Are Protists Monophyletic? 5 Facts You Should Know


In the 5-kingdom classification of the modern classification system, are “Protists” monophyletic? It is an umbrella term describing all single cell organisms. It can be further divided into prokaryotes and eukaryotes.

Protists are usually classified by their mitochondria, molecular similarities, trophic habits, process of locomotion etc. Phylogeny of protists are constantly changing as we gain more information.

What are monophyletic groups?

Monophyletic groups are a set of organisms that are derived from any single common ancestor.

In other words, monophyletic taxon is a ‘natural group’ of species that are related to one another through a single evolutionary lineage. The presence of homologous characters is often indication of existing common ancestor that shared the same characteristics.

e.g., Mammalia and Aves are monophyletic taxa. Humans, ape, new world monkeys are also monophyletic taxa.

are protists monophyletic
Fig: “A depiction of monkeys and Old World monkeys defined in a monophyletic way” by JoePhin is licensed under Creative Commons Attribution-Share Alike 4.0 International

What are polyphyletic groups?

Species that are grouped under polyphyletic taxa are descendants of multiple different ancestors and do not share evolutionary lineage.

Polyphyletic taxa are considered to be an ‘unnatural group’ of organisms that share same characteristic as a result of convergence of characteristics as a result of parallel development. Organisms are generally grouped in the polyphyletic taxa based on their morphological similarities.

e.g., various aquatic mammals such as, walrus, seals, dolphins, manatees, whales etc. all stemmed from different ancestors, following specific evolutionary lineages.

are protists monophyletic
Fig: “A cladistic depiction of marine mammals in a polyphyletic way” by Peter Southwood is licensed by  Creative Commons Attribution-Share Alike 4.0 International.

Difference between monophyletic and paraphyletic groups-

Organisms that are characterized as monophyletic or polyphyletic group is divided based on the presence of their lineage of common ancestor and all their descendants.

Monophyletic group consist most recent ancestor and all its descendants whereas, polyphyletic group is made of organisms from different phylogeny.

 Monophyletic taxaPolyphyletic taxa
AncestorsThey share a common ancestor.They are made of unrelated organisms that do not share same common ancestors.
Evolutionary explanationBased on synapomorphic characters, I.e., homology derived from common ancestors.Based on convergence of characters showing incomplete homology because the phylogeny in different.
NatureIt is a natural group of organisms.It is an unnatural assembly of organisms.
Fig: A table of differences between monophyletic taxa and polyphyletic taxa.

Why are protists not monophyletic?

Monophyletic taxa are made of ancestral species together with their subsequent descendants. Whereas kingdom protists consist all single cell organisms, both eukaryotes and prokaryotes.

Despite being unicellular they do not have many things in common nor do they share common ancestry.

Why are all protists polyphyletic?

Although classified as a kingdom, organisms that are called protist are not necessarily closely related through common evolutionary lineage, and are spread across the ‘phylogenetic tree of life’.

Some protists are rather closely related to different multicellular complex organisms than they are to the other single cell organisms that are also classified as protists. E.g., slime molds share common ancestry with animals and fungi; red and photosynthetic algae are much closely related to land plants than they are to brown algae; dinoflagellates are closer to ciliates and apicomplexans than they are to choanoflagellates; amoeboid share common ancestry with plasmodial slime molds.

are protists monophyletic
Fig:A cladistic view of the eukaryotic tree of life with relations between supergroups” by CNX OpenStax is licensed under the Creative Commons Attribution 4.0 International.

Conclusion:

Protists are an unnatural assemblage of different organisms irrespective of their phylogenetic lineage. With the help of genomics and other characteristic protists are now subdivided into supergroups for better access, although their phylogenetic accuracy is debatable.

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