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Branching Tree Diagram: Definition & Examples

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

There are different ways to describe how organisms are related to each other. One way is to explore evolutionary relationships with a branching tree diagram. In this lesson you'll learn about these diagrams, and even learn how to build one of your own.

Organizing Life

If you were asked to create an organizational scheme of your family members, how would you go about doing it? When you stop and think about it, there are several ways to accomplish this. You could outline how relatives are related to each other, or maybe you would sort them based on where they lived. You might even organize them based on characteristics that they have in common, such as hair color, eye color, and height.

Just like with your family, we can organize the organisms on Earth in different ways too. One common way is to use a branching tree diagram, which groups organisms together based on shared derived characteristics. These are characteristics that are shared by all organisms in that group, and these homologous, or similar ('homo' = 'same') structures help us understand the evolutionary relationships between those organisms.

Homologous structures, such as these various arms, are similar even though they belong to different animals
homologous structures

Evolutionary Characteristics

Before we dive into how a branching tree diagram works, let's take a moment to better understand the components of the diagram. Each group that branches off is called a clade. Each clade has those shared derived characteristics. The 'derived' part means that the characteristic evolved over time and is present now, but wasn't present in a past ancestor.

Homologous structures can be subtle, but once you see them clearly they're easier to understand. Take, for example, your arm. Your arm has a bone structure that is very similar to the arm of a dog or cat, with two long bones, a joint in the middle, and a 'hand' with digits at the end. But now take a look at the arm of a bat, which also has two long bones, a joint, and even long fingers that extend outward to create the wing. The bone structure of all of these is very similar, despite being in animals that look very different on the outside. It's these shared, homologous structures that help us decide where to place organisms along a branching tree diagram.

Branching Tree Structure

Now that you've got the pieces for your tree, let's take a look at how you would go about putting it together. The structure of a branching tree diagram is just what is sounds like - a set of branches that break off to represent different shared characteristics. A branching tree diagram is a set of groups within groups, with the organisms at the bottom having the fewest shared characteristics and the ones at the top having the most.

A simple branching tree diagram
 branching tree diagram

For example, if you wanted to construct a simple branching tree diagram of the vertebrates, you would start at the bottom with simple chordates, which don't have a backbone but instead have a dorsal, hollow nerve cord. The next branch occurs with craniates, which are chordates with heads. The next branch after this is jawed vertebrates, followed by a branch for lungs, one for fins, and one for lobed appendages. After this you have a branch for tetrapods, or animals that can walk on all four legs. And finally, we end our tree with amniotes, or animals that produce eggs (including humans!).

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