Why do cladists focus on a group’s most recent common ancestor?

Cladists focus on a group’s most recent common ancestor because they define biological taxa based on shared evolutionary history rather than general physical similarity. By identifying the most recent common ancestor, cladists can establish monophyletic groups—taxa that include an ancestor and all of its descendants.12

This approach relies on the use of synapomorphies, which are uniquely shared derived characteristics that indicate a group evolved from a single ancestral species. By focusing on these specific traits, cladists can infer branching patterns in the "tree of life" and classify organisms based on their actual genealogical relationships. This method provides a more objective and systematic framework for understanding evolutionary relatedness compared to earlier approaches like phenetics, which grouped organisms based on overall similarity regardless of ancestry.123