WHAT IS A PARAPHYLETIC GROUP? - BIOLOGY - 2025

The parafiléticos groups in phylogenetic taxonomy or cladística, are those that include a common ancestor and not all of his descendants. The largest group is said to be paraphyletic with respect to the excluded subgroup (s).

A paraphyletic group is not a clade. It is just a relative complement of one or more subclades within a clade. That is, it is not a natural group due to the exclusion of some element (s).

Example of paraphilia in reptilia (Reptiles). Taken and edited from evolutionism.org

The typical example of a paraphyletic group is that of the Reptilia (reptiles). This taxon contains the last common ancestor of reptiles and almost all of the descendants of that ancestor.

It includes all current organisms traditionally called reptiles, as well as all extinct synapsids. However, it excludes mammals and birds. Reptiles are then paraphyletic with respect to birds and mammals.

Controversies between cladist and evolutionary or traditional schools

According to the phylogenetic taxonomy, no descendant can be excluded from a group that contains their ancestors, for this group to be considered valid (monophyletic). If exclusion occurs, the result would be an unnatural (paraphyletic) group.

The school of evolutionary taxonomy explicitly requires that very different descendants of their ancestors must be included in separate groups. Both schools, however, often use the same terms, such as "monophyly," to designate different ideas.

Criteria

Evolutionary taxonomy then requires the consideration of two criteria: similarity and common ancestry for classification. These two criteria allow the taxa to be grouped and classified according to the Linnaean hierarchy system. Cladistics, for its part, accepts only one criterion, that is, common ancestry for the definition of taxa.

Rules

Evolutionary taxonomy has developed a series of standards, such as the International Code of Zoological Nomenclature. The cladists seem to want to use these tools, but under their own rules.

They accuse the codes of being too legalistic, as well as too permissive. In the first case, because it forces all taxa to fit into arbitrary hierarchical categories. In the second case, because it must apply to both monophyletic and paraphyletic groups.

Main differences

Basically, the difference between cladistic and evolutionary classifications is that the former accepts a single analytical method and a single criterion for classification, while the latter tries to incorporate several methods and accepts a combination or alternative use of the criteria for classification..

The former has the advantage of strict consistency and simplicity. The second has the advantage of better reflecting the diversity and complexity of evolutionary processes.

Some implications of the strict use of cladistics

If we accept the fact that only monophyletic groups should be considered valid, and refuse to exclude very different descendants of their ancestors, we could come to disturbing conclusions.

For example, we could say that we are all "bony fish." In effect, we are descendants of bony fish with lobed fins.

Parental groups, in some cases, have persisted along with their descendants. The strict application of monophyly as a criterion to carry out taxonomic rankings in such cases would be unmanageable.

It would simply artificially divide the older well-defined monophyletic groups, due to the increase in descendants. Or it would force you to create groups of descendants that contain parts of older groups.

That is, taxa defined according to the criterion of monophyly would not necessarily be more "natural" than paraphyletic taxa.

Grouping parental taxa with descendant taxa will create heterogeneous monophyletic taxa in many characters. Such taxa will not be easy to diagnose, which will reduce the feasibility of applying taxonomic tools.

The most prominent example is the breakdown of the traditional group "Reptilia", as well as the creation of the term "Bird Dinosaurs" for birds.

A strict application of monophyly to group taxa is thus problematic. Commonly used tree building methods result in too strong abstraction. Additionally, they allow an overly simplified visualization of evolutionary processes.

Some authors even point out that if paraphyletic taxa are rejected, the entire classification will collapse at the level of family, genus, and ultimately, species.

A possible solution"

Taxonomists Mayr and Bock proposed, in 2002, an alternative concept of "Darwinian" evolutionary classification. According to this, two criteria must be considered: similarity and common descent.

Therefore, the ordered grouping of groups of organisms into classes will be carried out according to their similarity with their "inferred evolutionary descendant". The incorporation of both criteria avoids the creation of clusters through the use of similarities derived from a parallel or convergent evolution.

However, the problem remains that allows the recognition of similarities of an older parental group that coexists in parallel with the derived group.

According to this proposal, then, monophyly would not be “the criterion” to be used in the definition of taxonomic groups, but would be one more tool.

This criterion could be used alternatively or in addition to other criteria. Subsequently, its form of use must be decided in case.

Some examples of paraphyletic groups

Prokaryotes (unicellular forms lacking a nucleus), are a paraphyletic group. The Eukaryotes (organisms with a true nucleus), descend from an ancestor that lacked a nucleus.

The "nucleusless" character is then plesiomorphic (ancestral) and the "nucleus" character is apomorphic (derived from the ancestral state). The exclusion of any group of organisms with nucleated cells from the group of Prokaryotes, then makes the latter group paraphyletic with respect to the excluded group.

According to recent studies, crustaceans are a paraphyletic group because it does not include the Hexapods (Insects). DNA analysis shows that terrestrial insects (Hexapoda) are more closely related to aquatic crustacea, than to terrestrial centipedes and millipedes (Myriapoda). The Hexapods would form a sister group to the Xenocarida (Cephalocarida and Remipedia).

Pancrustacea. Source: self made

The various phyla within the kingdom of fungi (Fungi) will be paraphyletic with respect to the polyphyletic group of the Deuteromycetes. Deuteromycetes, or imperfect fungi, are organisms whose sexual phase of reproduction is unknown.

It is an artificial group, which contains the species that have not been able to be located in the other groups of fungi because their classification is based mainly on characters present only in the sexual phase.

Until the "natural" taxonomic location of all the species still included within this taxon is elucidated, the remaining taxa cannot be considered monophyletic.

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. RK Brummitt (2006). Am I a bony fish? Taxon.
4. E. Hörandl (2006). Paraphyletic versus monophyletic taxa-evolutionary versus cladistic classifications. Taxon.
5. Paraphyly. On Wikipedia. Recovered from: en.wikipedia.org/wiki/Paraphyly
6. CJ Regier, WJ, Shultz, A. Zwick, A. Hussey, B. Ball, R. Wetzer, JW Martin & CW Cunningham (2010). Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature.