Dissimilarities and similarities among different groups of organisms are the result evolution. This change occurs in the characteristics of different biological populations with the passage of time. The four forces of evolution result in speciation. Speciation is the process in evolution through which new species arise. It could result from any of the four forces of evolution. These are genetic drift, mutation, gene flow, and selection. They lead to slight changes in existing species, as well as the formation of new species.
This is the main source of evolutionary change. It occurs randomly, when mistakes take place in copying