"" Movements of populations have been studied in the past by
comparing the cell's working parts, the proteins whose structure
is coded in DNA. But a coding region of DNA cannot change very
much, since mutations will alter the protein it specifies, often
with fatal consequences for the individual. Most of the genome,
however, is composed of noncoding DNA, where mutations make no
difference to the individual since most noncoding DNA has no
evident purpose. Mutations in noncoding DNA are ideal for the
population geneticist, since they accumulate at a fairly regular
rate, yielding the best data as to the diversity and age of
different populations. ""