Synapomorphy

A synapomorphy is a derived trait shared by members of a group and inherited from their most recent common ancestor. It is evidence that the group forms a clade.

Cladistics: A method of classifying organisms by shared derived traits and branching evolutionary relationships. Synapomorphies are the evidence cladistics uses to recognize clades.
Phylogenetic tree: A branching diagram representing hypotheses about evolutionary relationships. Synapomorphies help determine which taxa belong together on its branches.
Symplesiomorphy: A shared ancestral trait that does not uniquely identify a more recent group. It is shared, but unlike a synapomorphy it cannot establish the group's exclusive common ancestry.
Amniotic egg: An embryo-containing structure with protective membranes that enables reproduction away from open water. Its evolutionary origin is a classic shared derived feature of amniotes.
Incomplete lineage sorting: The persistence of ancestral genetic variation through rapid speciation, causing gene trees to differ from species trees. A gene variant shared by taxa may predate their common ancestor as a group.
Homology: Similarity between biological features caused by inheritance from a common ancestor. Shared ancestry distinguishes a synapomorphy from a superficial resemblance.
Parsimony: A principle favoring explanations that require the fewest evolutionary changes. Parsimony often guides reconstruction of the tree that best explains observed shared traits.
Homoplasy: Similarity between traits that did not arise through inheritance from their most recent common ancestor. Independent evolution or reversal can mimic a shared derived trait.
Feathers: Keratin structures that characterize birds and also occur in other theropod dinosaurs. Their distribution helps reveal relationships among birds and extinct dinosaurs.
Horizontal gene transfer: The transfer of genetic material between organisms outside parent-to-offspring inheritance. Transferred genes can create shared traits that do not track the organismal tree.

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