Symplesiomorphy

Part of speech: noun

Definitions

1. A trait inherited from a distant ancestor that is shared by several species yet does not reveal which of them are most closely related
2. An ancestral characteristic held in common by a group of organisms that fails to distinguish a specific subgroup from the rest
3. The retained, older form of a feature that several lineages possess, which therefore cannot by itself show true evolutionary kinship

Etymology: Symplesiomorphy is a technical term from evolutionary biology, built entirely from Ancient Greek parts. "Morphē" means "form" or "shape," "plesios" means "near" or "close," and "syn-" means "together." The German entomologist Willi Hennig, the founder of phylogenetic systematics, assembled this family of terms to label different kinds of shared characters. The term describes a trait that two or more organisms share because they inherited it from a distant common ancestor, even though it does not show that they are closely related. Backbones are a classic example. Fish and reptiles both have them, but that shared feature reflects an ancient inheritance that both lineages kept, not a recent common ancestor. The name signals the contrast. A "synapomorphy" is a shared derived trait that does point to close kinship, so Hennig's vocabulary separates the two kinds of shared resemblance. Only the second is useful for grouping species.