WHAT IS AN APOMORPHY? (WITH EXAMPLES) - BIOLOGY - 2025

An apomorphy, in cladistic terminology, is a state derived from a character. This state could be classified as "novel" if it is compared with a close ancestral group.

If the apomorphic character is shared between two or more groups, it is known as a synapomorphy, while if the character is unique to a group, it is called autapomorphy. Synapomorphies are key elements of cladism.

In mammals, hair is considered an apomorphy.

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The opposite concept of apomorphy is plesiomophy, which refers to an ancestral or primitive character.

It would be incorrect to define a character as apormóphic absolutely, since these concepts apply in a relative way. That is, they require comparison with another group, in order to define the status of the character.

For example, the vertebral column is an apomorphic character from the group of vertebrates. But if we take the position of this structure in a bird, in relation to the other vertebrates, the trait is plesiomorphic.

This terminology is widely used in the area of ​​evolutionary biology and is very useful when describing the existing phylogenetic relationships between organic beings.

What is an apomorphy?

An apomorphy refers to a state derived from a certain character, that is, to an evolutionary novelty within a group, if it is compared with another nearby ancestral taxon that lacks the characteristic under study.

These characteristics arise in the most recent common ancestor of the group in question or it is a characteristic that evolves recently and appears only in a group of related species.

In contrast, the opposite term is plesiomorphy. In these, the characters arise in a distant common ancestor, so they are labeled as primitive.

However, the terms "advanced" and "primitive" are often avoided by evolutionary biologists, as they imply a scale of perfection, which has no place under the prism of evolution.

In fact, plesiomorphies can be considered as apomorphies that are "deeper" in the phylogeny. This will become clearer with the examples that will be discussed in the next section.

Synapormorphies and autopomorphies

When mentioning apomorphies, it is necessary to distinguish between the terms that are derived from it: synapormorphies and autopomorphies.

When a characteristic is an apomorphy, and it is also shared by the members of a group, the term synapormophy or shared derived characters is used.

On the other hand, when the derived character is unique to a taxon, it is called self-morphing. For example, one such non-anatomical character is speech in humans, since we are the only group with this peculiar characteristic.

Source: Biotoscano, from Wikimedia Commons

Examples of apomorphy

Apomorphies in birds

Birds are flying vertebrates made up of about 18,000 species. Several apomorphies can be distinguished that allow the distinction of birds from the rest of the vertebrates.

The feathers are considered an apomorphy on the wings. As they are unique to the Aves class, they are autapomorphies. If we take a group within the birds, suppose, some family or some genus, the feathers would be an ancestral character.

Apomorphies in mammals

Mammals are a group of amniotic vertebrates comprising almost 5,500 species. Within this group there is a series of evolutionary novelties that undoubtedly characterize the group.

The hair of mammals is considered an apomorphic character, since it allows distinguishing mammals from other groups of vertebrates, such as reptiles, for example.

As hair is a characteristic shared by all mammals, it is also a synapomorphy of mammals in general. The same occurs with the mammary glands or with the three small bones of the middle ear.

Within mammals, there are several groups. Each of these orders has its own apomorphies. For example, in primates we can clearly distinguish that the opposable thumb is a derived feature, which is not found in any other group of mammals.

However, as we saw, the distinctions of apomorphies and other character states are relative. What we consider an apomorphic character for a large clade can be considered plesiomorphic if we see it from the point of view of a smaller clade nested within the larger one.

Apomorphies in insects

In insects, there is a subclass called Pterygota, defined by the presence of wings. In fact, the term "Pterygota" derives from the Greek pterygous, which means "winged."

In this way, in the mentioned subclass, the wings represent an apormorphic character. If we go to the insect order Lepidoptera, the wings are a plesiomorphic character.

Cladism and synapomorphies

What is cladism?

Cladism - also known as phylogenetic systematics or phylogenetic classification - is a school of classification that bases its system on the shared derived characteristics of individuals.

In this way, organic beings that share specific derived characters are grouped and separated from those groups that do not possess the characteristic in question.

The groups that are formed using this methodology are known as clades, and they consist of the most recent common ancestor and all of its descendants.

These relationships are expressed graphically in a hierarchical branching pattern (or tree) called a cladogram. Clades can be nested, one inside the other.

Monophyletic, paraphyletic, and polyphyletic groups

Now, using the previous example of winged and non-winged insects we can understand how cladism relates to the terms discussed in this article.

The critical aspect for recognizing monophyletic groups are synapomorphies, not plesiomorphies. Therefore, grouping based on plesiomorphies produces paraphyletic groups.

For example, the wings are synapomorphies that unite winged insects in the monophyletic group Pterygota. Before the evolutionary novelty of wings emerged, insects clearly lacked them. Thus the absence of wings is a primitive character.

If we group the insects using the characteristic of the absence of wings, we will obtain the paraphyletic group Apterygota.

Why is it paraphyletic? Because some wingless insects are more closely related to winged insects than other wingless insect species.

Finally, polyphyletic groups are based on convergent characters that do not share a common evolutionary derivation. If we form a group of flying animals, with insects, birds and bats, it would clearly be a polyphyletic group - these three animal groups did not inherit air locomotion from a common ancestor.

References

1. Choudhuri, S. (2014). Bioinformatics for beginners: genes, genomes, molecular evolution, databases and analytical tools. Elsevier.
2. Grimaldi, D., Engel, MS, & Engel, MS (2005). Evolution of the Insects. Cambridge University Press.
3. Hawksworth, DL (2010). Terms used in bionomenclature. GBIF.
4. Losos, JB (2013). The Princeton guide to evolution. Princeton University Press.
5. Singh, G. (2016). Plant Systematics: An Integrated Approach. CRC Press.