Does a longer branch mean a species has been around longer?

Not necessarily. Unless the tree has a specific time scale drawn on the side, branch lengths just show the pattern of splitting. All living species at the tips of the tree have been evolving for the exact same amount of time since their common ancestor.

Are humans at the top of the phylogenetic tree?

No. In biology, there is no 'top' or 'goal' of evolution. A phylogenetic tree can be drawn with humans on the far left, the far right, or in the middle. All species currently at the tips of the tree are equally 'modern' and have successfully adapted to their environments.

An outgroup is a species or group of species that is closely related to the group you are studying, but branched off earlier. Tutors and scientists use outgroups as a reference point to figure out where the root of the tree should go and which traits are older.

How do you read a phylogenetic tree?

A phylogenetic tree is a diagram that shows the evolutionary relationships among different species. Think of it like a family tree, but instead of tracing grandparents and cousins, it traces the history of life and how species share common ancestors. <br><br> To read one, you do not look at the order of the species across the top. Instead, you trace the branches downward to find where they connect. The closer two species connect at a shared intersection, the more closely related they are.

The Anatomy of the Tree

A phylogenetic tree is made of roots, branches, and nodes. The 'root' at the base represents the oldest common ancestor of all the organisms on the tree. The 'branches' represent lineages of species as they evolve over time. The 'nodes' are the intersection points where one branch splits into two. Each node represents a most recent common ancestor that the branching lineages share.

Clades and Sister Taxa

When two branches pop out of the exact same node, those species are called 'sister taxa.' They are each other's closest relatives. A 'clade' is a grouping that includes a single common ancestor and all of its descendants. An easy way to spot a clade is the 'snip test': if you could cut one branch of the tree and a whole section falls off together, that section is a valid clade.

Where Students Slip Up

A very common mistake is reading the tips of the tree from left to right to decide who is more advanced or closely related. The order at the tips does not matter because branches can rotate at the nodes, much like a mobile hanging over a crib. The only thing that determines relatedness is how far back you have to trace the branches to find a shared node.

Worked through

Imagine a tree with three species at the tips: A, B, and C. Species A and B connect at a node near the top. To reach species C, you have to trace the line from the A/B node further down to a lower node where C's branch connects. Is species A more closely related to species B or species C?

Species A is more closely related to species B. If you trace the branches from A and B, they intersect at a node very quickly. This node represents their most recent common ancestor. To connect A to C, you have to trace much further down the tree to an older, deeper node. Because A and B share a more recent common ancestor with each other than either does with C, they are more closely related.

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Where this comes from: Campbell Biology, 12th Edition · OpenStax Biology 2e, Chapter 20: Phylogenies and the History of Life · Khan Academy: Phylogenetic trees