Plesiomorphy and symplesiomorphy

In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades.

Plesiomorphy, symplesiomorphy, apomorphy, and synapomorphy all mean a trait shared between species because they share an ancestral species.

Apomorphic and synapomorphic characteristics convey much information about evolutionary clades and can be used to define taxa. However, plesiomorphic and symplesiomorphic characteristics cannot.

The term symplesiomorphy was introduced in 1950 by German entomologist Willi Hennig.

A backbone is a plesiomorphic trait shared by birds and mammals, and does not help in placing an animal in one or the other of these two clades. Birds and mammals share this trait because both clades are descended from the same far distant ancestor. Other clades, such as snakes, lizards, turtles, fish, and frogs, all have backbones and none are either birds or mammals. ^{[citation needed]}

Being a hexapod is plesiomorphic trait shared by ants and beetles, and does not help in placing an animal in one or the other of these two clades. Ants and beetles share this trait because both clades are descended from the same far distant ancestor. Other clades, e.g. bugs, flies, bees, aphids, and many more clades, all are hexapods and none are either ants nor beetles. ^{[citation needed]}

Elytra are a synapomorphy for placing any living species into the beetle clade, Elytra are plesiomorphic between clades of beetles, e.g. they do not distinguish the dung beetles from the horned beetles. The metapleural gland is a synapomorphy for placing any living species into the ant clade. ^{[citation needed]}

Feathers are a synapomorphy for placing any living species into the bird clade, hair is a synapomorphy for placing any living species into the mammal clade. Note that some mammal species have lost their hair, so the absence of hair does not exclude a species from being a mammal. Another mammalian synapomorphy is milk. All mammals produce milk and no other clade contains animals which produce milk. Feathers and milk are also apomorphies. ^{[citation needed]}