mutation
Mutations are permanent, transmissible changes to the genetic material (usually DNA or RNA) of an organism. Mutations can be caused by copying errors in the genetic material during cell division and by exposure to radiation, chemicals, or viruses. Mutations often lead to the malfunction or death of a cell and can cause cancer in higher organisms. Mutations are considered the driving force of evolution, where less favorable mutations are removed by natural selection, but favorable ones tend to accumulate. Neutral mutations do not affect the organism and can accumulate over time, which might result in what is known as Punctuated Equilibrium; a modern variation on classic evolutionary theory.