Plesiomorphy and Symplesiomorphy in Phylogenetic Analysis

In the study of phylogenetics—the analysis of evolutionary relationships among biological species—scientists must distinguish between different types of shared traits to accurately map the tree of life. Central to this process are the concepts of plesiomorphy and symplesiomorphy, terms that describe ancestral characteristics that can often mislead researchers if not properly identified.

A plesiomorphy (meaning "near form") or symplesiomorphy refers to an ancestral character state shared by all members of a clade. While these traits are common to the group, they are not unique to it; therefore, they cannot be used to distinguish one specific clade from another.

Phylogenies showing the terminology used to describe different patterns of ancestral and derived trait states.[1]

Key Facts

Plesiomorphy and symplesiomorphy are synonyms for ancestral traits shared by members of a clade.
These traits cannot be used to define specific taxa or distinguish one clade from another.
The term symplesiomorphy was introduced in 1950 by German entomologist Willi Hennig.
Unlike plesiomorphies, apomorphies (derived traits) and synapomorphies (shared derived traits) provide the necessary information to define evolutionary clades.
Whether a trait is considered ancestral or derived is relative to the context of the comparison.

Ancestral vs. Derived Characteristics

To understand why plesiomorphies are limited in their utility, it is helpful to compare them with derived traits. While plesiomorphies are inherited from a distant ancestor, apomorphies are derived characteristics that appear in a more recent ancestor. When a derived trait is shared by a group of species, it is called a synapomorphy.

Because synapomorphies represent a "new" evolutionary step, they allow biologists to define specific taxa. In contrast, because a plesiomorphic character can appear anywhere in a phylogenetic tree, its presence does not reveal specific relationships within that tree.

Imaginary cladogram.[2] The yellow mask is a plesiomorphy for each living masked species, because it is ancestral.[2] It is also a symplesiomorphy for them. But for the four living species as a whole, it is an apomorphy because it is not ancestral for all of them. The yellow tail is a plesiomorphy and symplesiomorphy for all living species.

Examples in Nature

The distinction between these traits is clearly visible when examining various animal groups:

The Backbone: A backbone is a plesiomorphic trait shared by birds and mammals. Because snakes, lizards, fish, and frogs also have backbones, this trait cannot be used to separate birds from mammals.
Hexapody: Being a hexapod (six-legged) is a plesiomorphic trait shared by ants and beetles. Since flies, bees, and aphids are also hexapods, this trait does not help in distinguishing ants from beetles.
Defining Traits (Synapomorphies): In contrast, feathers are a synapomorphy for birds, and milk production is a synapomorphy for mammals. Similarly, elytra (hardened wing covers) serve as a synapomorphy for the beetle clade, and the metapleural gland is a synapomorphy for ants.

The Relativity of Evolutionary Traits

A critical aspect of these definitions is that they are relative. A trait can be a plesiomorphy in one context and an apomorphy in another. For example, while a backbone is plesiomorphic when comparing birds to mammals, it is an apomorphy when comparing vertebrates (birds and mammals) to invertebrates (insects).

The Danger of Morphological Similarity

Grouping species based solely on similarity can lead to scientific errors. A prime example is found in Sauropsida (the clade containing lizards, turtles, crocodiles, and birds). Lizards, turtles, and crocodiles are all ectothermic (cold-blooded), while birds are endothermic (warm-blooded).

If a researcher grouped crocodiles with lizards and turtles based on being cold-blooded, they would be making an error. Cold-bloodedness is a symplesiomorphy—an ancestral trait. In reality, crocodiles are more closely related to birds than they are to lizards or turtles.

Comparison of Phylogenetic Character States
Term	Definition	Utility in Cladistics	Example
Plesiomorphy	Ancestral character state	Cannot define a specific clade	Backbone (for mammals/birds)
Symplesiomorphy	Shared ancestral character state	Cannot distinguish between clades	Cold-bloodedness (for crocodiles/lizards)
Apomorphy	Derived character state	Can define a new trait	Feathers (in birds)
Synapomorphy	Shared derived character state	Used to define and group taxa	Milk production (in mammals)

Frequently Asked Questions

What is the difference between a plesiomorphy and a synapomorphy?

A plesiomorphy is an ancestral trait inherited from a distant ancestor and shared by many groups, making it useless for defining a specific clade. A synapomorphy is a derived trait shared by a specific group, which allows scientists to identify and define that particular evolutionary clade.

Who introduced the term symplesiomorphy?

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

Can a trait be both a plesiomorphy and an apomorphy?

Yes, but only in different contexts. For instance, a backbone is a plesiomorphy when comparing mammals and birds, but it is an apomorphy when comparing those vertebrates to invertebrates.

Why can't scientists group animals based on shared ancestral traits?

Because ancestral traits (symplesiomorphies) can appear anywhere in a phylogenetic tree, they do not provide information about the specific relationships between species. Grouping by these traits would lead to incorrect classifications, such as grouping crocodiles with lizards instead of birds.

Does the loss of a trait affect its status as a synapomorphy?

Not necessarily. For example, hair is a synapomorphy for mammals. Even though some mammal species have lost their hair through evolution, the absence of hair does not exclude them from the mammal clade.