Phylogenetic tree

A phylogenetic tree, or phylogeny, is a diagram showing how a set of species, populations or genes are related through common ancestors. The base, or root, stands for the ancestral lineage, and time runs from the root toward the tips, which represent the living or extinct groups being compared. Each branch point, also called a node, marks a split where one lineage gave rise to two or more descendant lineages. Two groups that arise from the same node are called sister taxa.

Trees are built from shared inherited characteristics, today most often DNA or protein sequences, using methods such as maximum parsimony, which prefers the tree that needs the fewest evolutionary changes. Because no one can watch the past, every tree is a hypothesis that is revised as new data appear. A common ancestor together with all its descendants forms a clade, and clades nest inside larger clades.

Reading trees takes care. Rotating branches around a node changes nothing, and groups drawn side by side are not necessarily close relatives. Unless a scale is given, branch lengths usually show only the order of splits, not elapsed time.