The differences between clades, taxa and monophyletic groups

What are the differences between these terms used in biology when studying phylogeny?

Various sources indicate that there are at least 8.7 million species of living beings on the planet. Of these, it is estimated that 86% of terrestrial and 91% of marine species are still to be discovered. To date, we only know of 1.3 million living beings, so we still have a long way to go.

Humans need tools to compartmentalize the enormous amount of information 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 organisms.

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

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 phylogenetics is the discipline of phylogenetics is the discipline of biology in charge of discovering these relationships..

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

In general, we can summarize the process in the following steps: tissue is extracted from a living being and its DNA, whether nuclear, plastidial (in plants) or mitochondrial, is isolated, amplified and subsequently sequenced. This DNA sequence is compared with those of its possible relatives and, by means of genetic homology analysis (i.e. similar sequences of the same evolutionary origin), a series of DNA sequences are generated with computer programs. a series of phylogenetic trees are generated with computer programs..

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

Differences between clades, taxa and monophyletic groups

This introduction was necessary, because it is impossible to understand the terms that concern us today without spending some time understanding what a phylogenetic tree is and how it is constructed. 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 consists of a species and all its descendants..

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

2. Taxon

Things get complicated, since we are dealing with words that may seem quite similar at first glance. A taxon is defined as a group of related organisms, which in a given classification have been grouped in a hierarchy of inclusion, where each level encompasses other (in general) lower levels. The fundamental taxonomic categories are, from highest to lowest: domain, kingdom, phylum, class, order, family, genus and species.. Let's take 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 defines us completely as a species. We are eukaryotic beings because we are formed by cells with a true nucleus, we are also chordate animals, since we present an embryo with characteristics common to the rest of animals and we are also hominid primates.

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

On the other hand, an artificial taxon is one that does not occur in nature, i.e., the individuals collected in such a taxon are not found in nature. individuals collected in such a taxon need not have a common ancestor.. An example of this are the protozoa, which have similar characteristics together but have very remote ancestors among them. This is a linguistic convention that allows us to group living beings in a sort of "catch-all" to better understand each other.

Thus, flowers with yellow petals may form their own taxon, or aquatic animals may be separated from terrestrial animals by an artificial taxonomic grouping. These living beings may not have common ancestors, but they are grouped together in order to understand a series of specific characteristics or a shared lifestyle.

3. Monophyletic group

A group of living beings is monophyletic if all the organisms included in it have evolved from a single ancestral population or species and all the 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 descendants of this. 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 is no half-measures here: clade and monophyletic group are synonyms. 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, with which they share a common ancestor, are left out. Since they do not resemble 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. Therefore, the reptile group lacks a common ancestor, the reptile group 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 having direct common ancestors, due to the common characteristics and lifestyles they have among them.

So: what makes them different?

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

On the other hand, we emphasize the term "complete natural taxon".. A taxon does not always have to 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 and predictive classifications. This is also the case of artificial taxa (polyphyletic groups), among which we find the groups of algae or protozoa already mentioned.

Summary

You may get a headache after so much terminology and convoluted concepts, but the general message is simple: a clade and a monophyletic group can be considered synonyms, whereas a taxon does not always have to correspond to the cladeSometimes modifications are made in order to generate groupings that are more intuitive and easier to understand.

This is the case of the aforementioned reptiles, for example. Instead of reptiles and birds separated into two distinct groups, it would be more correct to speak of the clade sauropsida (modern reptiles+birds), since it is a monophyletic group with a common ancestor. Are birds reptiles, then? No. They are both sauropsids, some flying and some not.

Bibliographical references:

• The number of species on the planet is estimated at 8.7 million, BBC News. Retrieved November 8 from https://www.bbc.com/mundo/noticias/2011/08/110824_especies_censo_am.
• Clado, chili etymologies. Retrieved November 8 from http://etimologias.dechile.net/?clado.
• Phylogenies, sesbe.org. Retrieved 8 November from https://www.sesbe.org/evosite/evo101/IIBPhylogeniesp2.shtml.html#:~:text=A%20clade%20is%20a%20grouping%20aggregation%20C3%B3n,has%20been%20can%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. Retrieved November 8 from 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%20birds%20are%20reptile%3F,system%20taxon%C3%B3mico%20de%20Carlos%20Linn%20C3%A9.
• Taxon, EcuRed. Retrieved November 8 from https://www.ecured.cu/Tax%C3%B3n.
• WHAT IS MOLECULAR PHYLOGENY AND WHAT IS IT FOR?, Manual for molecular phylogenetic analysis. Retrieved November 8 from https://formacion.uam.es/pluginfile.php/122717/mod_resource/content/2/1_W_Tema_1.pdf