The differences between clade, taxon and monophyletic group

Various sources suggest that there are at least 8.7 million species of living beings on the planet. Of all of them, it is estimated that 86% of the terrestrial and 91% of the marine ones remain to be discovered. As of today, we only know of 1.3 million living beings, so we still have a long way to go.

Human beings require tools to compartmentalize the enormous amount of information that we collect and the physiological variety that surrounds us, and phylogenetics is a discipline of biology that helps us to do so in the field of living beings.

Unfortunately, we are dealing with a tool that is complex to understand and, therefore, it's normal for the average citizen to get lost in terms like "clade", "taxon", "monophyletic group" and many other words with complex connotations. That is why we are here, because we will see what the differences are between these terms.

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The importance of phylogeny

We cannot describe these terms without first making special mention of phylogeny and phylogenetics. Phylogeny is defined as the kinship relationships between species and, for its part,

Phylogenetics is the discipline of biology responsible for discovering them..

In other times, these phylogenetic relationships were inferred from morphological characters and to a lesser extent anatomical and chemical measure, since there was no other way to relate living beings beyond patterns observables. Today and after the discovery of PCR (Polymerase Chain Reaction), much more reliable relationships can be established.

In general, we can summarize the process in the following steps: the tissue of a living being is extracted and its DNA is isolated, whether nuclear, plastid (in plants) or mitochondrial, it is amplified and subsequently sequence. This DNA sequence is compared with those of its possible relatives and, through the analysis of genetic homologies (that is, similar sequences for the same evolutionary origin) a series of phylogenetic trees are generated with computer programs.

This type of diagram presents the evolutionary relationships between the groups of living things that are being analyzing, taking the common ancestor as the base or trunk and the different species as subsequent ramifications. It should be noted that, in many cases, these trees are not completely reliable and there is no single option to take as a dogma. We are facing a series of more or less probable hypotheses, but in very few cases are definitive facts.

Differences between clade, taxon and monophyletic group

This introduction was necessary, since it is impossible to understand the terms that concern us today without spending a good deal of time understanding what a phylogenetic tree is and how it is built. Once we have paved the way, we present each of the terms separately and then discuss the main differences between them.

1. clade

A clade refers in biology to a group of living beings formed by a phylogenetic branch that is made up of a species and all its descendants.

If we make a single "cut" in a phylogenetic tree in a grounded way, we will include the common ancestor (at the base) and all its descendants in subsequent branches. All these species encompassed as a result of the common ancestor form a single branch on the tree of life.

2. taxon

Things get complicated, because we are dealing with words that may seem quite similar at first glance. For its part, a taxon is defined as a group of related organisms, which in a classification given have been grouped into an inclusion hierarchy, where each level encompasses other minor ones (in general). The fundamental taxonomic categories are, from highest to lowest: domain, kingdom, phylum, class, order, family, genus, and species.. Let's give an example:

Human being: Domain Eukaryota- Kingdom Animalia- Phylum Chordata- Class Mammalia- Order Primates- Family Hominidae- Genus Homo- species Homo sapiens.

Thus, this taxonomic classification completely defines us as a species. We are eukaryotic beings because we are made up of cells with a true nucleus, we are also chordate animals, since we present an embryo with characteristics common to other animals and we are also hominid primates.

The key to a taxon, unlike a clade, is that it may or may not be natural.. A natural taxon follows the guidelines of a clade, since it only represents the living things that are found within a branch of the phylogenetic tree of life, that is, that they come from a common ancestor and present evolutionary relationships clear.

On the other hand, an artificial taxon is one that does not occur in nature, that is, that individuals listed in such a taxon need not have a common ancestor. An example of this is the protozoa, which have similar characteristics together but have very remote ancestors from each other. It is a linguistic convention that allows us to group living beings in a sort of “mixed bag” so that we understand each other better.

Thus, the flowers that present the color of their yellow petals can form their own taxon, or the aquatic animals can be separated from the terrestrial ones by means of an artificial taxonomic grouping. These living beings may not have common ancestors, but they do group together in order to understand a series of specific characteristics or a shared lifestyle.

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3. monophyletic group

A group of living things is monophyletic if all the organisms included in it have evolved from a single ancestral population or species. and all descendants are within this group. It is necessary to differentiate it from two other terms that usually accompany it:

Paraphyletic group: includes the common ancestor of all members, but not all of its descendants. Polyphyletic group: does not include the most recent common ancestor of the groups. It is constituted by an artificial selection of branches of the evolutionary tree.

There are no half measures here: clade and monophyletic group are synonymous. Similarly, a paraphyletic group is a clade from which a group has been subtracted for explanatory or scientific purposes. For example, reptiles are a paraphyletic group, since birds are left out, with which they share a common ancestor. Not resembling the rest of the animals in this group, it has been decided to create an artificial split that does not respond to the fidelity of the evolutionary tree. Thus, the group of reptiles lacks taxonomic validity from a strict point of view.

On the other hand, and building more bridges, a polyphyletic group could also be considered an artificial taxon. Using the same example as before, protozoa are selected from different branches of the evolutionary tree without have direct common ancestors, due to the common characteristics and lifestyles that they present among themselves they.

So: what differentiates them?

If you came looking for discrepancies, you may be disappointed. A clade, a complete natural taxon and a monophyletic group come to express the same thing: a common ancestor and all its descendants.

On the other hand, we emphasize the term "complete natural taxon". A taxon should not always correspond to a clade because, as we have seen, there are researchers in the world of taxonomy who have reasons to propose and use paraphyletic groups that are more intuitive than the actual monophyletic groups that encompass them, thus generating more useful classifications and predictive. This is also the case of artificial taxa (polyphyletic groups), among which we find the groups of algae or protozoa already named.

Summary

You may have a headache from all the convoluted terminology and concepts, but the general message is simple: a clade and a monophyletic group can be considered synonymous, while a taxon does not always have to correspond to the clade, since sometimes modifications are made in order to generate more intuitive and easy-to-understand groupings.

This is the case of the already named reptiles, for example. Instead of reptiles and birds separated into two different groups, the most correct thing to do would be to talk about the clade sauropsida (modern reptiles + birds), since it is a monophyletic group with an ancestor common. Are birds reptiles, then? No. Both are sauropsids, some are flying and some are not.

Bibliographic references:

• They calculate the number of species on the planet at 8.7 million, BBC News. Collected on November 8 in https://www.bbc.com/mundo/noticias/2011/08/110824_especies_censo_am
• Clade, etymologies of chili. Collected on November 8 in http://etimologias.dechile.net/?clado
• Phylogenies, sesbe.org. Collected on November 8 in https://www.sesbe.org/evosite/evo101/IIBPhylogeniesp2.shtml.html#:~:text=Un%20clado%20es%20una%20agrupaci%C3%B3n, you have%20pruned%20form%20a%20clade.
• If birds are dinosaurs, then are birds reptiles? And why from the points of view of the Linnean and Cladistic classifications? Quora Magazine. Collected on November 8 in https://es.quora.com/Si-los-p%C3%A1jaros-son-dinosaurios-entonces-las-aves-son-reptiles-Y-por-qu%C3%A9-desde-los-puntos-de-vista-de-las-clasificaciones-Linneana-y-Clad%C3%ADstica#:~:text=%C2%BFLas%20aves%20son%20reptiles%3F, system%20taxon%C3%B3myco%20of%20Carlos%20Linn%C3%A9.
• Taxon, EcuRed. Collected on November 8 in https://www.ecured.cu/Tax%C3%B3n
• TOPIC 1. WHAT IS A MOLECULAR PHYLOGENY AND WHAT IS IT FOR?, Manual for molecular phylogenetic analysis. Collected on November 8 in https://formacion.uam.es/pluginfile.php/122717/mod_resource/content/2/1_W_Tema_1.pdf