Tracing evolutionary relationships between organisms is called as phylogeny while studying structures of various DNAs to establish evolutionary relationships between them is called as, while studying structures of various DNAs to establish evolutionary relationships between them is called as molecular phylogeny.
The sequence of base pairs in a DNA molecule decides the characteristics of an organism. A slight change in this sequence can bring very large changes in certain organisms.
For e.g. - DNA of Homo sapiens (i.e. humans) are 99.9% same. That is, only a 0.1 % difference in humans' DNA can create so many variations. Similarly, population of different unrelated species is very different in their molecular organization too.
DNA of humans has some similarity with DNA of apes, and due to this reason, it is assumed that they might have some common ancestors.
