BIO.3.9.4

More Information About This Text

9 BIODIVERSITY AND ITS CELLULAR BASIS

4 Modern classification systems are based on evolutionary relationships

The nucleus is a cellular organelle that contains the genomic DNA. The nucleus is surrounded by a nuclear envelope consisting of a double layer of membranes. It is the site of gene transcription and most RNA processing.
A prokaryote, also called a bacterium, is an unicellular organism that lacks membrane-bound organelles such as a nucleus.
A eukaryote is an organism that is composed of one or more cells, each containing a nucleus. The term eukaryote means "true nucleus."
Archaebacteria (Archaea) are bacteria that differ from eubacteria in the chemistry of their cell wall, ribosomal RNA, lipids, and certain enzymes. This group of bacteria includes halophiles that live in salty environments, methanogens that live in oxygen-free environments, and thermoacidophiles that live in hot, acidic environments.
An empire is a taxonomic group used in Cavalier-Smith's six-kingdom classification system. A kingdom belongs to one of two empires, Prokaryota or Eukaryota.
An endosymbiont is an organism that lives inside the cells of another organism.

A more and more accurate understanding of true evolutionary relationships is emerging from comparing the DNA codes of organisms and taking into account the fossil record.
The cells of organisms within each kingdom share features that distinguish them from the cells of organisms in other kingdoms.

Figure 9.3

Sections through a prokaryotic and a eukaryotic cell, as seen with the electron microscope. Eukaryotic cells have their genetic material enclosed by a nuclear membrane; prokaryotic cells do not.

Over the past 50 years, classification systems have reflected the distinction made between organisms that have or do not have a nucleus in their cells. The nucleus is a membrane-bound structure that contains the genome of the organism. An organism that does not possess a nucleus is called a prokaryote (literally, "before the nucleus"). An organism that does have a nucleus is called a eukaryote ("true nucleus"). The cell structures of these organisms are illustrated in Figure 9.3. (For other differences in cell structure, see Prokaryotes, the simplest cells and Eukaryotic cells.)

Figure 9.4

Two modern classification systems involving five and six kingdoms: the five-kingdom system, as proposed by Robert Whittaker, and the six-kingdom system proposed by Thomas Cavalier-Smith and used in this textbook. In each system, organisms increase in complexity as you read from the bottom toward the top of the figure.

Figure 9.4 diagrams two systems of classification — one that includes five kingdoms, and another that includes six. Both systems place all prokaryotes in one kingdom, but differ in the way the eukaryotes are divided among other kingdoms. Later, we will explore a third classification system (see Molecular evolution and biodiversity). This system argues that a particular group of prokaryotes, called archaebacteria or Archaea, is sufficiently different from other prokaryotes and eukaryotes that it merits its own grouping.

A popular classification scheme for about the past thirty years has been the "five-kingdom" system proposed by Robert Whittaker. Using this system, the kingdoms are Monera (prokaryotes), Protista (unicellular eukaryotes), Fungi (fungi), Plantae (plants), and Animalia (animals). All unicellular eukaryotes are in the kingdom Protista, except for unicellular green algae, which are in Plantae. One criterion that Whittaker used to divide the multicellular eukaryotes was the manner of obtaining nourishment. Fungi absorb, animals ingest, and plants photosynthesize. Therefore, the five-kingdom system stresses ecological rather than evolutionary relationships.

The more recent, "six-kingdom" system introduced by Thomas Cavalier-Smith highlights evolutionary relationships among organisms, based on similarities in cell structure and DNA sequences. In this system, the kingdoms are Bacteria (instead of Monera), Protozoa (instead of Protista), Fungi, Plantae, Animalia, and Chromista. Because the next few chapters of our textbook focus on cells and on the processes within them, we will use this classification scheme, emphasizing cell structure.

Cavalier-Smith's six-kingdom system contains a few basic changes from Whittaker's five-kingdom system. Cavalier-Smith emphasizes the fundamental difference in cell structure between prokaryotes and eukaryotes by creating two supergroupings called empires, Prokaryota and Eukaryota. The kingdom Bacteria is in empire Prokaryota; the other kingdoms are in empire Eukaryota. He also creates a new kingdom called Chromista. This kingdom contains photosynthetic organisms (e.g., brown algae), whose chloroplasts differ from those found in plants. This kingdom also contains nonphotosynthetic organisms that superficially resemble fungi. These chromists are now known to have evolved from photosynthetic organisms. Finally, to highlight the evolutionary relationships among organisms in a kingdom, Cavalier-Smith places numerous unicellular organisms (formerly in Protista) into the other eukaryotic kingdoms.

Figure 9.5

Classification of diverse life forms derived from the six-kingdom system of Cavalier-Smith. Possible evolutionary relationships are indicated by branching patterns.

Figure 9.5 diagrams the evolutionary relationships among the six kingdoms. Branches indicate separate lines of evolution. The arrangement of branch points suggests the hypothetical sequence in which different groups of organisms might have arisen during evolution. For example, chloroplasts and mitochondria probably originated as endosymbionts of primitive eukaryotes. Only some of the major groups within each kingdom are listed, and representative organisms are depicted. The following sections give an overview of the six-kingdom system.

Last Revised on April 28, 2004

Instructors: More Information About This Text | Jones and Bartlett Biological Science Titles