Relatedness of organisms

• The theory of evolution suggests that widely separated groups of organisms share a common ancestor. Therefore, it would be expected that they share basic features, so their similarities should show how closely related they are. 
• The more similar two organisms are, the more recently they are assumed to have diverged. 

Assessing relatedness with common features 

• Biologists look for homologous structures. These may have different functions, but a similar form and developmental origin. 
• E.g. the pentadactyl limb in amphibians, reptiles, birds and mammals. 
• The structure is the same but the uses are different: grasping, walking, swimming and flying.
• Provides an example…