What is a synapomorphy?

A synapomorphy is a shared derived trait that is present in a common ancestor and all of its descendants. It is used in cladistics, a method of classifying organisms based on their evolutionary relationships, to determine which species are more closely related to each other. Synapomorphies are important in constructing phylogenetic trees and can help researchers trace the evolutionary history of a group of organisms. By identifying shared derived traits, scientists can infer how different species are related and how they have evolved over time.