THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

17. Protists: Advent of the Eukaryotes

17.4. Classifying the Protists

Protists are the most diverse of the four kingdoms in the domain Eukarya. The 200,000 different forms in the kingdom Protista include many unicellular, colonial, and multicellular groups. Protists were the first cells to contain a nucleus, as described in section 17.1—indeed, an organized internal membrane system that creates organelle compartments is the key feature that distinguished protists and other eukaryotes from archaea and bacteria. The evolution of early protists, like the fossil algae seen in figure 17.7, was one of the most important steps in life’s evolutionary journey.

Figure 17.7. Early eukaryotic fossil.

Fossil algae that lived in Siberia 1 billion years ago.

Probably the most important statement we can make about classifying the Kingdom Protista is that it is an artificial group; as a matter of convenience, single-celled eukaryotic organisms have typically been grouped together into this kingdom. This lumps many very different and only distantly related forms together. A taxonomist would say that the kingdom Protista is not monophyletic—that it contains many groups that do not share a common ancestor.

Traditionally, biologists have grouped protists artificially into functionally related categories, much as was done in the 19th century. Protists were typically grouped into photosynthesizers (algae), heterotrophs (protozoa), and absorbers (funguslike protists).

New applications of a wide variety of molecular methods are providing important new insights into the evolutionary relationships among the different groups of protists. For the first time, we can begin to see how many are related. Molecular taxonomists have assigned 12 of the 17 major protist phyla to 7 monophyletic groups, or “clades” All members within each clade share the same common ancestor. The seven groups can be tentatively arrayed on the protist phylogenetic tree as pictured in figure 17.8, although much remains uncertain. The lineages connecting the seven groups may change as we learn more.

Figure 17.8. The major protist clades.

Five major protist phyla cannot yet be placed on this tree with any confidence, but they are described in table 17.1, along with the other protist phyla. These five phyla include some of the more familiar protists such as amoebas. As researchers carry out more detailed DNA-level comparisons, our understanding of these five groups will increase.

TABLE 17.1. KINDS OF PROTISTS

It seems likely that over the next few years, the traditional “matter of convenience” kingdom Protista will be replaced by a more illuminating arrangement that expresses the evolutionary relationships among the members of this very diverse kingdom.

Key Learning Outcome 17.4. 12 of the 17 protist phyla can be assigned positions on the protist phylogenetic tree; the relationship of five others is still being worked out.