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I. Linnaean Taxonomy

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Brainard SHUMOCK

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Printed: August 21, 2011

AuthorsJean Brainard, ELISABETH SHUMOCK

ContributorsBarbara Akre, ELISABETH SHUMOCK

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Contents

1 I. Linnaean Taxonomy 11.1 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

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Chapter 1

I. Linnaean Taxonomy

Figure 1.1

Why classify? To study the great diversity of organisms, biologists must give each organism a name.Biologists must also attempt to organize living things into groups that have biological meaning. Thediscipline known as taxonomy allows scientisits to classify an assign each organism on Earth a universallyaccepted name. By using scientific names for organisms scientists can ensure that everyone is discussingthe same organism. Taxonomists classify organisms into biologically significant groupings based upon theirunique characteristics and evolutionary history. The pictures above show a vast variety of organisms givingthem common names and their taxonomic names or scientific names. Earth Forms and Life Begins

1.1 ClassificationLesson Objectives

• Explain how living things are organized for study.• Describe and define binomial nomenclature.• Explain Linnaeus’ system of classification and how it differs from phylogenetic classification.• Be apply to classify organisms using a dichotomous key.

Vocabulary• binomial nomenclature• clade

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• class• derived characteristics• dichotomous key• domain• family• genus• kingdom• Linnaean classification system• order• phylogenetic tree• phylogeny• phylum• species• taxa• taxonomy

IntroductionThe evolution of life on Earth over the past 4 billion years has resulted in a huge variety of species. Formore than 2,000 years, humans have been trying to classify the great diversity of life. The science ofclassifying organisms is called taxonomy. Classification is an important step in understanding the presentdiversity and past evolutionary history of life on Earth.

Linnaean ClassificationAll modern classification systems have their roots in the Linnaean classification system. It was de-veloped by Swedish botanist Carolus Linnaeus in the 1700s. He tried to classify all living things thatwere known at his time. He grouped together organisms that shared obvious physical traits, such asnumber of legs or shape of leaves. For his contribution, Linnaeus is known as the ‘‘father of taxonomy.”You can learn more about Linnaeus and his system of classification by watching the video at this link:http://teachertube.com/viewVideo.php?video_id=169889http://teachertube.com/viewVideo.php?video_-id=169889. The Linnaean system of classification consists of a hierarchy of groupings, called taxa (singular,taxon). There are seven taxa in the Linnaean system ranging from the species to the kingdom (see Figure1.2). The species is the smallest and most exclusive grouping. It consists of organisms that are similarenough to produce fertile offspring together. Closely related species are grouped together in a genus.Groups of genus that share many of the same characteristics are grouped togethr into a family. Groups ofsimilar families make up an order. Orders that share a great deal of similarites are grouped into a class.Groups of very closely related classes are placed into a phylum. The kingdom is the largest and mostinclusive grouping in the Linnaean system of classification. It consists of organisms that share just a fewbasic similarities. Examples are the plant and animal kingdoms.

Binomial NomenclaturePerhaps the single greatest contribution Linnaeus made to science was his method of naming species. Thismethod, called binomial nomenclature, gives each species a unique, two-word Latin name consisting ofthe genus name and the species name. The scientific name is always written in italics. The first word iscapitialized, and the second word is lowercased. An example is Homo sapiens, the two-word Latin namefor humans. It literally means ‘‘wise human.” This is a reference to our big brains. Why is having two

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Figure 1.2: Linnaean Classification System: Classification of the Human Species. This chart shows thetaxa of the Linnaean classification system. Each taxon is a subdivision of the taxon below it in the chart.For example, a species is a subdivision of a genus. The classification of humans is given in the chart as anexample.

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names so important? It is similar to people having a first and a last name. You may know several peoplewith the first name Michael, but adding Michael’s last name usually pins down exactly whom you mean.In the same way, having two names uniquely identifies a species.

Revisions in Linnaean Classification: The Three Domain SystemLinnaeus published his classification system in the 1700s. Since then, many new species have been discov-ered. The biochemistry of organisms has also become known. Eventually, scientists realized that Linnaeus’ssystem of classification needed revision. A major change to the Linnaean system was the addition of a newtaxon called the domain. A domain is a taxon that is larger and more inclusive than the kingdom. Mostbiologists agree there are three domains of life on Earth: Bacteria, Archaea, and Eukarya (see Figure 1.3).Both Bacteria and Archaea consist of single-celled prokaryotes. Domain Bacteria contains the kingdom ofEubacteria; domain Archaea contians the kingdom of Archaebacteria. Eukarya consists of all eukaryotes,from single-celled protists to humans. This domain includes the kingdoms of Animalia (animals), Plantae(plants), Fungi (fungi), and Protista (protists). Table 1.1 below summarizes the key evidence used inclassifying organisms into these major taxonomic groups.

Figure 1.3: Three-Domain Classification. This diagram shows the three domains of organisms that cur-rently live on Earth.

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Table 1.1: Table 1.1: Classification of Living Things

DOMAIN Bacteria Archaea EukaryaKINGDOM Eubacteria ArchaebacteriaProtista Fungi Plantae AnimaliaCELLTYPE

Prokaryote Prokaryote Eukaryote Eukaryote Eukaryote Eukaryote

CELLSTRUC-TURES

Cell wallswith pepti-doglycan

Cell wallswithoutpeptidogly-can

Cell walls ofcellulose insome; somehave chloro-plasts

Cell walls ofchitin

Cell wallsof cellulose;chloroplasts

No cellwalls orchloroplasts

NUMBEROF CELLS

Unicellular Unicellular Most uni-cellular;some colo-nial; somemulticellular

Most mul-ticellular;some unicel-lular

Multicellular Multicellular

MODE OFNUTRI-TION

Autotrophor het-erotroph

Autotrophor het-erotroph

Autotrophor het-erotroph

Heterotroph Autotroph Heterotroph

EXAMPLES Streptococcus,Escherichiacoli

Methanogens;halophiles

Amoeba,Parame-cium, slimemolds, giantkelp

Mushrooms,yeasts

Mosses,ferns, flow-ering plants

Sponges,worms, in-sects, fishes,mammals

Phylogenetic ClassificationLinnaeus classified organisms based on obvious physical traits. Basically, organisms were grouped togetherif they looked alike. After Darwin published his theory of evolution in the 1800s (discussed in one of thefollowing chapters), scientists looked for a way to classify organisms that showed phylogeny. Phylogenyis the evolutionary history of a group of related organisms. It is represented by a phylogenetic tree, likethe one in Figure 1.4.One way of classifying organisms that shows phylogeny is by using the clade. A clade is a group oforganisms that includes an ancestor and all of its descendants. Clades are based on cladistics. This isa method of comparing traits in related species to determine ancestor-descendant relationships througha cladistic analysis that identifies and considers only characteristics of organisms that are evolutionaryinnovations. Characteristics that appear in recent parts of a lineage but not in its older members are calledderived characteristics. Clades of derived charcteristics are represented by cladograms, like the one inFigure 1.5. This cladogram represents the mammal and reptile clades. The reptile clade includes birds.It shows that birds evolved from reptiles. Linnaeus classified mammals, reptiles, and birds in separateclasses. This masks their evolutionary relationships.

Using Dichotomous KeysWhat tools are available to help people identify unfamiliar organisms? One is a field guide, a book withillustrations that highlight differences between similar-looking organisms. Another tool used to identifyorganisms is a dichotomous key. A dichotomous key is a series of paired statements that describe physical

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Figure 1.4: Phylogenetic Tree. This phylogenetic tree shows how three hypothetical species are related toeach other through common ancestors. Do you see why Species 1 and 2 are more closely related to eachother than either is to Species 3?

Figure 1.5: Mammal and Reptile Clades. This cladogram classifies mammals, reptiles, and birds in cladesbased on their evolutionary relationships.

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characteristics of different organisms. In the Figure 1.6 you can see an example of a dichotomous key.If you were given a round, chewy piece of wrapped in white paper, could you follow this key to find itsscientific name? It really quite simple, you always begin at number 1. 1a applies the candy is chewy, thisis followed by a number 2 which is telling us to look at item 2 and find the characteristic that applies. 2astates that the candy is wrapped and refers us to number 3. 3a has a listing for round candy and refers usto number 4. 4a states that the wrapper is all white and gives us the scientific name of Saltus taffinia.

Figure 1.6: This key could be used to classify a variety of pieces of candy.

Lesson Summary• Classification is an important step in understanding life on Earth. All modern classification systems

have their roots in the Linnaean classification system. The Linnaean system is based on similaritiesin obvious physical traits. It consists of a hierarchy of taxa, from the kingdom to the species. Eachspecies is given a unique two-word Latin name. The recently added domain is a larger and moreinclusive taxon than the kingdom.

• Phylogeny is the evolutionary history of group of related organisms. It is represented by a phylogenetictree that shows how species are related to each other through common ancestors. A clade is a groupof organisms that includes an ancestor and all of its descendants. It is a phylogenetic classification,based on evolutionary relationships.

• A dichotomous key is a tool that can be used to help us identify unfamililar organisms.

Lesson Review Questions

Recall1. What is taxonomy?2. Define taxon and give an example.3. What is binomial nomenclature? Why is it important?4. What is a domain? What are the three domains of life on Earth?

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5. What is cladistics, and what is it used for?

Apply Concepts6. Create a taxonomy, modeled on the Linnaean classification system, for a set of common objects, suchas motor vehicles, tools, or office supplies. Identify the groupings that correspond to the different taxa inthe Linnaean system.7. Dogs and wolves are more closely related to each other than either is to cats. Draw a phylogenetic treeto show these relationships.8. Using the Dichotomous key presented in this chapter, find the scientific name for a piece of candy thathas the characteristics listed below. Be sure to write the name in the proper scientific name when you findit.

• You are given a ball-shaped piece of hard candy that is not wrapped.

Think Critically8. Compare and contrast a Linnaean taxon, such as the family or genus, with the clade.9. Explain why reptiles and birds are placed in the same clade.

Points to ConsiderThis chapter gives you a glimpse of 4 billion years of evolution on Earth. In the next chapter, you will readabout the forces that bring about evolution. Natural selection is one of these forces. It generally results ina population or species becoming better adapted to its environment over time.

• How does natural selection work? How does it bring about evolutionary change?• What might be the other forces of evolution?

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