Natural selection can acts on the basis of two different modes, positive purifying which included the process of eliminating a damaging allele, the second mode is balancing selection, which comprise of heterozygote advantage and frequency dependent selection. Each of the selection modes is a response to the environment or intrinsic selective pressure and operates to shift allele frequencies and leaves a signature on variation in the genome. Positive selection and balancing selection act in contrast manner, for instance positive selection decreases the genetic variation by favoring the advantageous or adaptive alleles while the balancing selection maintains tends to maintain polymorphism whereby overdominance selection favors heterozygotes and
