Monophyletic Groups: Definition & Examples

Instructor: Ashli Wilson

Ashli has a Master's Degree in Biology and has taught biology at different grade levels including college, elementary, and middle school.

This lesson describes monophyletic groups and the characteristics that define monophyletic groups. This lesson details the importance and usefulness of monophyletic groups and how monophyletic groups are visually represented.

Monophyletic Groups

What do you, your siblings, and your cousins all have in common? You all are descendants of your grandparents. Since all of you share common ancestors (your grandparents), you, your siblings, and your cousins are a monophyletic group (also knows as a clade), which is simply defined as a group of organisms that has a shared ancestor. You all share some of the same DNA that is specific to your monophyletic group, and this is known as a synapomorphy, which is simply a shared characteristic. It is weird to think of your family in scientific terms, but this is exactly how we classify organisms and how they are related to each other.

In monophyletic groups, some organisms share more than one common ancestor. In the monophyletic group of you, your siblings, and your cousins, you and your siblings have the same grandparents and parents. You will share some of the same DNA with your siblings that you get from your grandparents in addition to the same DNA you and your siblings share from your parents. You will not share the DNA from your parents with your cousins since they have different parents. This indicates that you and your siblings have two shared ancestors, your grandparents and parents, while you and your cousins only share one common ancestor, your grandparents.

Monophyletic groups are important to understand how animals are classified. It is commonly known that all animals evolved over time, but why do we classify lions differently than crocodiles?

Cladograms

It is pretty hard to visualize how you, your siblings, and cousins are related but it becomes a lot easier when you make a family tree. A cladogram is a family tree for organisms. Cladograms are visual representations of monophyletic groups and are used to help understand the shared traits or synapomorphies of the organisms in a monophyletic group.

Let's look at a simple cladogram.

Cladogram of Earthworms, Birds, and Snakes

Each line is called a branch. Where two branches meet is called a node. Each node represents a shared ancestor. At the end of each branch, there are taxa. Taxa are simply groups of organisms. Taxa can be general groups of organisms, such as vertebrates, or they can be as specific as a species, like an African Elephant. On a cladogram, you will commonly see hash marks that indicate a synapomorphy or shared common trait.

The node circled in blue indicates that earthworms, birds, and snakes all have a common ancestor. Since all these organisms share a common ancestor, they also share a synapomorphy, which is that earthworms, birds, and snakes all reproduce sexually. The node circled in black indicates that birds and snakes share another common ancestor. This ancestor is only shared by birds and snakes; earthworms do not share this ancestor with birds and snakes. Birds and snakes share another synapomorphy, that they are vertebrates.

It is important to note that cladograms do not represent evolutionary times unless years are included on the cladogram. This cladogram does not mean that earthworms evolved before birds and snakes.

Let's take a look at a more complex cladogram.

Cladogram of Lions, Echidnas, Komodo dragons, and Crocodiles

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