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The allele frequency in a population can be affected in the following ways -:
i. Genetic migration or Genetic drift. - When a section of population migrates to some other region then there is change in the gene frequencies and new allele or gene are added to the population. 
ii. Mutation Mutation leads to the formation of new species and hence change in allelic frequencies.
iii. Natural selection - is the process in which heritable variations that enable better survival are chosen and the organism is enable to produce and leave more number of progeny.