It is a field of biology that studies genetic composition of biological population and the changes in the genetic composition that result from the operation of various factors, including natural selection. For a population of individuals to succeed over evolutionary time, it must contain genetic variability. Evolutionary genetics is the broad field of studies that resulted from the integration of genetics and Darwinian evolution, called the ‘modern synthesis’. The force of mutation is the ultimate source of new genetic variation within populations. Although most mutations are neutral with no effect on fitness or harmful, some mutations have a small, positive effect on fitness and these variants are the raw materials for gradualistic adaptive evolution. Within finite populations, random genetic drift and natural selection affect the mutational variation.