• If different organisms have common ancestors then it is possible that they have similar characteristics due to inheritance. These similarities are used to determine their close relations in evolutionary terms. For example – forelimbs of bird and forelimb of humans have similar basic structure of bones which suggests that they have a common ancestor i.e. they are closely related in evolutionary terms. Over time these structures have developed to function as per the requirement. Thus homologous organs help in determining closeness in evolution.

• During evolution, the changes that take place in DNA of species are accumulated over time. If these changes are very less in two species then those 2 species are said to be closely related. On the other hand, if these changes in DNA of 2 species are large, then the species are said not to be closely related in evolutionary terms.