Difference Between Apomorphy and Plesiomorphy

Main Difference – Apomorphy vs Plesiomorphy

A clade or taxon refers to a taxonomic level that represents a group of organisms with common characters. It can be either a phylum, order, family, genus or a geographic population. Apomorphy and plesiomorphy are two terms used to describe unique characters or traits of a clade. The main difference between apomorphy and plesiomorphy is that apomorphy refers to any character unique to a particular clade and its descendants whereas plesiomorphy refers to any character homologous within the clade but, not unique to all members. Snakes are a suborder of reptiles. The absence of legs in snakes is an example of apomorphy. The presence of legs within reptiles is an example of plesiomorphy as all reptiles do not have legs.

Key Areas Covered

1. What is Apomorphy
– Definition, Types, Examples
2. What is Plesiomorphy
– Definition, Facts, Examples
3. What are the Similarities Between Apomorphy and Plesiomorphy
– Outline of Common Features
4. What is the Difference Between Apomorphy and Plesiomorphy
– Comparison of Key Differences

Key Terms: Apomorphy, Autapomorphy, Clade, Plesiomorphy, Symplesiomorphy, Synapomorphy

What is Apomorphy

Apomorphy refers to a novel evolutionary character, unique to a particular clade and all its descendants. An apomorphy restricted to a single species is referred to autapomorphy. Apomorphy can be used to define that particular clade. As an example, the class Aves is defined by the presence of feathers. The absence of legs in snakes is another example of apomorphy. But, apomorphy alone is unable to provide any information about the phylogenetic relationship of that species. Therefore, apomorphy indicates the degree of divergence of a species relative to its closest relatives. For example, speech is a unique characteristic in humans but, not in other primates.

Figure 1: Example of Apomorphy – absence of legs in snakes

An apomorphy shared by two or more species is referred to as synapomorphy. The synapomorphic characters can be used to strictly define monophyletic clades. This is the base of cladistic classification systems.

What is Plesiomorphy

Plesiomorphy refers to an evolutionary character homologous within a particular clade but, not unique to all members of that particular clade. It is also known as symplesiomorphy. As an example, bony fish have gills for respiration but, they are closely related to vertebrates that lack gills. Typically, reptiles are ectothermic but its relatives such as birds are endothermic; this is another example of plesiomorphy. Similarly, though other reptiles have legs, snakes do not have legs. Different types of reptiles with and without legs are shown in figure 2.

Difference Between Apomorphy and Plesiomorphy

Figure 2: Reptiles

It is important to know that, in plesiomorphy, the plesiomorphic character is shared by two different clades that have an earlier common ancestor.

Similarities Between Apomorphy and Plesiomorphy

Apomorphy and plesiomorphy are two terms used to describe similar characters of a clade.
Both apomorphy and plesiomorphy are derived from evolution.

Definition

Apomorphy: Apomorphy refers to a novel evolutionary character unique to a particular clade and all its descendants.

Plesiomorphy: Plesiomorphy refers to an evolutionary character homologous within a particular clade but, not unique to all members of that particular clade.

Significance

Apomorphy: An apomorphic character is similar throughout the clade.

Plesiomorphy: A plesiomorphic character is not similar throughout the clade.

Types

Apomorphy: The two types of apomorphy are autapomorphy and synapomorphy.

Plesiomorphy: Plesiomorphy is not divided further.

Type of Character

Apomorphy: Apomorphy is a derived or specialized character.

Plesiomorphy: Plesiomorphy is a primitive or ancestral character.

Defining Character

Apomorphy: An apomorphic character can be used as a defining character for the clade.

Plesiomorphy: A plesiomorphic character cannot be used as a defining character for the clade.

Examples

Apomorphy: The absence of legs is an example of an apomorphic character in snakes.

Plesiomorphy: The presence of legs is an example of a plesiomorphic character in reptiles.

Conclusion

Apomorphy and plesiomorphy are two terms used to describe similar characters or traits within a clade. An apomorphic character is similar to all members of the clade while the plesiomorphic character is not similar in all members of the clade. The main difference apomorphy and plesiomorphy is the occurrence of the character throughout the clade.

Reference:

1. “Apomorphy.” Oxford Index, Available here.
2. “Posts about Plesiomorphy on abirdingnaturalist.” Abirdingnaturalist, Available here.

Image Courtesy:

1. “Guatemalan Milksnake (Lampropeltis triangulum abnorma)” By Amdubois – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “State reptile collage cropped” By State_reptile_collage.jpg: *American_Alligator.jpg: User:Postdlfderivative work: TCO (talk)derivative work: TCO (talk) – State_reptile_collage.jpg (CC BY-SA 3.0) via Commons Wikimedia