Why are protozoans described as a paraphyletic group?

Protozoans are described as a paraphyletic group because they do not represent a single, exclusive evolutionary lineage that includes a common ancestor and all of its descendants.12

In biological classification, a monophyletic group (or clade) consists of an ancestor and all of its descendants. In contrast, a paraphyletic group includes an ancestor and only some of its descendants, while excluding others. Because organisms that fit the contemporary, descriptive definition of a "protozoan" are found scattered across all major groups of protists, the term does not correspond to a single, natural evolutionary branch.21

Historically, protozoans were grouped based on shared morphological or physiological traits, such as being single-celled, motile, or heterotrophic. However, modern molecular data and phylogenetic analyses have revealed that these traits evolved independently multiple times across the tree of life. Consequently, many organisms once classified as protozoans are more closely related to multicellular groups—such as animals, plants, or fungi—than they are to other "protozoans," rendering the group paraphyletic.1345