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Protists do not form a natural group, or clade, but are a polyphyletic grouping of several independent clades that evolved from the last eukaryotic common ancestor. Protists were historically regarded as a separate taxonomic kingdom known as Protista or Protoctista.
The former kingdom Protista is included within the domain Eukaryota (along with plants, animals, and fungi of course), and the reason it is no longer recognized as a kingdom is because we can now very clearly see that it does not refer to a natural group.
Protists are eukaryotic organisms belonging to the Kingdom Protista. Protists are highly diverse and do not have much in common with one another. They are grouped together because they don’t fit into any other kingdom; put simply, protists are all the eukaryotes that are not animals, plants, or fungi.
This particular eukaryote is one of the smallest, simplest organisms in the domain, called a protist. It’s scientific name is Giardia lamblia. As a human parasite, it can make us sick.
Under Copeland’s arrangement, the kingdom Protista thus consisted of nucleated life that was neither plant nor animal. The following decade he revived the name Protoctista, using it in favour of Protista.
For this reason, protist lineages originally classified into the kingdom Protista have been reassigned into new kingdoms or other existing kingdoms. The evolutionary lineages of the protists continue to be examined and debated.
Since then, the kingdom Protista has been refined and redefined many times. Different organisms moved in and out (notably, bacteria moved into a taxonomic kingdom of their own).
For this reason, protist lineages originally classified into the kingdom Protista have been reassigned into new kingdoms or other existing kingdoms. The evolutionary lineages of the protists continue to be examined and debated.
In the span of several decades, the Kingdom Protista has been disassembled because sequence analyses have revealed new genetic (and therefore evolutionary) relationships among these eukaryotes. …
The kingdom Protista has been abandoned by many taxonomists and scientists because it is a paraphyletic group, meaning it does not include all descendants of a common ancestor.