Transcript: Natural selection operates at the level of species interacting with their environment. At the microscopic level, DNA copies itself, a mechanism that is generally extremely efficient and effective, but its not perfect. The human cell copies the entire information in the human genome, three billion bases, in only a few hours. The error rate is tiny, one in a billion. However errors do occur, errors in transcription, alterations caused by ultraviolet radiation, and even the effects of dangerous chemicals that we call carcinogens. All these lead to mutations or changes in the DNA that are then propagated to successive generations. Some mutations are utterly harmless. Others can be fatal to the organism, for example, when they prevent a vital protein from being created. However some mutations will be beneficial, and when those are passed on, through the operation of natural selection we see that mutation is an essential part of the biological process.