The DNA copying mechanism, also called the DNA replication processes, are highly precise, but they are not completely accurate. Each time when the DNA is copied, some changes or mistakes might take place in formation of the new DNA strand. These mistakes account for the variations that occur amongst the individuals in a population. In sexual reproduction, these changes increase in rate of occurrence due to involvement of two individuals with different patterns of accumulation of variations. The variations hence created, are unique in nature. Thus, the DNA copies generated are similar, but not identical to the original.