A phylogenetic tree is a visual representation of the branching patterns of evolution. Through a phylogenetic tree, readers can trace the evolutionary relationships between various organisms.
To understand evolutionary relationships, one important concept to understand is common ancestry. If two lineages have common ancestry, it means that they descended from the same ancestral lineage, and share some percentage of their genes. However, common ancestry does not mean that two lineages are exactly the same. For example, you and your cousins share common ancestry through your grandparents, but you probably don’t look exactly like them.
A monophyletic group is a collection of organisms, including one ancestor organism and all the descendants of that single common ancestor. A monophyletic group can be a single branch of a tree or a collection of connected branches.
The first image to the right represents a monophyletic group because it includes different species and their common ancestor. Every species in the group highlighted in green is related to each other in some way.
The second image to the left does not represent a monophyletic group because it excludes the common ancestor of all the species highlighted in orange. Although the species highlighted are all related to each other, you can't tell by looking at only the highlighted groups because their common ancestor is excluded.
Relatedness refers to how closely two different species are connected by a common lineage. Two species are more closely related if they have a more recent common ancestor. To identify how closely related two species are, start at the branch tips that hold the two species of interest, then trace the branches of the tree backward until the two branches converge. This point of convergence represents the two species’ most recent common ancestor.
Phylogenetic trees can be presented in a lot of different ways. Although the format may look different, remember that it’s the branches that really matter. All of these trees communicate the same information!
Rotating a tree’s structure doesn’t change the relationship of species in the tree. Different trees may present the structure of the tree in different ways, but the relationships between species remain the same. In this tree, species 3 and 4 are the most closely related species in the tree, no matter how the branches of the tree are rotated.
Sometimes, when scientists don’t have enough information to determine the branching order, they end up with three or more species coming off the same branch point. These branch points are called polytomies. Later, if scientists learn some new information, they may be able to resolve these polytomies.
