Plesiomorphy : A primitive character state

As biologists, we all wonder what our ancestors looked like. Plesiomorphy refers to a primitive or ancestral character state, whereas a shared plesiomorphy is known as a symplesiomorphy. Plesiomorphy refers to a primitive or ancestral characteristic that is more broadly distributed than in a specific group of interest. These are primitive for the group under consideration and cannot give proof for the group. In phylogenetics, plesiomorphy (“near form”) and symplesiomorphy are synonyms for an ancestral trait shared by all members of a clade but which does not differentiate the clade from others.

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Plesiomorphic Characters

An evolutionary characteristic that is homologous within a certain group of organisms but not unique to members of that group. Hence it cannot be utilized as a diagnostic or distinguishing characteristic for the group. For example, vertebrae may be found in zebras, cheetahs, and orangutans, but the common ancestor from whom this characteristic originated is so remote that many other creatures share it. Thus, ownership of vertebrae gives little insight into the evolutionary relationships of these three taxa.

Synapomorphies differ from plesiomorphies, which are ancestral traits that do not give grouping information. Suppose an investigation ingroup consisted of a lion, a tiger, and a bear. If hair were utilized as a character in such research, it would constitute a plesiomorphy because all three creatures have the trait, which is common to mammals. The existence of hair alone does not indicate whether a lion is more closely related to a tiger or a bear, while a number of other characteristics (synapomorphies) do distinguish the two huge cats from the bear.

Examples of Plesiomorphy

Plesiomorphy refers to any attribute that a group got from its predecessor. Reptiles, for example, are exothermic, which means that their internal body temperature does not remain constant. They have this trait because the progenitor of all reptiles was exothermic. This varies from synapomorphy in that certain descendants of the earliest reptiles are not exothermic (birds are endothermic). In other words, this quality is inherited but not shared by all of the forebears, or descendants.

Hair, for example, is a distinct mammalian trait that develops with mammals. Following mammalian evolution, distinct mammalian groupings and subgroups evolved in response to evolutionary novelties. Primates, for example, are a relatively recent mammalian group. As a result, hair is a plesiomorphy (ancestral trait) for primates. Hair, being an ancient mammalian trait, is shared by all primates. This makes it a symplesiomorphy (shared plesiomorphy) for primates as a whole. Because all snails have a feeding apparatus known as a radula, the existence of this trait is ineffective in detecting connections between snail groups. In this context, radula is a plesiomorphic character.

Discussion on Plesiomorphy

Plesiomorphy addresses the dangers of classifying species based solely on morphological or genetic resemblance, without differentiating between ancestral and derived character states. Because a plesiomorphic trait acquired from a common ancestor might occur anywhere in a phylogenetic tree, its existence provides no information about the connections within that tree.

• For example, possessing a backbone is plesiomorphic between birds and mammals but apomorphic between them and insects.
• Birds and mammals, for example, have a backbone as a distinguishing synapomorphic feature, but insects lack one. Species should not be classified only based on their morphological or genetic similarities.
• Because a plesiomorphic trait acquired from a common ancestor might occur anywhere in a phylogenetic tree, its existence provides no information about the tree’s connections.
• Thus, classifying species involves differentiating between ancestral and derived character states.
• An example is thermo-regulation in Sauropsida, a group that includes lizards, turtles, crocodiles, and birds. Lizards, turtles, and crocodiles are ectothermic (cold-blooded), whereas birds are endothermic (warm-blooded).
• Lizards, turtles, and crocodiles are all coldblooded, however, they do not form a clade since crocodiles are more closely related to birds than lizards or turtles.
• Thus, using coldbloodedness as an apomorphic characteristic to group crocodiles with lizards and turtles is incorrect; instead, it is a plesiomorphic trait shared by all three clades due to their distant common ancestor.

Conclusion

Plesiomorphy, symplesiomorphy, apomorphy, and synapomorphy all refer to a characteristic that species share because they share an ancestor species. All of these concepts are relative by definition, meaning that a characteristic might be plesiomorphic in one context and apomorphic in another. Apomorphic and synapomorphic traits reveal a lot about evolutionary clades and can help designate taxa. However, plesiomorphic and symplesiomorphic traits cannot.

REFERENCES:

1. Choudhuri, S. (2013). Fundamentals of Molecular Evolution. Bioinformatics for Beginners, 27-53. https://doi.org/10.1016/B978-0-12-410471-6.00002-5
2. Cabej, N. R. (2011). Evolution by Reverting to Ancestral Characters. Epigenetic Principles of Evolution, 623-650. https://doi.org/10.1016/B978-0-12-415831-3.00015-X