"Evolutionary change is change in the genetic constitution of a population. This type of change can come about as a result of a number of factors. Immigration and emigration of organisms will bring new alleles and new allelic combinations into the population and will cause others to disappear. Mutation will lead to the formation of new alleles. The major claim of a Darwinian theory of evolution is that the principal factor of change is natural selection: The most important evolutionary changes come about because some allelic pairs are fitter than others, and these obtain greater representation for their constituents alleles in subsequent generations."