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Plesiomorphy and Symplesiomorphy in Phylogenetic Classification

Plesiomorphy and Symplesiomorphy in Phylogenetic Classification In the study of phylogenetics—the branch of biology that deals with evolutionary relationships—scientists must distinguish ...

Plesiomorphy and Symplesiomorphy in Phylogenetic Classification

In the study of phylogenetics—the branch of biology that deals with evolutionary relationships—scientists must distinguish between traits that define a group and traits that are simply inherited from a distant ancestor. To do this accurately, researchers use specific terminology to describe whether a biological characteristic is ancestral or derived. Two critical terms in this process are plesiomorphy and symplesiomorphy.

Understanding these terms is essential for building accurate evolutionary trees. Without distinguishing between shared ancestral traits and shared derived traits, scientists might incorrectly group species together based on superficial similarities rather than true evolutionary lineage.

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

Defining Ancestral and Derived Traits

A plesiomorphy (meaning "near form") refers to an ancestral character state. When this ancestral trait is shared by all members of a specific clade (a group consisting of a common ancestor and all its descendants), it is known as a symplesiomorphy. The term symplesiomorphy was introduced in 1950 by the German entomologist Willi Hennig.

While these terms describe shared history, they have limitations in classification. Because a symplesiomorphy is an ancestral trait shared by many different groups, it cannot be used to distinguish one specific clade from another. In contrast, apomorphies (derived traits) and synapomorphies (shared derived traits) provide the necessary information to define specific taxa (biological groups) because they represent evolutionary innovations unique to certain lineages.

Key Differences in Phylogenetic Terms

Comparison of Phylogenetic Character States
Term Definition Utility in Defining Clades
Plesiomorphy An ancestral character state. Low; does not distinguish clades.
Symplesiomorphy An ancestral character shared by all members of a clade. Low; shared by many different groups.
Apomorphy A derived character state. High; indicates evolutionary change.
Synapomorphy A shared derived character state. High; used to define specific taxa.

Practical Examples in Nature

To see these concepts in action, we can look at how different animals are categorized. For instance, having a backbone is a plesiomorphic trait shared by both birds and mammals. Because fish, frogs, lizards, and snakes also possess backbones, this trait does not help scientists place an animal specifically into the bird clade or the mammal clade; it only indicates they share a very distant ancestor.

Similarly, being a hexapod (an animal with six legs) is a plesiomorphic trait shared by ants and beetles. While they share this trait due to a common ancestor, it does not distinguish them from other hexapods like flies, bees, or aphids.

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.
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.
: 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.

Identifying Clades via Synapomorphies

To accurately group species, biologists look for synapomorphies—traits that are unique to a specific group. Examples include:

  • Elytra: These hardened forewings are a synapomorphy for the beetle clade, though they are plesiomorphic when comparing different types of beetles (such as dung beetles versus horned beetles).
  • Metapleural glands: A synapomorphy used to place species within the ant clade.
  • Feathers: A synapomorphy that defines the bird clade.
  • Milk: A mammalian synapomorphy; all mammals produce milk, and no other clade does.
  • Hair: A synapomorphy for mammals, though it is important to note that the absence of hair does not exclude a species from being a mammal if it belongs to that lineage.

The Relativity of Evolutionary Traits

It is vital to remember that these terms are relative. A trait can be a plesiomorphy in one context and an apomorphy in another. For example, a backbone is plesiomorphic when comparing birds to mammals, but it is an apomorphy when comparing vertebrates to insects.

A common error in classification is grouping species based on shared ancestral traits rather than shared derived ones. Consider Sauropsida, the clade containing lizards, turtles, crocodiles, and birds. While lizards, turtles, and crocodiles are all ectothermic (cold-blooded), this trait is a symplesiomorphy and does not mean they form a single exclusive clade. In fact, crocodiles are more closely related to birds (which are endothermic, or warm-blooded) than they are to lizards. Using "cold-bloodedness" to group crocodiles with lizards would be an evolutionary error.

Key Facts

  • Plesiomorphy refers to an ancestral trait, while apomorphy refers to a derived trait.
  • Symplesiomorphy is an ancestral trait shared by all members of a clade.
  • Symplesiomorphies cannot be used to define specific taxa or clades.
  • Synapomorphies are shared derived traits and are essential for defining evolutionary groups.
  • Evolutionary terms are relative and depend on the group being compared.

Frequently Asked Questions

What is the main difference between a plesiomorphy and a synapomorphy?

A plesiomorphy is an ancestral trait inherited from a distant ancestor, whereas a synapomorphy is a derived trait that is shared by a specific group and helps define them.

Why can't we use symplesiomorphies to group animals?

Because symplesiomorphies are shared by many different lineages, they do not provide enough specific information to distinguish one unique group from another.

Can a species be a mammal even if it lacks hair?

Yes. While hair is a mammalian synapomorphy, some species have lost the trait through evolution; however, they remain part of the mammal clade.

Why is being cold-blooded not a reliable way to group crocodiles with lizards?

Being cold-blooded is a symplesiomorphy. Because crocodiles are actually more closely related to warm-blooded birds than to lizards, grouping them by temperature regulation ignores their true evolutionary relationships.

Who introduced the term symplesiomorphy?

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

References

  1. For a dissenting view, see "About nothing" by Brower and de Pinna 2014.[3]
  2. Roderick D.M. Page; Edward C. Holmes (14 July 2009). Molecular Evolution: A Phylogenetic Approach. John Wiley & Sons. ISBN 978-1-4443-1336-9.
  3. Freeman, Scott, 1955- (2015). Evolutionary analysis. Herron, Jon C., 1962- (5th ed.). Harlow. ISBN 9781292061276. OCLC 903941931.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  4. Brower, Andrew V. Z.; de Pinna, M. C. C. (2014). "About Nothing". Cladistics. 30 (3): 330–336. doi:10.1111/cla.12050. PMID 34788975. S2CID 221550586.
  5. Patterson, Colin (1982), "Morphological characters and homology", in Joysey, Kenneth A; Friday, A. E. (eds.), Problems in Phylogenetic Reconstruction, Systematics Association Special Volume 21, London: Academic Press, ISBN 0-12-391250-4.

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