Phylogenetic tree, a diagram showing the evolutionary interrelations of a group of organisms derived from a common ancestral form. The ancestor is in the tree “trunk”; organisms that have arisen from it are placed at the ends of tree “branches.” The distance of one group from the other groups indicates the degree of relationship; i.e., closely related groups are located on branches close to one another. Phylogenetic trees, although speculative, provide a convenient method for studying phylogenetic relationships.