i Classification

Search

INSIDE:•18.1 Finding Order in Diversity

18.2 Modern Evolutionary Classification

• 18.3 Building the Tree of Life

• Untamed Science Video • Chapter Mystery

CHAPTER

MYSTEPY♦ f

GRIN ANDBEAR ITIf you simply looked ata polar bear and brownbear, you would probablynever doubt that they aremembers of different species.Polar bears grow much largerthan brown bears, and their paws haveadapted to swimming long distancesand to walking on snow and ice.Their white fur camouflages them, butthe coats on brown bears are, well,

brown—and their paws aren't adaptedto water.

Clearly polar bears and brownbears are very different physically.But do physical characteristics tellthe whole story? Remember thedefinition of species: "a group ofsimilar organisms that can breed andproduce fertile offspring." Well, polarbears and brown bears can mate and

produce fertile offspring. They must bemembers of the same species, then.But are they? As you read this chapter,look for clues to whether polar bearsare a separate species. Then, solvethe mystery.

Never Stop Exploring Your World.Solving the mystery of scientificclassification is only the beginning.Take a video field trip with theecogeeks of Untamed Science to seewhere the mystery leads.

^-IffitaaBU !ij nee

Classification 509

Key Questions

E3 Whatare thegoals ofbinomial nomenclature andsystematics?

CS How did Linnaeus groupspecies intolarger taxa?

Vocabularybinomial nomenclature •

genus • systematics • taxon •family • order • class •phylum • kingdom

Taking NotesPreview Visuals Before you read,look at Figure 18-5. Notice allthe levels of classification. As youread, refer to the figure again.

Hk' •

Finding Orderin Diversity

THINK ABOUT IT Scientists have been trying to identify, name, andfind order in the diversity of life for a long time. The first scientificsystem for naming and grouping organisms wasset up long beforeDarwin. In recentdecades, biologists havebeen completinga changeover from that older system of names and classification to a newerstrategy that is based on evolutionary theory.

Assigning Scientific NamesWhat are me goals ofbinomial nomenclature andsystematics?

The first step in understandingand studyingdiversity is to describe andname each species. To be useful, each scientific name must refer to oneand only one species, and everyone must use the same name for thatspecies. But what kind of name should be used? Common names can beconfusing, because theyvaryamonglanguages and from place to place.Theanimal in Figure 18-1,forexample, canbe called a cougar, a puma,a panther, or a mountain lion. Furthermore, different species maysharea common name. In the United Kingdom, the word buzzard refers to ahawk, whereas in the United States, buzzard refers to a vulture.

Back in the eighteenth century, European scientists recognized thatthese kinds of common names were confusing, so theyagreed to assignLatin or Greek names to each species.Unhappily, that didn't do muchto clear up the confusion. Early scientific names often described speciesin greatdetail, so the names could be long.For example, the Englishtranslation of the scientific nameof a tree mightbe "Oakwith deeplydivided leaves that have no hairs on their undersides and no teeth

around their edges." It was also difficult to standardize these names,becausedifferent scientists focused on different characteristics. Manyofthese same characteristics can still be used to identify organisms whenusing dichotomous keys, as you can see in Figure 18-2.

FIGURE 18-1 Common Names You might recognize thisas a cougar, a puma, a panther, or a mountain lion—allcommon names for the same animal. The scientific namefor this animal is Felis concolor.

• Lesson Notes

la

lb

2a

2b

3a

3b

4a

4b

5a

5b

6a

6b

[*]{•

USING A DICHOTOMOUS KEYFIGURE 18-2 A dichotomous key is used to identify organisms.It consists ofa series of paired statements or questions thatdescribe alternative possible characteristics of an organism.The paired statements usually describe the presence or absenceof certain visible characteristics or structures. Each set of choicesis arranged so that each step produces a smaller subset.

Suppose you found a leaf that you wanted to identify. The leaflooks like the one shown here. Use the key to identify this leaf.

Leaf Characteristics

Compound leaf (leaves divided into leaflets) . .go to Step 2

Simple leaf (leaf not divided into leaflets) . . .go to Step 4

Leaflets all attached at a central point

Leaflets attached at several points ... go to Step 3

Buckeye• vt/

Because your leaf is asimple leaf, you skipahead to Step 4.

Leaflets tapered with pointed tips

Leaflets oval with rounded tips

Veins branched out from one central point. . .go to Step 5

Veins branched off main vein in middle ofthe leaf... go to Step 6

Heart-shaped leaf

Star-shaped leaf

Leaf with jagged edges

Leaf with smooth edges

•4 Pecan

Locusts /

Redbud^

Sweet gum

Birch

Magnolia^

l9i.com Search GO • Art In Motion t

Continue reading thestatements until youdetermine the identityof your leaf.

Because your leaf hasjagged edges, youdetermine that it's froma birch tree.

511

MYSTERY

CLUEPolar bears andbrown bears interbreedand producefertile hybrids in zoos,but theyvery rarelyinterbreed in nature.What do you thinkthis means about therelationship betweenthem?

FIGURE 18-3 BinomialNomenclature The scientific nameof the polar bear is Ursus maritimus,which means "marine bear." Thescientific name of the red maple isAcer rubrum. The genus Acer consistsof all maple trees. The species rubrumdescribes the red maple's color.

512 Chapter 18 • Lesson 1

Binomial Nomenclature In the 1730s, Swedish botanist,Carolus Linnaeus, developed atwo-word naming system calledbinomial nomenclature. G3 In binomial nomenclature, eachspeciesis assigned a two-part scientificname. Scientific namesare written in italic. The first word begins with a capital letter, andthe second word is lowercased.

The polar bear in Figure 18-3 is called Ursus maritimus. Thefirst part ofthe name—Ursus—is the genus towhich the organism belongs. Agenus (plural: genera, jen ur uh) isa group ofsimilar species.tThe genus Ursus contains five other species ofbears, including Ursus arctos, the brown bear or "grizzly."

The second part of a scientific name—in these examples,maritimus or arctos—is unique to each species.A species, remember, is generally defined as a group of individuals capable ofinterbreeding and producing fertile offspring. The species nameis often a description of an important trait or the organism'shabitat. The Latin word maritimus, refers to the sea, because polarbears often live on pack ice that floats in the sea.

- ^ In Your Notebook-^ names.'

The word binomial means "having twoHow does this meaning apply to binomial nomenclature?

Classifying Species Into Larger Groups Inaddition tonaming organisms, biologists also try to organize, or classify, living andfossil species into larger groups that have biological meaning. Ina useful classification system, organisms in a particular group aremore similar to one another than they are to organisms in othergroups. The science of naming and grouping organisms is calledsystematics (sis tuh mat iks).C3 The goal of systematics is toorganize living things into groups that have biological meaning.Biologists often referto these groups as taxa (singular: taxon).

Whether you realize it or not, you use classification systems allthe time. You may, for example, talk about "teachers" or "mechanics." Sometimes you refer to a smaller, more specific group, suchas"biologyteachers" or "auto mechanics." When you do this, yourefer to these groups using widelyaccepted names and characteristicsthat many people understand. In the same way, whenyou hear the word bird, you immediately think of an animal withwings and feathers.

Classifying Fruits US Jk0 Obtain five different fruits.

0 Use a knife to cut each fruit open andexamine its structure. CAUTION: Be carefulwithsharp instruments. Do not eatanyof the

0 Construct a table with five rows and fourcolumns. Label each row with the name of a

different fruit.

O Examine the fruits, and choose fourcharacteristics that help you tell the fruits apart.Label the columns in your table with the names ofthese characteristics.

0 Record a description of each fruit in the table.

Analyze and Conclude1. Classify Based on your table,which fruits mostclosely resemble one another?

The Linnaean Classification SystemG3 How did Linnaeus group species into larger taxa?In addition to creating the system of binomial nomenclature, Linnaeusalso developed a classification system that organized species into taxathat formed a hierarchy or set of ordered ranks. Linnaeus's originalsystem had just four levels. CHS Over time, Linnaeus's originalclassification system expanded to include seven hierarchical taxa: species,genus, family, order, class, phylum, and kingdom.

We've alreadydiscussedthe two smallest categories, species and,genus.Now let's work our way up to the rank of kingdom by examininghow camels are classified.The scientific name of a camel with two humpsis Camelus bactrianus. (Bactria was an ancient country in Asia.) As youcan see in Figure 18-5, the genus Camelus also includes another species,Camelus dromedarius, the dromedary, which has only one hump.In deciding how to place organisms into these larger taxa, Linnaeusgrouped speciesaccording to anatomical similarities and differences.

• Family The South American llama bears some resemblance toBactrian camels and dromedaries. But the llama is more similar to

other South American species than it is to European and Asian camels.Therefore, llamas are placed in a different genus, Lama; their speciesname is Lamaglama. Several genera that share many similarities, likeCamelus and Lama, are grouped into a larger category, the family—inthis case, Camelidae.

• Order Closely related families are grouped into the next largerrank—an order. Camels and llamas (family Camelidae) are groupedwith several other animal families, including deer (family Cervidae)and cattle (family Bovidae), into the order Artiodactyla, hoofed animals with an even number of toes.

FIGURE 18-4 Carolus Linnaeus

EnHDVocabularyMULTIPLE MEANINGS The wordsfamily, order, class, and kingdomall have different meanings inbiology than they do in commonusage. For example, in systematics,a family is a group of genera.In everyday usage, a family is agroup of people who are relatedto one another. Use a dictionaryto find the common meanings oforder, class, and kingdom.

Classification 513

• Class Similar orders, in turn, are grouped into the next larger rank,a class. The order Artiodactyla is placed in the class Mammalia, whichincludes allanimals that arewarmblooded, have body hair, and produce milk for their young.

• Phylum Classes are grouped into a phylum. A phylum includesorganisms that are different but share important characteristics. Theclass Mammalia isgrouped withbirds (class Aves), reptiles (class Rep-tilia), amphibians (class Amphibia), and all classes offish into thephylum Chordata.These organisms share important body-planfeatures, among them a nerve cord along the back.

• Kingdom The largest and most inclusive of Linnaeus's taxonomiccategories is the kingdom. All multicellular animals are placed in thekingdom Animalia.

FIGURE 18-5 From Species to KingdomThis illustration shows how a Bactriancamel, Camelus bactrianus, is groupedwithin each taxonomic category. Only somerepresentative organisms are illustratedfor each taxon above the genus level.Interpret Visuals What phylum doesCamelus bactrianus belong to?

SPECIES Camelus bactrianus

I GENUS Camelus

FAMILY Camelidae

ORDER Artiodactyla

CLASS Mammalia ^^

PHYLUM Chordata

KINGDOM AnimaliaBactrian camel Dromedary Llama Giraffe Abert's squirrel Coral snake Sea star

514 Chapter 18 • Lesson 1

Problems Wilh Traditional Classification In a sense, membersof aspecies determine which organisms belong tothat species by deciding withwhom they mate and produce fertile offspring. There is thus a"natural"definition of species. Researchers, on the otherhand, define Linnaeanranks above thelevel of species. Because, overtime,systematists haveemphasized a variety ofcharacteristics, some ofthese groups have beendefined in differentways at different times.

For example, Linnaeus's strategy of classifying organisms accordingto visible similarities and differences seems simple at first. But howshould scientists decide which similarities and differences are most

important? If you lived in Linnaeus's time, for example,how wouldyou have classified the animals shown in Figure 18-6? Adult barnaclesand limpets live attached to rocks and have similar-looking shells.Adult crabs look quite unlike both barnacles and limpets. Based onthese features, would you place limpets and barnacles together, andcrabs in a different group? As biologists attempted to classifymoreorganisms over time, these kinds of questions arose frequently.

Linnaeus was a good scientist, and he chose his characteristicscarefully. Many of his groups are still valid under modern classificationschemes. But Linnaeus worked more than a century before Darwinpublished his ideas about descent with modification. Modern systematists apply Darwin's ideas to classification and try look beyondsimple similarities and differences to ask questions about evolutionary relationships.Linnaeus grouped organisms strictly accordingtosimilarities and differences. Scientists today try to assign species to alarger group in ways that reflect how closely members of those groupsare related to each other.

FIGURE 18-6 Barnacles, Limpets,and Crabs Problems can arise whenspecies are classified based oneasilyobserved traits. Look closelyat the barnacles (top), the limpets(bottom), and the crab (left). Noticetheir similarities and differences.Compare and Contrast Whichanimals seem mostalike? Why?

Review Key Concepts G31. a. Review Identify two goals of systematics.

b. Explain Why do the common names oforganisms—like daisy or mountain lion—often cause problems for scientists?c. Classify The scientific nameof the sugarmaple is Acersaccharum. What does eachpart of the name designate?

2. a. Review List the ranks in the Linnaeansystem of classification, beginning withthe smallest.

b. Explain Inwhich group oforganisms are themembers more closely associated—all of theorganisms in the same kingdom or all of theorganisms in the sameorder? Explain youranswer.c. Apply Concepts What do scientists meanwhen theysay that species is the only"natural"rank in classification?

OCY.corn Search Lesson 1 8.

Apply the Big idea

Unity and Diversity of Life3. Which category has more biological meaning—

all brown birds or all birds descended from ahawklike ancestor? Why?

Self-Test • Lesson Assessment , ,„,,.„.„,»„,,

Classification 515

f *% Modern Evolutionary\Q%JU Classification

Key Questions

G_ What is thegoal ofevolutionary classification?

C_) Wifjar is a cladogram?

£3 Howare DNA sequencesused in classification?

Vocabularyphylogenyclademonophyletic groupcladogramderived character

Taking NotesOutline Make an outline ofthis lesson using the green headings as the main topics and theblue headings as subtopics. Asyou read, fill in details undereach heading.

I32JJ33VocabularyWORD ORIGINS The wordcladogram comes from twoGreek words: klados, meaning"branch," and gramma,meaning "something that iswritten or drawn." A cladogramis an evolutionary diagram witha branching pattern.

THINK ABOUT IT Darwin's ideas about a"tree oflife" suggests anewwayto classify organisms—not just based on similarities and differences,but insteadbasedon evolutionary relationships. Under this system, taxaare arrangedaccording to how closely related they are. When organismsare rearranged in this way, some of the old Linnaean ranks fall apart. Forexample, the Linnaean classreptilia isn't valid unless birds are included—which means birds are reptiles! And not only are birds reptiles, they'realso dinosaurs! Wondering why? To understand, we need to look at theway evolutionary classification works.

Evolutionary ClassificationGD What is the goal of evolutionary classification?The concept of descent with modification led to the study ofphylogeny (fy lahj uh nee)—the evolutionary history of lineages.Advances in phylogeny, in turn, led to phylogenetic systematics.GS The goalof phylogenetic systematics, or evolutionaryclassification, is to group species into larger categories that reflectlines of evolutionary descent, rather than overall similaritiesand differences.

Common Ancestors Phylogenetic systematics places organisms intohigher taxawhose members aremore closely related to one anotherthan they are to members of anyother-group. The larger a taxon is,the farther back in time all of its members shared a common ancestor.

This is true all the way up to the largest taxa.

Clades Classifying organisms according to these rules places them intogroups called clades. Aclade isa group of species that includes a singlecommon ancestor and all descendants of that ancestor—living andextinct. How are clades different from Linnaean taxa? A clade must be amonophyletic (mahnoh fy let ik) group. Amonophyleticgroup includesa single common ancestor and allof itsdescendants.

Some groups oforganisms defined before the advent of evolutionaryclassification aremonophyletic. Some,however, are paraphyletic, meaningthat the groupincludes a commonancestor but excludes one or moregroups of descendants. These groups are invalid underevolutionaryclassification.

In Your Notebook In your own words, explain what makes aclade monophyletic or paraphyletic. _ _______ -

516 Search Lesson 18.2 S JiO • Lesson Overview • Lesson NotesimiiiiiiM ii MMi^iiiiiiiiiiiiiiiiiiiiiiiwiiiiiiiMnlHiMil'ilii'iliilii u—i iwililirilliiirnriini

CladogramsGD What is a cladogram?Modern evolutionary classification uses amethod called cladistic analysis. Cladisticanalysis compares carefully selected traits todetermine the order in which groups of organisms branched off from their common ances

tors. This information is then used to link clades

together into a diagram called a cladogram.GS Acladogram linksgroupsoforganismsby showing how evolutionary lines, or lineages,branched off from common ancestors.

Building Cladograms To understand howcladograms are constructed, think back tothe process of speciation. A speciation event,in which one ancestral species splits into twonew ones, is the basisNaf each branch point, ornode, in a cladogram. That node representsthe last point at which the two new lineagesshared a common ancestor. As shown in part 1of Figure 18-7, a node splits a lineage into twoseparate lines of evolutionary ancestry.

Each node represents the last point atwhich species in lineages above the nodeshared a common ancestor. The bottom, or"root" of a cladogram, represents the commonancestor shared by all of the organisms in thecladogram. A cladogram's branching patternsindicate degrees of relatedness among organisms. Look at part 2 of Figure 18-7. Becauselineages 3 and 4 share a common ancestormore recently with each other than they dowith lineage 2, you know that lineages 3 and4 are more closely related to each other thaneither is to lineage 2. The same is true forlineages2, 3, and 4. In terms of ancestry, theyare more closely related to each other thanany of them is to lineage 1.Look at the cladogram shown in part 3 of Figure 18-7. Does itsurprise you thafe amphibians are more closelyrelated to mammals than they are to ray-finned fish? In terms of ancestry, it's true!

FIGURE 18-7 Building a Cladogram A cladogram showsrelative degrees of relatedness among lineages.

Splitting eventAncestral lineage

© Cladograms are diagrams showing howevolutionary lines, or lineages, split from eachother over time. This diagram shows a singleancestral lineage splitting into two. The point ofsplitting is called a "node" in the cladogram.

Descendants

XJ How recently lineages share a commonancestor reflect how closely the lineages arerelated to one another. Here, lineages 3 and4 are each more closely related to each otherthan any of them is to any other lineage.

tJ This cladogram shows the evolutionaryrelationships among vertebrates, animals withbackbones.

Search Lesson 18.2 i&X • InterActive Art 517

FIGURE 18-8 Derived CharactersThe coyote and lion share severalcharacters—hair, four limbs, andspecialized shearing teeth. Theseshared characters put them in theclades Tetrapoda, Mammalia, andCarnivora. The lion, however, hasretractable claws. Retractable clawsis the derived character for the cladeFelidae.

Derived Characters In contrast to Linnaean taxonomy, cladisticanalysis focuses on certain kinds of characters, called derived characters, when assigning organisms into clades. A derived character isa trait that arose in the most recent common ancestor of a particular lineage and was passed along to its descendants.

Whether or not a character is derived depends on the level atwhich you're grouping organisms. Here's what we mean. Figure 18-8shows several traits that are shared by coyotes and lions, bothmembers of the clade Carnivora. Four limbs is a derived character

for the entire clade Tetrapoda because the common ancestor of alltetrapods had four limbs, and this trait was passed to its descendants. Hair is a derived character for the clade Mammalia. But for

mammals, four limbs is not a derived character—if it were, onlymammals would have that trait. Nor is four limbs or hair a derived

character for clade Carnivora. Specialized shearing teeth, however,is. What about retractable claws? This trait is found in lions but

not in coyotes. Thus, retractable claws is a derived character for theclade Felidae—also known as cats.

Losing Traits Notice above that four limbs is a derived characterfor clade Tetrapoda. But what about snakes? Snakes are definitelyreptiles, which are tetrapods. But snakes certainly don't have fourlimbs! The ancestors of snakes, however, did have four limbs.

Somewhere in the lineage leading to modern snakes, that trait waslost. Because distantly related groups of organisms can sometimeslose the same character, systematists are cautious about using theabsence of a trait as a character in their analyses. After all, whalesdon't have four limbs either, but snakes are certainly more closelyrelated to other reptiles than they are to whales.

Specializedshearing teeth

y|

f€JTFour limbs

518 Chapter 18 • Lesson 2

VISUAL SUMMARY

INTERPRETING A CLADOGRAM

FIGURE 18-9 This cladogram shows the evolutionary historyof cats. In a cladogram, all organisms in a clade share a setof derived characters. Notice that smaller clades are nestedwithin largerclades. Interpret Visuals For which clade in miscladogram is an amniotic egg a derived character?

Clade TetrapodaClade Amniota

Clade Mammalia

Clade CarnivoraClade Felidae

Amphibians

Four

limbs

Amniotic egg(egg withmembranes)

A clade consists of a single commonancestor and all the groups thathave descended from that ancestor.

Specializedshearing teeth

A node is a point at whichtwo groups branched off fromeach other. It also representsthe last point at which thosetwo groups shared a commonancestor. This node showsmarsupials branching off fromother mammals.

Interpreting Cladograms We can now put this informationtogether to "read" a cladogram. Figure 18-9 shows a simplified phylogeny of the cat family. The lowest node represents the last commonancestor of all four-limbed animals—members of the cladeTetrapoda.The forks in this cladogram showthe order in which various groupsbranched off from the tetrapod lineage over the course of evolution.The positions of various characters in the cladogram reflect the orderin which those characteristics arose in this lineage. In the lineageleading to cats,for example, specialized shearingteeth evolved beforeretractable claws. Furthermore, each derived character listed alongthe main trunk of the cladogram defines a clade. Hair, for example,is a defining character for the clade Mammalia. Retractable claws is aderived character shared only by the clade Felidae. Derived charactersthat occur"lower" on the cladogram than the branch point for a cladearenot derived for that particularclade. Hair, for example, isnot aderived character for the clade Carnivora.

<ts- In Your Notebook List the derived characters in Figure 18-9and-•^ explain which groups in the cladogram have those characters.

Retractable

claws

A derived character, likeretractable claws, is a traitshared by all members ofa clade and onlybymembers of that clade.

Classification 519

FIGURE 18-10 Clade or Not? A clade includes anancestral species and all its descendants. Linnaeanclass Reptilia is not a clade because itdoes not includemodern birds. Because it leaves this descendant groupout, the class is paraphyletic. Clades Reptilia andAves, however, are monophyletic and, therefore, validclades. ApplyConcepts Would a group that includedallof clade Reptilia plus amphibians be monophyleticorparaphyletic? Explain.

CLASS REPTILIA:

NOT A CLADE

CLADE

AVES

Constructing a CladogramO Identify the organism in the table that is leastclosely related to the others.

@ Use the information in the table to construct acladogram of these animals.

Analyze and Conclude1. Interpret Tables What trait separates the leastcloselyrelated animal from the other animals?

2. Apply Concepts Do you haveenoughinformation to determine where a frog should beplaced on the cladogram? Explain your answer.

520 Chapter 18 • Lesson 2

Clades and Traditional Taxonomic Groups Whichof the Linnaean groupings form clades, and which donot? Remember that a true clade must be monophyletic,which means that it contains an ancestral speciesand allof its descendants—it can't leave any out. Italso cannot include any species which are not descendants of that original ancestor. Cladistic analysis showsthat many traditional taxonomic groups do form validclades. For example, Linnaean class Mammalia corresponds to clade Mammalia (shown in Figure 18-9).Members of this clade include all vertebrates with hair

and several other important characteristics.In other cases, however, traditional groups do not

form validclades. Figure 18-10 showswhy. Today'sreptiles are all descended from a common ancestor. Butbirds, which have traditionally not been considered partof the Linnaean class Reptilia, are also descended fromthat same ancestor. So, class Reptilia, without birds, isnot a clade. However, several valid clades do include

birds: Aves (the birds themselves), Dinosauria, and theclade named Reptilia.So, is it correct to call birds reptiles? An evolutionary biologist would say yes!

You may be wondering: class Reptilia, clade Reptilia, who cares? But the resulting names aren't asimportant as the concepts behind the classification.Evolutionary biologists look for links between groups,figuring out how each is related to others. So the nexttime you see a bird, thinking of it as a member of aclade or class isn't as important as thinking about itnot just as a bird, but also as a dinosaur, a reptile, atetrapod, and a chordate.

Derived Characters in OrganismsOrganism Derived Character

Backbone Legs Hair

Earthworm Absent Absent Absent

Trout Present Absent Absent

Lizard Present Present Absent

Human Present Present Present

3. DrawConclusions Does your cladogramindicate that lizards and humans share a more

recent common ancestor than either does

with an earthworm? Explain your answer.

DNA in ClassificationCD How are DNA sequences usedin classification?The examples of cladistic analysis we've discussed so far are basedlargely on physical characteristics like skeletons and teeth. But the goalof modern systematics is to understand the evolutionary relationshipsof all life on Earth—from bacteria to plants, snails, and apes. How canwe devise hypotheses about the common ancestors of organisms thatappear to have no physical similarities?

Genes as Derived Characters Remember thatall organisms carrygenetic information in their DNA passed on from earlier generations. A wide range of organisms share a number of genes and showimportant homologies that can be used to determine evolutionaryrelationships. For example, all eukaryotic cells have mitochondria, andall mitochondria have their own genes. Because all genes mutate overtime, shared genes contain differences that can be treated as derivedcharacters in cladistic analysis. For that reason, similarities and differences in DNA can be used to develop hypotheses aboutevolutionary relationships. Q In general, the morederived genetic characters two species share, the morerecently they shared a common ancestor and the moreclosely they are related in evolutionary terms.

NewTechniques Suggest NewTrees The use ofDNAcharacters in cladistic analysis has helped to make evolutionary trees more accurate. Consider, for example, thebirds in Figure18-11.TheAfrican vulturein the top photograph looks a lot like the American vulture in the middlephotograph. Both were traditionally classified in the falconclade.But American vultures have a peculiar behavior: Whentheygetoverheated, theyurinateon their legs, relying onevaporation to cool them down. Storks share this behavior,while African vultures do not. Could the behavior be a clue to therelationships between these birds?

Biologists solved the puzzle byanalyzing DNA from all three species. Molecular analysis showed that the DNA from American vulturesis more similar to the DNA of storks than to the DNA of Africanvultures. DNA evidence therefore suggests that Americanvultures andstorks share a more recent common ancestor than the American andAfrican vultures do. Molecular analysis is a powerful tool that is nowroutinely used bytaxonomists to supplement data from anatomy andanswer questions like these.

BIOLOGY.com

FIGURE 18-11 DNA and Classification Scientists usesimilarities in the genetic makeup oforganisms to helpdetermine classification. Traditionally African vultures andAmerican vultures were classified together in the falcon family.But DNA analysis suggests that American vultures are actuallymore closely related to storks.

Search 18. GO Data Analysis 521

MYSTERY •

CLUEDNAcomparisons showthat some populations ofbrown bears are more

closely related to polarbears than they are toother brown bears. What

do you thinkthis meansfor the classification ofpolar bears?

Often, scientists use DNA evidence when anatomical traits

alone can't provideclearanswers. Giant pandas and red pandas,for example, have given taxonomists a lot of trouble. These twospecies share anatomical similarities with both bears and raccoons, and both of them have peculiar wrist bones that work like ahuman thumb. DNAanalysis revealedthat the giant panda sharesa more recent common ancestor with bears than with raccoons.

DNA places red pandas, however,outside the bear clade.So pandashave been reclassified, placed with other bears in the clade Ursidae,as shown in Figure 18-12. What happened to the red panda? Itis now placed in a different clade that also includes raccoons andother organisms such as sealsand weasels.

0 9Red pandas Grant pandas BearsRaccoons

Common Ancestor

Assessment

Review Key Concepts G31. a. Explain How does evolutionary classification differ

from traditional classification?

b. ApplyConcepts To an evolutionary taxonomist, whatdetermines whether two species are in the same genus?

2. a. Explain What is a derived character?b. Interpret Diagrams Along any one lineage, whatdo thelocations of derived characters on a cladogram show? Inyour answer, use examples from Figure 18-9.

3. a. Review How do taxonomists use the DNA sequences ofspecies to determine howclosely two species are related?b. RelateCause and Effect Explain why the classificationof American vultures has changed.

FIGURE 18-12 Classification ofPandas Biologists used to classify thered panda and the giant panda together.However, cladistic analysis using DNAsuggests that the giant panda shares a morerecent common ancestor with bears than witheither red pandas or raccoons.

VISUAL THINKING

4. Examine the cladogram.

a. Interpret Diagrams Whichgroups—X and Y, or X , Y, andZ—have the most recent common

ancestor?

b. Infer Which species—X and Y,or X and Z—share more derived

characters?

BIOLOGY.com Search Lesson T8.2 GO, • Self-Test • Lesson Assessment 1-

522 Chapter 18 • Lesson 2

•S0j Vi

Building the Tree of Life

THINK ABOUT IT The process of identifying and naming all knownorganisms, living and extinct, is a huge first step toward the goal ofsystematics. Yet naming organisms is only part of the work. The realchallenge is to group everything, from bacteria to dinosaurs to bluewhales, in a way that reflects their evolutionaryrelationships. Overthe years, newinformationand ways of studying organisms haveproduced major changes in Linnaeus's original scheme for organizingliving things.

Changing Ideas About KingdomsC3D What are the six kingdoms oflife as they are now identified?During Linnaeus's time, the onlyknown differences among livingthings were the fundamental characteristics that separated animalsfrom plants. Animals were organisms thatmoved from place to placeand used food for energy. Plants were green organisms that generallydid not move and got their energy from the sun.

As biologists learned more about the natural world, they realizedthat Linnaeus's two kingdoms—Animalia and Plantae—did not reflectthe full diversity oflife. Classification systems have changed dramatically since Linnaeus's time, as shown in Figure 18-13. And hypothesesabout relationships among organisms are still changing today as newdata are gathered.

Kingdoms of Life, 1700s-1990s

Key Questions

C3 Whatare the six kingdomsof life as mey are nowidentified?

G3 What does the tree of lifeshow?

Vocabularydomain «

Archaea

Bacteria «

• Eukarya

Taking NotesConcept Map As you read, construct a concept map describingthe characteristics of the threedomains.

First Introduced Names of Kingdoms

1700s4

Plantae Animalia

Late 1800sL

Protista Plantae Animalia

1950s Monera Protista Fungi Plantae Animalia

1990s Eubacteria Archaebacteria Protista Fungi Plantae Animalia••BHHHB

Search Lesson 18.3

FIGURE 18-13 From Two to Six Kingdoms Thisdiagram shows some of the ways in which organismshave been classified into kingdoms since the 1700s.

523