Why does cladistics focus on shared derived characteristics?

Cladistics focuses on shared derived characteristics, known as synapomorphies, because they provide the most reliable evidence for inferring monophyly—the descent of a group of species from a single, most-recent common ancestor.123

Unlike phenetics, which classifies organisms based on overall physical similarity regardless of evolutionary history, cladistics seeks to reconstruct true evolutionary relationships. By identifying traits that were uniquely acquired by a common ancestor and passed down to its descendants, scientists can determine which species are more closely related to each other than to others.13

For example, if a specific character evolves in an ancestral species, that trait serves as a marker for all subsequent lineages branching from that ancestor. Using these derived traits allows researchers to build phylogenetic trees that accurately reflect the branching patterns of life, rather than grouping organisms based on superficial similarities or parallel evolution, which can be misleading.23