Protists are considered paraphyletic because they represent a group of eukaryotes that includes the common ancestor of protists but does not include all of the descendants of that ancestor. Specifically, protists exclude the multicellular kingdoms of animals, plants, and fungi, which evolved from within groups of protists. This makes protists a grouping based on exclusion rather than on containing all descendants of a single common ancestor.
Explanation of Paraphyly in Protists
- Protists are mostly unicellular or simple multicellular eukaryotes and were historically grouped as a kingdom (Protista) based on shared morphological traits like unicellularity.
- However, multicellularity evolved multiple times independently within eukaryotes, leading to animals, plants, and fungi, which descend from different protist lineages.
- Therefore, while protists include the common ancestor of these groups, they exclude the animal, plant, and fungal lineages, making them paraphyletic.
Modern Understanding
- In modern phylogenetic classifications, protists are not considered a natural, monophyletic group but are spread across various eukaryotic supergroups.
- The grouping of protists is practical but does not reflect a full evolutionary lineage as the group does not contain all descendants from a single stem species.
- The paraphyletic nature stems from the retention of ancestral (plesiomorphic) traits like unicellularity and exclusion of derived multicellular kingdoms.
In summary, protists are paraphyletic because they include some but not all descendants of their last common ancestor, excluding animals, plants, and fungi that evolved from within protist lineages.
