A phylogenetic tree, also known as a tree of life or simply a phylogeny, describes branching relationships among species, showing which species shares its most recent common ancestor with which other species.

A phylogeny implicitly has a time axis, and time usually goes up the page. Phylogenetic relations have to be inferred using homologies because the splitting events and common ancestors existed in the past and cannot be directly observed.

There are two methods of phylogenetic inference:

1. Parsimony. Species are arranged in a phylogeny such that the smallest number of evolutionary changes is required.

2. Distance (or similarity.) Species are arranged in a phylogeny such that each species is grouped with the other species that it shares the most characters with.

Figure: a phylogenetic tree of the main vertebrate groups. Lizards and snakes share a more common ancestor than other species and so are grouped together.