-Objectives

Demonstrate how each successive

taxon is more specific than the previ-ous taxon.

. Discuss why the number of kingdoms

of organisms has changed over time.

14.2 ASystemofClassification-Think againabout classifying everyday objects like motor vehicles. In anysystem, you usually first assign the objects to very broad categories. Motorvehicles might be one broad group of objects used for travel. Other suchgroups might be wind-powered vehicles and animal-drawn devices. Beyondthe broad group, you can then assign each object to increasingly more spe-cific groups. You've seen, for example, that cars comprise subgroups suchas sedans and convertibles. You could classify sedans as two-door or four-door models. You could further group each of these by type of engine.With each subdivision, the object is classified into a more specific cate-gory. Within each subdivision, the members are more like each other.

Taxa

When an organism is assigned to a species, it has been classified as specif-ically as possible. The genus to which that species belongs is a slightlymore general taxon because several species may be members of thatgenus. The complete classification system includes five more taxa, for atotal of seven.

The seven taxa form a series. The broadest taxon is the kingdom.Kingdoms are divided into increasingly more specific taxa. These are, inorder, phylum, class, order, and family. Genus and species are last. Eachtaxon narrows the number of organisms of the previous group. Therefore,the number of organisms included in each taxon becomes smaller andsmaller. Each species is one certain type of organism.

That sequence is classification in its simplest form. But classification isoften more complex because each of these major groups may be furthersubdivided. For example, each phylum may be divided into several sub-

phyla. The phylum that vertebrates belong to contains three subphyla, ofwhich one is Vertebrata. A single species may be made up of several sub-species. Subspecies are sometimes called breeds, varieties, or races. Thevarious breeds of domestic dogs, for example, are all subspecies.

Figure 14-11 The seven taxa used to classifyorgan-isms include the division, a taxon at the phylum level.Organisms in the Plant and Fungi Kingdoms are classifiedin divisions rather than phyla.

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Species

Some Examples of Classification

What would the complete classification of a common house cat (FelisGalus) be? First, a cat is classified as an animal based on the very broadcharacteristics of all animals. Like all other animals, for example, a catdoesn't make its own food, but eats other organisms. Cats are also multi-cellular organisms. Using these and other characteristics, the cat is placedin Kingdom Animalia.

A cat is then put into the Phylum Chordata and, because it has a back-bone, into the subphylum Vertebrata. This subphylum includes the sevenclasses of fish, amphibians, reptiles, birds, and mammals. You can see therest of the classification of a common house cat in Table 14.1. Each grouprepresents more specific characteristics than the previous group.

Now compare the classification of a cat with that of a dog. Becausecats and dogs share the same broad characteristics, they are in the samekingdom, phylum, subphylum, class, and order. But at the family level,the classification diverges. The different families represent differentbranches of animal evolution. Each of the remaining groups is, of course,also different from the others.

----Minilab 2

What traits are used to classifyan unknown organism?

Your teacher will provide you with an

organism you've never seen before.Your task is to classify it by kingdom.

Observe and record as many traits

as you can see with the naked eye,

then observe under the microscope.

Record additional traits. Review your

list. Use your list to determine thekingdom to which you believe this

organism belongs.

Analysis What kingdom does this

organism belong to~ What traitshelped you decide which kingdom to

place the organism in~ Explain why.

Table 14.1 Classification of Some Animals

'--....

14.2 A SY ST EM 0 F C LA S S I Fie A T ION;" 377

Taxon House Cat Dog Human Grasshopper

Kingdom Animalia \ Animalia Animalia Animalia

Phylum Chordata Chordata Chordata Arthropoda-

.Subphylum Vertebrata Ver:tebrata Vertebrata,.

Class Mammalia Mammaia Mammalia Insecta-

Order Carnivora Carnivora Primates Orthoptera

Family .Felidae Canidae Hominidae Locustidae

. Genus Felis Canis Homo Schistocerco

Species Felis cotus Canis familiaris Homo sapiens Schistocerca americana

81

Visit the GlencoeScience Web site atscience.glencoe.comfor an Internet lab ontree characteristics.

Now compare the cat and dog with humans. How closely do humansseem to be related to dogs and cats? How many levels of classification dohumans have in common with dogs and cats? At what level do the classi-fications differ?

Finally, compare these three animals with the grasshopper. The onlycommon taxon among the grasshopper, cat, dog, and human is kingdom-all these organisms are classified as animals. Because the grasshopper'scharacteristics are different from those of the other animals, so, too, isits classification.

The grasshopper is classified in the Phylum Arthropoda. What majordifference among organisms would cause a taxonomist to place a newly-discovered animal into the Phylum Chordata rather than the PhylumArthropoda? How are you different from a grasshopper?

The KingdomProblem

You know from Chapter 2 that most biologists today use a five-kingdomsystem of classification. These kingdoms include the monerans, protists,fungi, plants, and animals. Examples of organisms in each kingdom areshown in Figure 14-12 on page 381. All organisms once were classified aseither plants or animals. This two-kingdom scheme was reasonable formost familiar, multicellular organisms. After all, it is not difficult to dis-

tinguish a plant like a cactus from an ani~allike a coyote.However, the discovery of microscopic organisms posed a problem.

Some unicellular forms could easily be classified into one kingdom or theother, but many did not fit neatly into either group. For example, anorganism called Euglena has characteristics common to both kingdoms.

Euglena is mobile, likean animal, yet autotrophic, like a plant. To con-fuse the matter further, at certain times Euglena may lose its chlorophylland become heterotrophic.

Other problems arise with a two-kingdom system. For example, howshould fungi such as mushrooms and molds be classified? They are likeplants in that they do not move, but they are heterotrophic like animals.What about slime molds? They are heterotropic and move like Amoeba,but they produce spores like fungi. Sponges, sea anemones, and coral allare animals, but they resemble plants.

Based on current knowledge of evolutionary relationships amongorganisms, the five-kingdom system has solved some of these problems.Identifying cells as prokaryotes or eukaryotes helped classify many micro-scopic organisms. Remember from Chapter 5 that prokaryotic cells haveno nucleus. All monerans are prokaryotes. However, other classificationsystems have been proposed. That's not surprising because biologistsoften interpret data in different ways. One proposed system is based onsix kingdoms. The sixth kingdom consists of a group of extremely ancientbacteria that are different in certain respects from other bacteria. Sometaxonomists, therefore, argue that these bacteria represent a differentmajor branch of evolution. You'lliearn about these bacteria in the nextchapter. However, this book will use the five-kingdom system, as pre-sented in Appendix A. Use it as a reference when needed.

Kingdom Moneraabout 4000 species

.Coccus

Kingdom Protistaabout50 000 species

Kingdom Animaliaabout 1 000 000 species

"Diatoms

Kingdom Fungiabout 100 000 species

Mushrooms

Kingdom Plantaeabout 265 000 species

Equisetum

Figure 14-12 The five kingdoms of life include the Monera, Protista, Fungi, Plant and Animal Kingdoms.

14.2 A SYSTEM OF. CLASSIFICATION I-. 381

Figure 14-13 Which of these is aplant and which is an animal?Structure alone is not enough to clas-sifyan animal like the sponge (a) and aloofah (b), the dried frnit of a plant inthe gourd genus Luffa.

a-LOOKINGAHEAD ;.,;., ;., .~ ~.,

b

1-. ;., ;.,;., ;.,

VARIOUSLINESof evidence-structural, biochemical, molecular-areused to establish evolutionary relationships. From this evidence, a five-kingdom system for classifying organisms has been developed. In Chapter15, you will learn about the diversity of and evolutionary relationshipsamong organisms of three kingdoms-monerans, protists, and fungi.You'll also study the unique viruses, which are not classified in anykingdom. je,

Section Review

Understanding Concepts

1. Explain why two organisms classi-fied in the same family must also bein the same order.

2. Based on Table 14.1, which taxa

would a gorilla and a human havein common? Defend your answer.

3. Relate the classification of an organ-ism to its phylogeny.

4. Skill Review-Classifying:UseAppendix A to classifya beedown to order. Formore help, seeOrganizing Informationin the SkillHandbook.

5. Thinking Critically: Whatstructural features put humans, cats,and dogs in the same taxa throughthe class level? What characteristics

place humans in a different orderfrom cats and dogs?

382 je, C LAS S I Fie A TION