Which classification system is based on common ancestry?

Cladistics is a classification system that focuses on organizing species based on their evolutionary relationships and common ancestry. It uses shared derived characteristics to establish groupings, or clades, which include a common ancestor and all descendants of that ancestor. This method emphasizes the tree of life, illustrating how different organisms are related through evolutionary history.

In contrast, phenetics classifies organisms based on overall similarity, often relying on morphological features without consideration for evolutionary relationships. Taxonomy is a broader term that encompasses various methods of classification, including both cladistics and phenetics, but does not inherently focus solely on common ancestry. Biogeography explores the geographical distribution of species and does not directly pertain to a classification system based on ancestry.

Thus, cladistics stands out as the method specifically tied to understanding and documenting evolutionary lineages based on common ancestry, making it the correct answer.