WHAT IS A MONOPHYLETIC GROUP? - BIOLOGY - 2024

A monophyletic group is a group of species that are related to each other through a unique descent history, that is, an ancestral species and all its descendants.

This term then designates a natural group. He opposes the terms polyphyletic and paraphyletic. The latter define artificial groups because they are incomplete (paraphyletic) or because they include descendants of different ancestors (polyphyletic).

Example of a monophyletic group. Taken and edited from coccinellidae.cl

Some authors argue that, being the only natural groupings, the mophyletic groups should be the only ones accepted. This point of view, however, is not unanimously shared by all taxonomists and systematists. Numerical taxonomy, for example, does not differentiate mono, para, or polyphyletic taxa.

The classification of organisms

Taxonomy is the science that is responsible for the classification of living things. According to this, organisms must be grouped into taxa that are mutually exclusive.

These taxa in turn are grouped into higher level taxa, also mutually exclusive for each of these levels or taxonomic categories.

In each taxon, organisms have attributes (characters) on which taxonomists rely to indicate their relationship with other organisms and thus delimit biological taxa.

There are different approaches (or schools) to evaluate and weigh the similarities (or differences) that exist between these characters and make the corresponding decisions.

Taxonomic schools

There are currently three main taxonomic schools:

Numerical or phenetic taxonomy

Proposed by RR Sokal and PHA Sneath in 1963. It is based on the similarity or dissimilarity of observable characters, without taking into account previous hypotheses about their phylogeny, to classify organisms.

All characters have the same "value" (global similarity), regardless of whether the similarities are due to homologies or homoplasies.

Evolutionary taxonomy

It is also known as traditional or Darwinian taxonomy. It uses phylogenetic relationships, parent-offspring relationships (serial descent), as well as the degree of evolutionary change to classify organisms.

It allows groups to be excluded from their parent taxa, considering paraphyletic taxa valid.

Phylogenetic or cladistic taxonomy

Proposed by Willie Hennig in 1966 in his book called Phylogenetic Systematics. It relies on shared derived similarities (homologies) or synapomorphies to establish evolutionary relationships between organisms.

It is the basis of most modern biological classification systems and seeks to group organisms by their evolutionary relationships. It only recognizes how valid monophyletic groups.

Controversy between schools

The phenetic taxonomy is currently followed, in its strict sense, by very few taxonomists, however, its tools are frequently used by either of the other two taxonomic schools.

According to Damien Aubert, the practice of systematic taxonomy has been hampered for too many years by profound differences about the foundations of this discipline.

Discrepancies

There are discrepancies about the type of information that should be incorporated or excluded in a proper classification of living things. Although the two main schools of systematics acknowledge evolution, they have opposite ideas.

Cladism asserts that the classification should only reflect the order in which the ramifications of the lineages occur on the tree of life.

Evolutionism, for its part, holds that the degree of modification, reflected as the length of the branches, must also be taken into consideration. According to this school, said length would reflect macroevolutionary jumps.

The cladistic school holds that no descendant of a group containing his ancestors should be excluded. For its part, evolutionary taxonomy explicitly requires that very different descendants of their ancestors must be included in separate groups.

Thus, both schools often use the same terms, such as "monophyly," to designate different ideas. This fact, according to Aubert, makes phylogenetic research globally erratic and the taxonomic classification, therefore, highly unstable.

Finally, we can infer that, if we want to perform an analysis to classify one or several taxa and use the postulates of the three schools separately, the results are most likely to be different.

Some basics

In order to properly understand the concept of monophyletic, you have to handle some basic terminology, according to the cladistic school, including:

Character: any observable attribute in an organism, whose different manifestations are called states, for example, presence of hair, feathers or scales; geographical distribution; behavior, etc.

Status of a character: each of the forms in which that character can be presented, either primitive or derived. For example, bipedal walking in humans is a derived condition (character), as opposed to 4-limb displacement (ancestral condition or character) of other hominids.

Plesiomorphic character: primitive or ancestral character shared by the entire monophyletic group.

Simplesiomorphy: plesiomorphy shared by two or more taxa.

Derived or apomorphic character: it is one that arises from the ancestral state, that is, it results from a transformation of the character within the group under study. It constitutes the beginning of a new clade.

Autapomorphy: non-shared derived character. It is present only in one taxon and is frequently used in microtaxonomy to differentiate species.

Synapomorphy: apomorphy or characteristic shared by two or more species or taxa.

Clado (monophyletic): group that includes an ancestral species and all its descendants.

Homology: condition of similarity due to the presence of a common ancestor.

Homologous character: similar characters or with different attributes, but that come from a common ancestral character.

Analogy: development of similar structures that fulfill the same function, but their embryonic origin is different.

Homoplasia: false similarity that is established by the presence of characters from different ancestors. It occurs by convergence, parallelism, or reversal.

Convergence: it is synonymous with analogy.

Parallelism: independent evolution of the same character state from the same ancestral character state.

Reversion: apomorphy that is subsequently lost (reverts to a plesiomorphic state) in any of the taxa of a monophyletic group.

Graphic representations according to taxonomic schools

Cladogram

The cladogram is the characteristic diagram of the cladistic school. In these, genealogical phylogenetic relationships are expressed, which must be natural or monophyletic, that is, they include the common ancestor and its descendants.

Cladogram illustrating the phylogenetic relationships between various groups of feathered dinosaurs. Taken and edited from Chiappe & Dyke (2002).

Phenogram

Phenograms are the diagrams that phenetic taxonomy uses to express classifications of organisms. This type of analysis accepts all three types of taxa: monophyletic, paraphyletic, and polyphyletic.

Although these diagrams are relatively similar to cladograms, they do not express phylogenetic relationships but rather apparent similarity or dissimilarity between organisms.

Phylogram or phyletic tree

The phylogenetic classifications proposed by the evolutionary or classical taxonomic school uses phyletic trees. These diagrams express genealogical relationships of descendant ancestors and accept two types of taxa: monophyletic and paraphyletic.

Philetic tree or phylogram proposed by Charles Darwin in "The Origin of Species". Taken and edited from es.wikipedia.org

References

1. D. Aubert (2015). A formal analysis of phylogenetic terminology: Towards a reconsideration of the current paradigm in systematics. Phytoneuron
2. D. Baum (2008). Reading a phylogenetic tree: The meaning of monophyletic groups. Nature Education
3. LM Chiappe & G. Dyke (2002). The mesozoic radiation of birds. Annual Review of Ecology and Systematics.
4. Cladistics. On Wikipedia. Recovered from: en.wikipedia.org/wiki/Cladistics
5. W. Hennig (1966). Phylogenetic Systematics. Univ. Of Illinois Press, Urbana
6. Monophyly. On Wikipedia. Recovered from: en.wikipedia.org/wiki/Monophyly
7. PA Reeves & CM Richards (2007). Distinguishing terminal monophyletic groups from reticulate taxa: Performance of phenetic, tree-Based, and network procedures. Systematic Biology