In today’s world of extremely vast biological diversity, it has become super hard to maintain the pace with the various biological studies.
With such a cause, the need for a proper and well-defined classification system is a must for properly categorizing the biological world.
There are millions of species on this planet earth. For example, if you just see at the largest Phylum Arthropoda you will find that it alone has millions of species in it.
So, not just Arthropoda, there are 36 recognized animal phyla, of which nine i.e. Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata contain the vast majority of described, existing species.
To study all of them properly, systematically, and with the proper arrangements, we need some system for categorizing them, based on some characters.
This can only be achieved when we follow the classification system of the biological world in a very orderly manner.
The 5 Kingdom Classification system proposed by R.H. Whittaker in 1969 is widely used in today’s world of biological studies.
So, in order to get an idea about this 5 Kingdom Classification system, here are lists of Merits and Demerits that can be used to know about its impact to the biological classification system.
- List of Merits of Five Kingdom Classification System:
- List of Demerits of Five Kingdom Classification System:
- 1. Algae and Protozoa is included under Kingdom Protista
- 2. Viruses, Viroids, Prions, and Lichens are not included in Five Kingdom Classification System
- 3. Archaebacteria differs from other Bacteria
- 4. Diverse groups are difficult to be kept together
- 5. Mycloplasma is different from Bacteria
- 6. Limitations in considering Phylogenetic Relationships to one common evolutionary ancestral group
List of Merits of Five Kingdom Classification System:
1. Drawbacks of Two Kingdom Classification System
Due to the limitations and drawbacks of the Two Kingdom Classification System proposed by Linnaeus in the year 1735.
It was seen that this classification system was unable to accommodate certain living beings as they neither had properties of animals and plants.
With the Two Kingdom Classification System, it was harder classify plants and animals as the first formed animals were neither plants nor animals.
Moreover, this system does not differentiate between unicellular & multicellular organisms, between eukaryotes & prokaryotes, and also between autotrophic & heterotrophic organisms.
With, this system of classification it was not possible to include every set of characters in a particular kingdom.
So, with these things in mind, there was an extreme need for a new system of Classification. Thus, the Five Kingdom Classification System came into place.
2. Separation of Prokaryotes from Eukaryotes
We all know that there are a lot of major structural advantages that Eukaryotes have over the prokaryotes.
Prokaryotes are very simple whereas, the Eukaryotes are evolved from the Prokaryotes and are a bit complex.
So, a proper comparison and classification of Prokaryotes from Eukaryotes is a must to carry out further biological studies into a new level.
This was made possible by the Five Kingdom Classification System which has well-classified both prokaryotes and eukaryotes into well-defined boundaries.
3. Introduction to Kingdom Protista
In the Five Kingdom classification system, there is Kingdom Protista which includes the unicellular, eukaryotic organisms.
It was introduced because in this vast biological world there are a vast majority of species that are transition and intermediate in nature that show the characteristics of both plants and animals but can’t be considered true plants or animals.
The species which can’t be declared as plants or animals or any other multicellular eukaryotic organisms, and so they are the intermediate forms. These are all included in the Kingdom Protista.
The presence of Kingdom Protista has made the modern-day classification system a lot of systemic and confusion-free as well.
4. Separation of fungi into an all-new Kingdom Fungi
Separation of Fungi (which was earlier included in the Plants Kingdom) was done in Kingdom Fungi with an all-new introduction in the Five Kingdom Classification System.
This Merit is one of the most prominent characteristics of the Five Kingdom Classification System by R.H. Wittaker.
Initially when the Fungi group was included in the Plants Kingdom, it created a lot of confusions during the classification of species.
This confusion was created because the fungi have a lot of differences from those of the plants. For example: One basic difference was that the Fungi cell wall was very much different from the cell walls of the plants.
In addition to that, it was seen that the biological and the morphological characteristics of Fungi was very much different from the other Eukaryotic organisms, and very specifically different from those of the plants.
5. Important criterias for classification
The Five Kingdom Classification System is currently in use because of its consideration of the various important criteria that very finely classify the vast biological world.
The main criteria for classification used by R.H. Whittaker are Cell Structure, Thallus Organization, Mode of Nutrition, Reproduction, and Phylogenetic Relationships.
Thus due to the consideration of such criteria, this Five Kingdom Classification System could accomodate more set of characters purifying the various confusions caused during the classification of species.
As a result, it was a lot easier to categorize all living beings with proper systemic position.
6. It opened the way for a better system of classification in the future
With the introduction to the Five Kingdom Classification System, an all new set of organization of the living organism came into place.
This has led to the classification of biological world into very well-defined boundaries considering various new characteristics into point. This has opened future options for better classification system.
Bacteria can be autotrophic as well as heterotrophic. It was a difficult question to categorize them either under plants and animals.
Thus, a separate Kingdom Monera was the most logical choice. Now so, all the future classifications of bacteria can be done easily under the Kingdom Monera.
Similarly, viruses are a borderline case between the living and non-living world. When they don’t get a host, they are non-living. Once they get inside their host, they become living.
Although the 5 Kingdom Classification does not have room for viruses but it opened the way for a better and more inclusive system of classification for the future.
List of Demerits of Five Kingdom Classification System:
1. Algae and Protozoa is included under Kingdom Protista
We all know that Protozoans are unicellular eukaryotes, but this statement doesn’t fit well in the case of algae which are also included in Kingdom Protista (are unicellular) and some in Kingdom Plantae (are multicellular).
The dermit here is that the common name ‘algae’ is used but these are considered differently in two different kingdoms.
Unicellular algae are kept in the kingdom Protista, whereas algae-like multicellular organisms are kept in Kingdom Plantae.
So, here it can be seen that the proper well-defined distinction between unicellular and multicellular organisms is not possible in the case of algae.
Some scientists do not agree that algae and protozoa should be placed into the same kingdom. They also say that it is not logical to consider algae in plants as well.
These scientists believe that all the algae should be considered into an all-new separate Kingdom.
2. Viruses, Viroids, Prions, and Lichens are not included in Five Kingdom Classification System
Viruses did not find a place in the classification system since they are not true living and they don’t have a proper cell structure. It’s because when they don’t get a host, they are non-living and, once they get inside their host, they become living.
Viroids are also not included because viroids are the smallest infectious pathogens that are even smaller than viruses. They are naked, circular, single-stranded RNA molecules that do not encode protein yet replicate autonomously when introduced into host plants.
Viruses are usually units consisting of nucleic acids and coat proteins called capsids. Viroids consist only of RNA, i.e. they contain no protein at all.
Prions are also not included. Prions are those proteins that can cause various diseases in humans and animals by causing several deadly transmissible diseases in humans and many other animals.
Prions are those various misfolded proteins that are finely able to transmit their misfolded shape into the various normal variants of the same protein. Prions convert the various correctly-folded proteins into the disease-causing forms.
Symbiotic associations are also not considered in this classification system. For example, lichens are organisms that are formed by the symbiotic association between fungi and algae.
Without Viruses, Viroids, Prions, and Lichens the biological classification system is yet to completed and can’t be considered a well-defined one. That’s why it’s a major demerit.
3. Archaebacteria differs from other Bacteria
Archaebacteria differs from other Bacteria (Eubacteria) which can be concluded according to modern-day scientific experiments.
Archaebacteria differs a lot from normal bacteria in many of its cases like they are physiologically also different. Archaebacteria differ from other bacteria in structure, composition, and physiology.
Archaebacteria are not true bacteria and are found only in harsh habitats like salty areas, hot springs, marshy areas, etc. Whereas, Eubacteria are true bacteria that are found in abundance in nature and these are unicellular prokaryotic microscopic cells.
The main demerit is that both these different types of bacteria are included under the same Kingdom Monera, which seems to be not properly classified.
4. Diverse groups are difficult to be kept together
There is so much diversity in the biological world that makes it super difficult to make such a classification where a single group contains organisms with the same kind of characteristics.
Biological diversity creates diverse groups that are super difficult to be kept together.
For example, Kingdom Monera and Protista contain both walled and wall-less organisms, photosynthetic and non-photosynthetic organisms, cellular or filamentous organisms.
If we are to classify anything, then it’s logical that we should always take care that the dissimilar organisms must not be kept in the same group.
But, due to the huge biological diversity, the Five Kingdom Classification system didn’t cater to this need up to a maximum extent which is a huge demerit.
5. Mycloplasma is different from Bacteria
Mycoplasma is quite different from bacteria where they have been placed along with prokaryotes under Kingdom Monera.
Mycoplasma can live independently with no cell wall, but other prokaryotes need the cell wall.
Their lacking of cell walls makes them naturally resistant to the various antibiotics that target cell wall synthesis. This makes them parasitic or saprotrophic in nature.
These Mycoplasmas are the smallest prokaryotic microbial cells. They can be easily extracted or isolated from the normal human and animal respiratory mucosa.
Mycoplasma is added as a genus of bacteria in this Classification System and it lacks a cell wall around its cell membranes.
This characteristic makes them naturally resistant to antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic.
Although Mycoplasma is similar to bacterias, there are a lot of dissimilarities between Mycoplasma and Bacteria as well. So, Mycoplasma is a type of “atypical” bacteria.
This makes it improper to add it under the Kingdom Monera with other prokaryotes.
6. Limitations in considering Phylogenetic Relationships to one common evolutionary ancestral group
Due to the diverse groups and polyphyletic conditions, many organisms cannot be considered Phylogenetically Related to one common evolutionary ancestral group.
Polyphyletic means that a group of organisms are derived from more than one common evolutionary ancestor or ancestral group and therefore not suitable for placing in the same taxon.
Similar organisms that show Phylogenetic Relationships are kept far from each other. For example, Unicellular and Multicellular algae.
With a very few exceptions, you will find that all organisms other than those in Kingdom Monera shows that they are Polyphyletic in origin.
It is a great shortcoming of five kingdom classification.