Reading Phylogenetic Trees and Cladograms

A phylogenetic tree is built from nodes and branches. Each node is a most recent common ancestor, the point where one lineage split into two or more. Two species are close relatives if they share a node close to the tips, meaning less time and fewer splits separate them from their last shared ancestor. Species sitting at neighboring tips on the page are not automatically close relatives: what counts is which node connects them, not how near their branches happen to be drawn. A cladogram shows this same branching pattern, the topology, without claiming anything about the amount of time or the amount of genetic change along each branch. A full phylogenetic tree often adds that information, using branch length to represent time elapsed or number of mutations.

Because only the branching pattern carries real information, you can rotate any branch around a node, swapping which lineage is drawn on the left and which is on the right, without changing a single relationship. This is why the species sitting at the far right of a tree is not the most evolved: every tip, no matter where it sits, has been evolving for exactly the same span of time since the root. Left-to-right position is just a drawing choice.

Trees are hypotheses, not fixed records. When new molecular data (new DNA or protein sequence comparisons) conflicts with a tree built from older morphological evidence, biologists redraw the topology to fit the strongest current evidence. This links directly to speciation: each node is itself a speciation event, so reading a tree correctly is how you trace the order in which lineages split apart over time.