What is a monophyletic group?

• A. A group defined by living species only
• B. A group that includes a common ancestor and all its descendants
• C. A group based on similar habitats
• D. A group that excludes extinct lineages

Answer: (B)

Monophyletic groups (clades) are defined by shared ancestry: they include a common ancestor and all of its descendants. In a phylogenetic tree, this forms a single, complete branch with no descendants left out. That’s why this description is the best choice—it captures a full evolutionary lineage from the ancestor to all its descendants.

If a group is defined only by living species, it ignores extinct descendants from that ancestor and thus isn’t a complete branch. If a group is based on similar habitats, it reflects similarities that can arise independently (convergent evolution) rather than true common ancestry. If a group excludes extinct lineages, it leaves out some descendants of the common ancestor, breaking the monophyletic criterion.