Final answer:
A clade, defined as a group encompassing a common ancestor and all its descendants, must be a monophyletic group, indicating a single shared origin point on the evolutionary tree.
Step-by-step explanation:
A clade is composed of a single common ancestor and all of its descendants, by definition, a clade must be a monophyletic group. This is because every organism within the clade shares a single point of origin on the evolutionary tree, meaning they have a common ancestor from which all the members of that clade are derived. For example, in Figure 20.11, a clade may encompass diverse groups, such as animals, fungi, and plants, or it might represent a more specific lineage like the flagellates. However, it's essential that all of the organisms in a clade come from a single evolutionary branch point. Using cladistics, scientists can determine these evolutionary relationships and represent them through cladograms, which help visualize how clades are composed of both an ancestor and all its descendants, reflecting their phylogeny.