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  Section: General Zoology » The Diversity of Animal Life
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Major Divisions of Life

Classification and Phylogeny of Animals
Linnaeus and the Development of Classification 
Taxonomic Characters and Phylogenetic Reconstruction 
Theories of Taxonomy 
Major Divisions of Life 
Major Subdivisions of the Animal Kingdom 

Major Divisions of Life
From Aristotle’s time to the late 1800s it was traditional to assign every living organism to one of two kingdoms: plant or animal. However, the twokingdom system had serious problems. Although it was easy to place rooted, photosynthetic organisms such as trees, flowers, mosses, and ferns among the plants and to place foodingesting, motile forms such as insects, fishes, and mammals among the animals, unicellular organisms presented difficulties (Protozoan Groups). Some forms were claimed both for the plant kingdom by botanists and for the animal kingdom by zoologists. An example is Euglena, which is motile, like animals, but has chlorophyll and photosynthesis, like plants. Other groups, such as bacteria, were rather arbitrarily assigned to the plant kingdom.

Several alternative systems have been proposed to solve the problem of classifying unicellular forms. In 1866 Haeckel proposed the new kingdom Protista to include all single-celled organisms. At first the bacteria and cyanobacteria (blue-green algae), forms that lack nuclei bounded by a membrane, were included with nucleated unicellular organisms. Finally, the important differences between the anucleate bacteria and cyanobacteria (prokaryotes) and all other organisms that have membrane-bound nuclei (eukaryotes) were recognized. In 1969 R. H. Whittaker proposed a five-kingdom system that incorporated the basic prokaryote- eukaryote distinction. The kingdom Monera contained the prokaryotes. The kingdom Protista contained the unicellular eukaryotic organisms (protozoa and unicellular eukaryotic algae). Multicellular organisms were split into three kingdoms on the basis of mode of nutrition and other fundamental differences in organization. The kingdom Plantae included multicellular photosynthesizing organisms, higher plants, and multicellular algae. Kingdom Fungi contained the molds, yeasts, and fungi that obtain their food by absorption. Invertebrates (except the protozoa) and vertebrates compose the kingdom Animalia. Most of these forms ingest their food and digest it internally, although some parasitic forms are absorptive.

All of these different systems were proposed without regard to the phylogenetic relationships that are needed to construct evolutionary or cladistic taxonomies. The oldest phylogenetic events in the history of life have been obscure, because the different forms of life share very few characters that can be compared among them to reconstruct phylogeny. Recently, however, a cladistic classification of all life forms has been proposed based on phylogenetic information obtained from molecular data (the nucleotide base sequence of ribosomal RNA, Figure 10-11). According to this tree, Woese, Kandler, and Wheelis (1990) recognized three monophyletic domains above the kingdom level: Eucarya (all eukaryotes), Bacteria (the true bacteria), and Archaea (prokaryotes differing from bacteria in membrane structure and ribosomal RNA sequences). They did not divide the Eucarya into kingdoms, although if we retain Whittaker’s kingdoms Plantae, Animalia, and Fungi, the Protista become a paraphyletic group (Figure 10-12). To maintain a cladistic classification, the Protista must be broken up by recognizing as separate kingdoms the Ciliata, Flagellata, and Microsporidia as shown in Figure 10-11, and phylogenetic information must be gathered for additional protistan groups, including the amebas. This taxonomic revision has not been made; however, if the phylogenetic tree in Figure 10-11 is supported by further evidence, revision of the taxonomic kingdoms will be necessary.
Some Evolutionary relationships among some major groups of living organisms as inferred from ribosomal RNA
Figure 10-11
Some Evolutionary relationships among some major groups of living organisms as inferred from ribosomal RNA
sequence comparisons, and used by Woese, Kandler and Wheelis (1990) to recognize domains Archaea, Bacteria
and Eucarya. Exact relationships among major lineages of Eucarya are uncertain; more recent data suggest that
choanoflagellates and fungi may be the closest phylogenetic relatives of animals, but this result is not well supported
statistically. Data are not available for all groups of organisms.

Until a few years ago, the animallike protistans were traditionally studied in zoology courses as the animal phylum Protozoa. Given current knowledge and the principles of phylogenetic systematics, this taxonomy commits two errors; “protozoa” are neither animals nor are they a valid monophyletic taxon at any level of the Linnaean hierarchy. The kingdom Protista is likewise invalid because it is not monophyletic. Animallike protistans, now divided into seven or more phyla, are nonetheless of interest to students of zoology because of their animal-like properties.
Whittaker’s five-kingdom classification
Figure 10-12
Whittaker’s five-kingdom classification superimposed on a phylogenetic tree showing living representatives of these
kingdoms. Note that the kingdoms Monera and Protista constitute paraphyletic groups and are therefore
unacceptable to cladistic systematics.


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