WHAT IS A SIMPLEIOMORPHY? (WITH EXAMPLES) - BIOLOGY - 2025

A simpleiomorphy, in cladistic terminology, refers to an ancestral character shared by two or more taxa. That is, this characteristic coincides with the one inferred to have been present in the common ancestor of both groups.

Simpleiomorphies are types of plesiomorphies, which are defined as ancestral characters. This term is opposed to apormorphy - derived characters or evolutionary novelties. In the same way, the term simplesiomorphy is opposed to synapomorphy - a shared derived character.

Source: Benjamín Núñez González, from Wikimedia Commons

According to the cladist classification school, ancestral derived characters should not be used to define groups, as it would result in a paraphyletic grouping.

What is a simpleiomorphy?

In cladism, a polarity is attributed to the different characteristics present in organic beings. Thus, there are derived characters and ancestral characters. The first of these is known as apomorphic, while the ancestral state is called plesiomorphic.

If more than one taxon presents the ancestral state, the character is a simpleiomorphy - because it is shared. Similarly, shared derived features are synapomorphies.

These terms are of relative use and depend on the "position" or depth of the phylogenetic tree that the reader is taking.

For example, in the division between mandibular and non-mandibular, the lack of structure represents the ancestral character, while the presence of mandibles is considered derivative. But, if I am comparing two groups of mammals, for example cats and dogs, the jaw will be an ancestral character.

Ancestral vs. ancestral species

Reading phylogenetic trees is subject to a number of misunderstandings. Among the most common is to assume that said graphic representation provides information about the primitive or advanced state of the species that are represented there.

In this way, phylogenetic trees provide us with information about the evolutionary history of organic beings, focusing on the characters. That is, it cannot tell us if a species is ancestral or derived, but we can infer these states of the character in question.

For example, imagine that we can sequence the amino acids in a protein that is present in gorilla, bonobo, chimpanzee, and human. In this hypothetical sequence, all of the organisms mentioned have the amino acid residue valine, while the chimpanzee has glutamic acid in its place.

In that case, we could assume that the most possible hypothesis - following the principle of parsimony, also called Occam's razor, which requires the least number of evolutionary changes - is that valine is the ancestral character and that everyone has inherited them from yet. common ancestor. However, in chimpanzees the character changed.

How to differentiate them?

An almost universal method for the differentiation between both states of the character is the comparison with an external group, following the following principle: if different states of a characteristic appear in two groups, it is highly probable that the manifestation found in its closest relative, is the ancestral.

Simpleiomorphies and paraphyletic groups

In cladism, phylogenetic relationships are deduced using strictly synapomorphies or shared derived characters.

The use of this characteristic leads to the formation of monophyletic groupings - the common ancestor of the group, plus all its descendants. The resulting phylogenetic hypothesis is expressed in a graph called a cladogram.

If we wanted to establish groupings using simpleiomorphies, the result would be paraphyletic. Take for example reptiles and winged and wingless insects

Reptiles

Scaly skin is an ancient characteristic shared by turtles, crocodiles, lizards, and the like. The scales contributed to misunderstandings in taxonomy for centuries. Today, fossil, molecular and morphological evidence has led to the conclusion that reptiles do not form a clade (a monophyletic group).

Why are reptiles paraphyletic? Because crocodiles are more related to birds, than to snakes and lizards, for example. Following this line of thought, it is more than clear that birds are part of the clade of reptiles.

Apterygota and Pterygota

In insects, we can establish a very intuitive division between the representatives that do not have wings and those that do - in Apterygota and Pterygota respectively.

In the course of evolution, insects, which previously did not possess wings, developed these structures. Therefore, having no wings is an ancestral character, while wings represent the derived state.

These two groups have no taxonomic validity. Apterygota represents a paraphyletic group, as it is based on a shared ancestral characteristic: the absence of wings.

As in reptiles, there are wingless insects that are more related to the winged variants than to other wingless species.

These examples clearly illustrate how the use of shared derived characters provides us with evidence of real kinship relationships, while the use of simpleiomorphies does not.

Examples

Hemichordates and chordates

The paraphyletic group of the "prochordates" is made up of hemichordates, urochordates and cephalochords. These organisms are classified by the presence of primitive characters.

If you want to form a monophyletic group, you have to take into account apomorphic characters, which clearly unify the urochordates, cephalochordates and vertebrates. These form the clade of the chordates.

Hemichordates are characterized by the presence of a stomochord, which was long believed to resemble a real notochord, but current evidence has made it clear that it is not. In addition, they have gill slits and a dorsal nerve cord.

In contrast, chordates are characterized by a notochord, hollow dorsal nerve cord, and gill slits. These characteristics may change throughout the individual's life, but remain diagnostic of the group.

The monotremes

Monotremes have an interesting mix of plesiomorphic characteristics, reminiscent of reptiles, and apomorphic, typical of mammals. However, these organisms are strongly adapted to a semi-aquatic or ant-consuming lifestyle, making character analysis difficult.

For example, the skull of the members of the group exhibits plesiomorphic characteristics, but they differ in the morphology of the beak. The muzzle features a long bone found in reptiles, therapsids, and xenarthras. The ventral surface of the skull has structures that could be remnants of reptilian features.

References

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