chapter outline

The Classification of Organisms (p. 320)
The Invention of the Linnaean System (p. 320; Fig. 15.1)
A. Any system that attempts to group and categorize organisms is called a classification scheme.
B. Prior to using the current system, which employs two-part names, or binomials, a variety of
other systems were in use from the time of Aristotle on.
C. The basic unit, called a genus (plural, genera), has been used since the time of the Greeks and
D. A particular type of organism became known as a species.
E. The Polynomial System
1. Until the mid-1700s, strings of long polynomials, up to 12 words long, were used to
classify organisms.
F. The Binomial System
1. The classification scheme in universal use today, a binomial system, was devised 200
years ago by Swedish biologist Carolus Linnaeus.
2. Two-part Latin names are used to name particular organisms.
Species Names (p. 321; Fig. 15.2)
A. By convention, the first part of a binomial name identifies the genus to which the species
belongs, and the second part distinguishes one species from others in the genus.
B. The two names together are called the scientific name and are written in italics.
Higher Categories (p. 322; Fig. 15.3)
A. The Linnaean system is a hierarchical system that uses groupings, with each one in succession
smaller and more specific than the one before it.
B. From largest to smallest, biologists use the groupings kingdom, phylum, class, order, family,
genus, and species.
C. Each category is associated with a specific type of characteristic, so each gives information
about the organisms grouped within it.
D. In addition, an eighth level of classification, called domains, is used.
What Is a Species? (p. 323; Fig. 15.4)
A. John Ray (1627–1705) was one of the first to propose a general definition of a species: all the
individuals that belong to it can breed with one another and produce fertile offspring.
B. The Biological Species Concept
1. Later, the biological species concept (contributed by evolutionist Ernst Mayr, see chapter
17) defined species more precisely as “groups of actually or potentially interbreeding
natural populations which are reproductively isolated from other such groups.”
2. It follows that hybrids occur rarely in nature, while members of the same species can
readily interbreed.
3. The biological species concept definition works well for animals, but many of the
organisms of the other kingdoms only rarely have sexual reproduction and outcrossing,
but instead often reproduce by asexual reproduction.
4. The biological species concept is not always employed for plants and other organisms.
C. How Many Kinds of Species Are There?
1. About 1.5 million species have been named, but scientists estimate there may be as many
as 10 million species on earth.
Inferring Phylogeny (p. 340)
How to Build a Family Tree (p. 324; Figs. 15.5-15.8)
A. Taxonomy is important because it helps scientists understand the relatedness of different
groups of organisms and enables them to construct a phylogeny, or family tree.
B. Cladistics
1. Several methods exist for constructing a phylogeny, but probably the simplest method is
to approach the problem with cladistics and make use of a clade, or group of organisms
related by descent.
2. Cladograms are not really family trees but comparisons of relative relationships.
3. Cladograms enable the scientist to determine the order in which a series of evolutionary
traits arose.
C. Traditional Taxonomy
1. If a researcher wishes to weigh the importance of some evolutionary trait (i.e., it gave rise
to flight as a means of locomotion), they must use phylogenetic systematics.
2. This is a much more time-intensive method that employs volumes of information about
the biology of a group of species, which allows for an informed decision about the
importance of the evolutionary trait being examined.
3. When much information is available, phylogenetic systematics is the favored approach.
4. When little information is available concerning the importance of the trait under
examination, cladistics is the best choice.
Kingdoms and Domains (p. 330)
The Kingdoms of Life (p. 330; Fig. 15.9)
A. The earliest classification schemes recognized only animals and plants, but most scientists
now use a six-kingdom system.
B. In the six-kingdom system, four kingdoms consist of eukaryotes and two of prokaryotes.
1. The kingdoms Animalia and Plantae contain organisms that are multicellular throughout
most of their lifespans.
2. The kingdom Fungi contains multicellular forms and single-celled yeasts.
3. Animals, plants, and fungi all have fundamental differences and probably each
descended from a different ancestor.
4. The kingdom Protista encompasses the algae and a large number of unicellular
5. The remaining two kingdoms, Archaea and Bacteria, consist of prokaryotes that are
vastly different from other living things.
C. Domains
1. Because the archaea are so different from all other organisms, the use of a higher
taxonomic category, called domains, is necessary.
2. Archaea, bacteria, and eukaryotes are each in a separate domain.
Domain Bacteria (p. 331; Table 15.1)
A. Bacteria are the most abundant organisms on earth, and they play critical ecological roles
throughout the biosphere.
B. There are many different kinds of bacteria, and the evolutionary links between them are not
readily understood.
C. Archaea appear to be more closely related to eukaryotes than they are to bacteria.
Domain Archaea (p. 332; Fig. 15.10)
A. The term archaea refers to the ancient origins of this group which probably diverged early
from the bacteria.
B. Archaea exist today in some of the most extreme environments on earth.
C. A type of archaea called methanogens obtains their energy by using hydrogen gas to reduce
carbon dioxide to methane gas.
1. These archaea must live in environments without oxygen, such as in swamps, marshes,
and in mammalian intestines.
2. Methanogens contribute 2 billion tons of methane gas into the atmosphere annually.
D. Extremophiles include thermophiles (heat lovers), halophiles (salt lovers), pH-tolerant
archaea, and pressure-tolerant archaea.
E. Nonextreme archaea grow in habitats similar to bacteria and are identified by their signature
Domain Eukarya (p. 333; Fig. 15.11)
A. The eukaryotes appeared in the fossil record 1.5 billion years ago, much later than the
B. Three Largely Multicellular Kingdoms
1. Because of the size and ecological dominance of plants, animals, and fungi, and because
they are mostly multicellular, they are recognized as kingdoms distinct from Protista.
C. A Fourth Very Diverse Kingdom
1. Protists are a fascinating and diverse group of eukaryotes that have great biological
D. Symbiosis and the Origin of Eukaryotes
1. Mitochondria and chloroplasts are both believed to have entered early eukaryotic cells
by endosymbiosis.
2. Each of these organelles is the size of a bacterium and bears its own DNA.
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