation
From RationalWiki
In biological evolution, mutation refers to a change in the sequence of base pairs in a chromosome. Such changes are usually random, caused by copying errors or some external cause such as radiation or chemical action. They are "random" in the sense that the changes are not preferentially distributed with respect to the result - they are not "goal-oriented".
The majority of these changes have no effect on fitness (that is, they are "neutral mutations") - examples of neutral mutations would be those which change between synonyms in the genetic code (sequences which produce the same protein), which affect non-coding regions of the chromosome (see gene expression), or which result in inconsequential changes (such as blood type or eye color in humans). Of the minority which are not neutral, most are harmful and a few are beneficial. Whether the mutation is harmful or beneficial (and to what degree) may depend upon what circumstances the resulting organism happens to live in (a standard example is the loss of sight being beneficial for a cave-dweller living where there is no light).
[edit] Creationists on mutations
It is an oft-quoted position of creationists that mutations can only be harmful, or at best, neutral. Evidence of useful mutations is studiously ignored by them.
Peppered moths anyone? Nylon eating bacteria, anyone? Antibiotic-resistant bacteria, anyone?
