During the course of an organism's life, changes to its underlying genetic code will be introduced by a variety of processes. Many of these changes are harmless and are the underlying current driving a species' evolution. Rarely, variation within the genome or combinations thereof will cause adverse phenotypes in individuals that carry the variation(s) [e.g. cancer]. The replicate-and-branch nature of the cell lifecycle means that genetic code within different parts of one's body evolve separately and slowly diverge from one another. This phenomenon can be observed more starkly when considering the difference in genetic code from humans separated by large geographic regions.