to randomly change a setting in the hard drive of a computer, the result would be harmful rather than helpful. Every once in a while, however, a mutation gives an organism a certain advantage over other individuals For example, webbed toes allow for faster swimming. Similarly, thicker coats allow for being able to live in cold temperatures. These changes help an organism live. The trait is then passed on to its offspring. These offspring are more likely to survive and produce other offspring that will also live. Thus, through mutations, a species can develop characteristics that allow it to live in different environments Scientists believe that most mutations are caused by mistakes in the DNA replication during reproduction or development. Mutations can also be a result of exposure to things in the environment. These are called mutagens. Some mutagens occur naturally. Ultraviolet radiation from the sun, for example, can damage DNA. It does so by changing the chemical structure of nucleotides. Many organisms have evolved ways to repair the damage by removing and replacing altered nucleotides. Too much exposure to ultraviolet radiation, however, is one of the leading causes of cancer. This is particularly true of skin cancer in humans other forms of radiations include cosmic rays, x-rays, and the emissions eo of radioactive materials. They also produce mutagenic effects.