classification of living things

contributions of linnaeus

Carl Linnaeus (1707-1778) is called the father of taxonomy. You may already be familiar with the classification system Linnaeus introduced.

linnaean classification system

You can see the main categories, or taxa (taxon, singular), of the Linnaean system in Figure 2.16. As an example, the figure applies the Linnaean system to classify our own species, Homo sapiens. Although the Linnaean system has been revised, it forms the basis of modern classification systems. The broadest category in the Linnaean system is the kingdom. Figure 2.16 shows the Animal Kingdom because Homo sapiens belongs to that kingdom. Other kingdoms include the Plant Kingdom, Fungus Kingdom, and Protist Kingdom. Kingdoms are divided, in turn, into phyla (phylum, singular). Each phylum is divided into classes, each class into orders, each order into families, and each family into genera (genus, singular). Each genus is divided into one or more species. The species is the narrowest category in the Linnaean system. A species is defined as a group of organisms that can breed and produce fertile offspring together.

taxonomy

Like you, scientists also group together similar organisms. The science of classifying living things is called taxon- omy. Scientists classify living things in order to organize and make sense of the incredible diversity of life. Modern scientists base their classifications mainly on molecular similarities. They group together organisms that have similar proteins and DNA. Molecular similarities show that organisms are related. In other words, they are descendants of a common ancestor in the past.

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how should viruses be classified

This question was posed at the beginning of the chapter. Should viruses be placed in one of the three domains of life? Are viruses living things? Before considering these questions, you need to know the characteristics of viruses. A virus is nothing more than some DNA or RNA surrounded by a coat of proteins. A virus is not a cell. A virus cannot use energy, respond to stimuli, grow, or maintain homeostasis. A virus cannot reproduce on its own. However, a virus can reproduce by infecting the cell of a living host. Inside the host cell, the virus uses the cells structures, materials, and energy to make copies of itself. Because they have genetic material and can reproduce, viruses can evolve. Their DNA or RNA can change through time. The ability to evolve is a very lifelike attribute. Many scientists think that viruses should not be classified as living things because they lack most of the defining traits of living things. Other scientists arent so sure. They think that the ability of viruses to evolve and interact with living cells earns them special consideration. Perhaps a new category of life should be created for viruses. What do you think?

binomial nomenclature

Linnaeus is also famous for his method of naming species, which is still used today. The method is called binomial nomenclature. Every species is given a unique two-word name. Usually written in Latin, it includes the genus name followed by the species name. Both names are always written in italics, and the genus name is always capitalized. For example, the human species is named Homo sapiens. The species of the family dog is named Canis familiaris. Coming up with a scientific naming method may not seem like a big deal, but it really is. Prior to Linnaeus, there was no consistent way to name species. Names given to organisms by scientists were long and cumbersome. Often, different scientists came up with different names for the same species. Common names also differed, generally from one place to another. A single, short scientific name for each species avoided a lot of mistakes and confusion.

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domains

When Linnaeus was naming and classifying organisms in the 1700s, almost nothing was known of microorganisms. With the development of powerful microscopes, scientists discovered many single-celled organisms that didnt fit into any of Linnaeus kingdoms. As a result, a new taxon, called the domain, was added to the classification system. The domain is even broader than the kingdom, as you can see in Figure 2.17. Most scientists think that all living things can be classified in three domains: Archaea, Bacteria, and Eukarya. These domains are compared in Table 2.3. The Archaea Domain includes only the Archaea Kingdom, and the Bacteria Domain includes only the Bacteria Kingdom. The Eukarya Domain includes the Animal, Plant, Fungus, and Protist Kingdoms. Trait Multicellularity Archaea No Bacteria No Cell Wall Yes Without peptidoglycan Yes With peptidoglycan Cell Nucleus (DNA inside a membrane) No No Eukarya Yes except for many pro- tists Yes for plants, fungi, and some protists No for animals and other protists Yes Trait Cell Organelles structures membranes) (other inside Archaea No Bacteria No Eukarya Yes The Archaea and Bacteria Domains contain only single-celled organisms. Both Archaea and Bacteria have cells walls, but their cell walls are made of different materials. The cells of Archaea and Bacteria lack a nucleus. A nucleus is membrane-enclosed structure for holding a cells DNA. Some Eukarya are also single-celled, but many are multicellular. Some have a cell wall; others do not. However, the cells of all Eukarya have a nucleus and other organelles. Archaea and Bacteria may seem more similar to each other than either is to Eukarya. However, scientists think that Archaea may actually be more closely related to Eukarya than Bacteria are. This view is based on similarities in their DNA.

instructional diagrams

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questions

___Linnaean classification has been completely replaced by modern classification systems.

a. true

-->  b. false

___Modern scientists classify organisms on the basis of molecular similarities.

-->  a. true

b. false

___The name of the human family is the Chordates.

a. true

-->  b. false

___Each genus is divided into one or more families.

a. true

-->  b. false

___Organisms that lack cell walls include plants and animals.

a. true

-->  b. false

___The cells of all Eukarya have a nucleus and other organelles.

-->  a. true

b. false

___All scientists agree that viruses should be considered living things.

a. true

-->  b. false

___broadest taxon in the Linnaean classification system

a. species

b. binomial nomenclature

c. order

d. domain

e. taxonomy

f. phylum

-->  g. kingdom

___science of classifying living things

a. species

b. binomial nomenclature

c. order

d. domain

-->  e. taxonomy

f. phylum

g. kingdom

___taxon that includes one or more classes

a. species

b. binomial nomenclature

c. order

d. domain

e. taxonomy

-->  f. phylum

g. kingdom

___taxon that includes one or more families

a. species

b. binomial nomenclature

-->  c. order

d. domain

e. taxonomy

f. phylum

g. kingdom

___group of organisms that can breed and produce fertile offspring together

-->  a. species

b. binomial nomenclature

c. order

d. domain

e. taxonomy

f. phylum

g. kingdom

___taxon broader than the kingdom in a modern taxonomic system

a. species

b. binomial nomenclature

c. order

-->  d. domain

e. taxonomy

f. phylum

g. kingdom

___method of naming organisms introduced by Linnaeus

a. species

-->  b. binomial nomenclature

c. order

d. domain

e. taxonomy

f. phylum

g. kingdom

The human order is the

a. animals.

b. chordates.

c. mammals.

-->  d. primates.

Which of the following is not a kingdom in the Linnaean system of classification?

-->  a. Archaea

b. Protist

c. Plant

d. Fungus

The second word in an organisms two-word Latin name is the name of its

-->  a. species.

b. genus.

c. domain.

d. family.

The domains of life include

a. Bacteri

b. Archaea.

c. Eukarya.

-->  d. all of the above

Which of the following statements about viruses is true?

a. A virus belongs to the Archaea Domain.

b. A virus is a single-celled organism.

c. A virus has a cell membrane made of proteins.

-->  d. A virus can evolve.

What is the name of the domain that contains four kingdoms?

a. Animal

b. Plant

c. Bacteria

-->  d. Eukarya

Which trait is found in bacteria?

a. multiple cells

-->  b. cell wall

c. cell nucleus

d. cell organelles

diagram questions

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