a feature from an ancestral species that is retained in more or less the same form in all its descendants

A synapomorphy is a shared derived trait that occurs in multiple organisms and is inherited from their closest common ancestor. Unlike persistent traits (symplesiomorphies) and traits with convergent evolution (homoplasy), synapomorphy is a key piece of evidence for common descent and helps identify and define groups of organisms (strains) in phylogenetic analyses.

Why is it important?

• Phylogenetics:
  Synapomorphies are an essential tool for constructing phylogenetic trees (cladograms) that show evolutionary relationships among organisms.
• Group identification:
  They help define monophyletic groups-groups that include a common ancestor and all of its descendants.
• Resolution:
  Allows you to distinguish organisms based on evolutionary innovations that distinguish them from other groups