Reading a Tree

Before I first started working at CMU, I could have told you what a family tree was and could trace the genealogy of my family back, seeing how distantly related I was to past relatives in the process. But a phylogeny?? What’s that? What information could possibly come from a plant systematics tree with no information on long-dead ancestors present on its branches?

Well, those ancestors are the links that show us how present day species are evolutionary connected. We don’t need to be told what color their flowers were, or how their leaves look, because we can make a pretty good guess based on what their lineages evolved into. By tracing character traits of morphology or genetics through a tree, we can make a decent hypothesis as to how a group of organisms is related to each other and apply this information in taxonomical and population studies.

But how do you read a tree if all you have are the tips of the branches? It all comes down to monophyletic groups, or clades (there’s a jeopardy word for you). Below you can see a pretty simple tree, showing the evolution through time to the organisms at present day, numbers 1-5. Each horizontal line is a lineage, evolving from a common ancestor at the “node”. We call the organisms that have evolved from an ancestor descendants.

As you can see from the diagram, there are two groups displayed, the ingroup and the outgroup. The ingroup is all the species you have chosen and that you want to understand relationships among. All the ingroup species are more closely related to each other than they are to the outgroup, which serves to root the tree. The outgroup should have enough similarities to help define relationships among ingroup taxa, while at the same time helping show where the ingroup falls on a bigger biodiversity tree.

A group made of ALL the descendants from a common ancestor is called a monophyletic group. This is similar to looking at a family of two parents with three children, and grouping all the children together. A group made of SOME BUT NOT ALL of the descendants from a common ancestor is known as a paraphyletic group. This is a little like looking at a family with five children, but grouping males and females separate, and disregarding all the males. This would not give a very accurate representation of the current generation of the lineages present from the parents, would it?

Monophyletic groups of organisms are those most closely related to each other, and aid in describing and organizing species. Not only that, but our herbarium is arranged according to the evolution of plants, so understanding a phylogeny of plants can help everyone find what they are looking for in our shelves!