What is a clade, and why is monophyly important in constructing phylogenies?

• A. A clade is a group defined by similar ecological roles; monophyly ensures groupings reflect current habitats
• B. A clade includes a common ancestor and all its descendants; monophyly reflects true evolutionary history
• C. A clade is any group sharing at least one trait; monophyly is unnecessary
• D. A clade excludes extinct species; monophyly refers to living members only

Answer: (B)

A clade is a group that includes a common ancestor and all of its descendants. This makes it a single branch on the evolutionary tree, representing a true lineage. Monophyly matters because it ensures the group reflects the actual evolutionary history: the lineage from one ancestor and every descendant remains intact, with nothing left out. When a group leaves out some descendants (paraphyly) or mixes species from different ancestors (polyphyly), the grouping no longer mirrors how life has diversified, which can mislead conclusions about relationships and trait origins. So, using monophyletic groups in phylogenies keeps the tree honest about ancestry and descent, allowing traits to be traced to a single origin.