Reasons for a separate kingdom Protista
- Basically the kingdom Protista is defined by the principle of exclusion i.e. all the members have certain characteristics that exclude them from all other four kingdoms. The kingdom Protista is composed of a large number of primarily aquatic organisms. These organisms have different body forms, types of methods of reproduction, modes of nutrition, and lifestyles. It is difficult to characterize them in a single group. There was difficulty in placing these organisms in any of the kingdoms. So, all such organisms are placed in Protista.
- There is another reason for placing them in a separate kingdom. All the protists are eukaryotes. They have evolved from prokaryotes. All other eukaryotic kingdoms have their evolutionary origin in Protista (evolved from protists). The protists are the ancestors of all other eukaryotic organisms. So it becomes difficult to place them in any of the kingdoms. The other eukaryotic kingdoms, Plantae, Animalia, and Fungi have evolved from protists in different ways.
- The protists are unicellular, colonial, or simple multi-cellular organisms. The protists are eukaryotes like the other three eukaryotic kingdoms, Fungi, Plantae, and Animalia. But the protists do not form blastula and embryos which are formed by animals and plants. They are also different from the prokaryotic kingdom Monera. So they are placed in separate kingdoms.
The following biologist played their role in the formation of Kingdom Protista:
1. John Hogg
He proposed the kingdom Protoctista for microscopic organisms in 1861.
2. Ernst Haeckel
He suggested creating the kingdom Protista in 1866. He included in this kingdom bacteria and all other microorganisms which did not fit in the plant and animal kingdoms like amoeba. However, he separated the blue-green algae and bacteria (prokaryotes) from the nucleated protists. He placed them in a separate group called Monera, within the kingdom Protista.
3. Herbert Copeland
He separated the prokaryotes from the Protista and raised the prokaryotes (Monera) to the status of the kingdom in 1938.
4. Robert Whittaker
He formed five kingdom systems in 1969. He placed only unicellular eukaryotes in Protista.
5. Margulis and Schwartz
They modified the five-kingdom system in 1982. They also included colonial and simple multicellular eukaryotes in Kingdom Protista. Now kingdom Protista has all unicellular, colonial, and simple multicellular organisms. Protists or Protoctista is one of the five kingdoms.
Diversity Among Kingdom Protista
Evolution has produced diversity in the Protista. They show diversity in the following characters:
- Size and structure.
- Means of locomotion.
- Ways of obtaining nutrients.
- Interactions with other organisms.
- Mode of reproduction.
Due to this diversity, most biologists take protists as polyphyletic groups of organisms. The protists do not have a single common ancestor. Margulis and Schwartz have formed 27 phyla of all the diverse organisms of the Protista.
Major Groups of Kingdom Protista
Protista has the following four major groups:
- Protozoans (single-celled).
- Unicellular algae.
- Multicellular algae.
- Fungus-like protists: Slime molds (Myxomycetes) and water mold (oomycotes).