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Why is the former kingdom Protista considered paraphyletic?

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A diagram showing the tentative evolutionary relationships among hominid species over the past four million years, with skull images and labeled species names connected by solid and dotted lines to indicate proposed evolutionary pathways. — phylogeny Tentative phylogenetic scheme for the evolution of the human lineage. Solid bars indicate the time ranges during which species are thought to have existed, based on fossil evidence. Dotted lines signify evolutionary relationships between hominin species that have been proposed on the basis of the fossil evidence.

The former kingdom Protista is considered paraphyletic because it does not include all the descendants of a common ancestor. In phylogeny, a monophyletic group (or clade) must contain an ancestor and every one of its descendants.12

Historically, the kingdom Protista was used as a "catch-all" category for eukaryotic organisms that did not fit neatly into the kingdoms of Plantae, Animalia, or Fungi. Because these diverse organisms—ranging from algae to protozoa—do not share a unique, exclusive evolutionary history that excludes the other eukaryotic kingdoms, the group is paraphyletic.341

Molecular evidence, particularly RNA sequencing, has revealed that many "protists" are more closely related to plants, animals, or fungi than they are to other protists. Consequently, many scientists have abandoned the use of the kingdom Protista in formal classification schemes because it fails to represent a natural, evolutionarily cohesive group.13