Mutations are the grist of evolution, and have accumulated in our DNA over time. When populations separate, each group accumulates their own unique set of DNA mutations.
DNA found in the mitochondrion of a cell differs in structure and is separate from the DNA found in the cell nucleus. Mitochondrial DNA, or mtDNA, exists as a circular loop of double-stranded DNA rather than the linear form found in nuclear DNA. However,
Unlike nuclear DNA, mtDNA is  inherited solely from the mother. (Mitochondria are predominantly from the motherâs egg and not from the fatherâs sperm.) Therefore, mtDNA sequences or orders of nucleotides generally remain constant over many generation
Seymour Benzer was able to map mutations within one phage gene and correlate the mutations with protein changes. This showed that the DNA sequence of a gene was aligned with protein sequence.