Kingdoms and Domains

Systematists work in the field of biological classification, in which they attempt to organize biodiversity into discrete and logical categories. The task of classifying life is much like categorizing books in a library—books can be divided into "fiction" or "nonfiction," and within each of these divisions, more precise categories can be made (for example, nonfiction can be divided into biography, history, science, etc.). The book-cataloguing system used in most public libraries, the Dewey Decimal System, is only one way of shelving books. For instance, academic and research libraries use a different system, developed by the United States Library of Congress. Librarians use the cataloguing system that is appropriate to the collection of books owned by the library and the needs and interests of the library's users; just as there are alternative methods of organizing books, there is more than one way to organize biodiversity to meet differing needs.

Biologists have traditionally subdivided living organisms into great groups that share some basic characteristic. Fifty years ago, most biologists divided life into two categories: plants, for organisms that were immobile and apparently made their own food, and animals, for organisms that could move about and relied on other organisms for food. When it became clear that too many organisms did not fit easily into this neat division of life, some scientists began to argue for a system of five kingdoms, in which organisms were categorized according to the type of cell they possessed and their method of obtaining energy. Table 3.1 (page 60) provides an overview of this system.

The five-kingdom system is not perfect either—for instance, the Protista kingdom contains a wide diversity of life forms that have only very superficial similarities. More recently, many biologists have argued that the most appropriate way to classify life is according to historical relationships among organisms. In Chapter 8 we will discuss the theory of evolution, which states that all modern organisms represent the descendants of a single set of common ancestors that existed nearly 4 billion years ago. Evidence for shared ancestry includes the universality of many aspects of the cell structures and cellular processes that were described in Chapter 2. The process of divergence from this set of ancestors to the diversity of modern species occurred as millions of branching events— leading biologists to describe it as "the tree of life" (Figure 3.4).

Monera

Eukarya protista

Monera

Eukarya

More recent divergence

tree exemplifies the current state of knowledge regarding relationships among living organisms. Note that the termination of branch tips are all on the same plane, representing the present time. Living organisms represent a small remnant of all the species that have appeared over

Figure 3.4 The tree of life. This

Earth's history.

Five-Kingdom System

Three-Domain System h

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