What causes changes in DNA?

The major cause of changes in DNA are mutations. They can be random or induced. Mutations can be either harmful or beneficial and the beneficial mutations are those that allow an animal to adapt better. They can occur during replication where a wrong base is added and thus the template is altered or even via a virus that injects its DNA into the host genome. Also, changes can occur during meiosis where the chromosomes do not separate completely (nondisjunction) and the child is born with a abnormal number of chromosome. This could result in physical distortions. Mutations can also occur due to the environment. For example, a pregnant mother that uses drugs can greatly effect the future mental and physical health of the child. The ones with the harmful mutations die out and the ones with the beneficial mutations gain an advantage and continue to live on and reproduce and pass on their characteristics.