Fig. 1
From: Teaching Tree-Thinking to Undergraduate Biology Students
Phylogenetic trees show the relationships of four species (A, B, C, and D). Each tree represents the same relationships of the four species: species C and D are the closest relatives, species B is equally related to both C and D, and species A is the outgroup. a A large arrow indicates the direction of time from the past to the present. The most recent common ancestor (MRCA) of all four species is indicated by a circle and small arrow. An arrow indicates the root of the tree. The tree on the right is a diagonal representation of the evolutionary relationships. b Two clades are indicated on each tree: clade 1 consists of species C and D, and it is nested within clade 2, consisting of species B, C, and D. c A fifth species (Z) has been added to the phylogeny, but the relationships of species A, B, C, and D have not changed. d A circles-within-circles diagram can represent the nested hierarchical relationships of species A, B, C, and D. e The branches of the tree have been rotated around various nodes, but they still depict the same evolutionary relationships