A group of species that includes a single common ancestor and all its descendants is called a monophyletic group, also known as a clade.

Here's a breakdown:

* Monophyletic: Meaning "single tribe" or "single origin".

* Clade: A group of organisms that share a common ancestor.

Think of it like a family tree. A clade would be a branch of the tree, including the original ancestor and all the branches that split off from it.