Classification schemes are used by biologists to place the huge number of organisms on Earth into natural groupings. Ideally, these groupings are made by taxonomists on the basis of shared distinguishing features. Today taxonomists use such features as anatomy, developmental stages, and biochemical similarities to categorize organisms. Early classification schemes placed all organisms into either the plant or animal kingdom. Later, close examination of the unique structure of fungi and the diversity of single-celled organisms made it necessary to propose additional kingdoms that recognized the fundamental differences among plants, animals, fungi, and unicellular prokaryotes (organisms whose cells do not have a distinct membrane- bound nucleus) and eukaryotes (organisms whose cells have a distinct membrane- bound nucleus). The current scheme consisting of five kingdoms— Monera, Protista, Animalia, Fungi, and Plantae—was devised in response to this need.
Scientists do not know how many species share our world. Each year 7000 to 10,000 new species are named. The total number of named species is currently around 1.4 million. However, many scientists believe that 7 million to 10 million species may exist, and estimates range as high as 30 million. Of all of the species that have been identified, about 5 percent are in the Monera and Protista kingdoms. An additional 22 percent are plants and fungi, and the rest are animals. This distribution has little to do with the actual abundance of these organisms and a lot to do with the size of the organisms, how easy they are to classify, and the number of scientists studying them.
The kingdom Protista, defined as comprising all single-celled eukaryotic organisms, is not a natural grouping, and scientists disagree about which organisms it should include. Plants, animals, and fungi all have close protistan relatives, and the separation of single-celled organisms from multicellular organisms is sometimes problematic. It is especially so for the algae, which have both single-celled and multicellular representatives within most smaller taxonomic groupings. Can closely related organisms be placed into separate kingdoms, Protista and Plantae, simply on the basis of multicellularity? If you look at different textbooks, you will see that the algae, photosynthetic organisms with simple reproduction，are sometimes placed entirely into Protista, and sometimes they are split between Protista and Plantae depending on whether they are single celled or multicellular. Some taxonomists split the multicellular algae into two kingdoms, placing the multicellular brown and red algae with the protists and the multicellular green algae into the plant kingdom. These different attempts to classify closely related organisms are good examples of how difficult it is to develop standard criteria for grouping organisms, even at the kingdom level.
One approach to this problem, enthusiastically endorsed by Lynn Margulis, a biologist at the University of Massachusetts, is the creation of the kingdom Protoctista. This taxonomic category would include single-celled organisms and their close descendants (for example, the multicellular algae but not the animals, fungi, and plants). Margulis describes the kingdom Protoctista as “the entire motley and unruly group of nonplant, nonanimal, nonfungal organisms representative of lineages of the earliest descendants of the eukaryotes.”
It is conceptually difficult to group one of the largest multicellular organisms in the world, the brown algae called giant kelps, with simple microscopic single-celled organisms. Kelps, some of which are up to 60 meters long, possess a tissuelike level of organization that is relatively complex and can transport materials over long distances, as can the tissues of higher plants. The cells in kelps and some other algae are specialized and show division of labor. However, kelps reproduce like other algae and differently from plants. Thomas Cavalier-Smith of the University of British Columbia has proposed that brown algae merit their own kingdom (kingdom Chromista) based on ultrastructural features and molecular comparisons of all algae. So, even among the algae, there are clear differences that some scientists believe are sufficient to justify the status of a separate kingdom.
As we learn more about the relationships between organisms and refine the criteria used to classify them, classification schemes will change. As the superficially simple question “In which kingdom should we place the algae?” illustrates, the taxonomic categories in textbooks are tentative and subject to revision as we continue to discover more about life on Earth.
