(i) Natural selection can operate and select most fit organisms from a population only if organisms of that population have variations or show differences.
(ii) Evolution can occur only if natural selection is able to select out most suitable organisms with better survival. Therefore, evolution and natural selection are the results or consequences of those processes that introduce variation in the individuals or the gene pool of the population and isolate the gene pools to conserve the changed gene pool.
These processes include
(a) mutations in the genetic material
(b) changes in the structure or composition of chromosomes
(c) immigration of new individuals to contribute to the gene pool
(d) emigration of individuals of a population
(e) change in the number of chromosomes and finally isolation of individuals to prevent gene exchange.