A study of the family relationships between living beings, with a view to understanding their evolution. It is based on the identification of homology, in other words the resemblances between organisms, at a morphological level (the only way of classifying fossil species whose DNA is recently preserved) or a molecular level (DNA, proteins). The results are represented by a phylogenetic tree or a cladogram in which each node corresponds to a common ancestor. Two organisms are all the more related if they share a common ancestor close by in the tree.