14 MARCH 7, 2005 SCIENCE WORLD 15

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LIFE HISTORY/ADAPTATIONS

SAMPLE THIS: Eldredge (top), aDarwin expert, studies fossils that hehas collected. Charles Darwin foundmany of his research specimens inthe Galápagos Islands (bottom).

hy don’t polar bears livein a steamy rain forest?

And why don’t pandas livein the Arctic? It may seem

obvious that animals have differenttraits that help them survive in different environments. But how didthe animals develop these physicalcharacteristics in the first place? Itwasn’t until the 19th century that scientists began to understand thisprocess. That’s when British natural-

ist Charles Darwin (1809–1882) introduced many ideas that changedpeople’s perceptions about the naturalworld, says Niles Eldredge, a paleontologist who studies fossils atthe American Museum of NaturalHistory in New York.

By examining nature, Darwinbecame one of the first scientists topiece together how so many differenttypes of organisms came about, andhow each one became suited to a particular environment. “His writingsare still fundamental to sciencetoday,” says Eldredge, a Darwinexpert. What were some of Darwin’sideas, and how did he develop them?

When Darwin was 22 years old, hewas offered the opportunity of a life-time. The captain of the HMS Beagle

invited him to join his ocean voyage toexplore South America. “Darwin hadan unstoppable fascination withnature, so he jumped at the chance toleave England and see another part ofthe world,” Eldredge says. Eldredgetells Science World how the eye-open-ing sights, on what turned out to be afive-year trip, inspired Darwin’s think-ing about the diversity of life on Earth.

How did Darwin conduct hisresearch during the ship’slengthy voyage?

Darwin was very prone to seasick-ness, so he got off the Beagle andonto dry land whenever he could. Hespent months exploring whateverareas they reached (see map, p. 16).Darwin was so full of curiosity thathe collected and catalogued all sortsof plant and animal specimens. Hetook notes on their traits, and evensent crates full of organisms back toEngland so that he could study themfurther upon his return.

VOYAGEA historic South American cruise led to

groundbreaking findings about the wild kingdom

W NilesEldredge

B Y M O N A C H I A N G

Discoveryof

Are there any funny storiesrelated to the specimens thatDarwin collected?

There are these large, flightless birdsin Argentina called rheas. Local rumorhad it that a different species of rheaexisted, but most people had neverseen it. Naturally, Darwin was anxiousto find and study the elusive bird.

For dinner one night, Darwin ate asmaller-than-usual rhea. In the middleof the meal, Darwin noticed that thebird’s bones didn’t look like the com-mon rheas’ bones. Suddenly, he went“oops!” He had eaten the mystery bird.

Yikes! Did anyone find anothersmall rhea?

Yes—as it turned out, the smallerrhea that Darwin had eaten for dinnerthat night was not in its usual habitat.Darwin later learned that the smallerbird normally lives in a region southof where it was caught. This gotDarwin wondering: Why would twobirds that are so closely related liveapart from each other, thriving indifferent environments?

Did Darwin find an answer?Yes, but first he made similar

observations about many other ani-mals. He was particularly inspired by his visit to the Galápagos Islands.Darwin noticed a pattern amongplants and animals living there: Manyanimals, such as mockingbirds andgiant tortoises, looked slightly differ-ent on one island than they did onanother island. The governor of the

Galápagos Islands even told Darwinthat he could tell which island a tor-toise is from just by looking at theshape and pattern of its shell.

What is so telling about theshape of the tortoises’ shells?

There are two main shell shapes(see photos, above). Saddleback tor-toises live on dry islands with littlegrass, but with many tall plants.

16 OCTOBER 3, 2005 SCIENCE WORLD 17

These tortoises have shells that allowtheir long necks to extend upward toreach the food.

On the other hand, tortoises withdome-shaped shells live on wetterislands where plants grow low to theground. This shell shape only allowsthe animal’s shorter neck to extendforward—fine for grabbing a bite of low-lying grass. Each islandappeared to have different condi-tions that made the resident tortoisedevelop a shell type that helpedincrease the animal’s chances to eatand thus survive there.

What else did Darwin discover?After Darwin returned to London,

he spent many years poring over thenotes he took and the specimens thathe had sent back from his five-year-

long journey. He was puzzled aboutsomething: What keeps one species ofplant or animal from over-populating the planet—and living in every corner of theworld?Darwin had anidea, but first he thought it throughusing an animal he was familiarwith—elephants: Suppose a pair ofelephants were to mate and have acouple of babies in their lifetime.When the babies matured, they wouldhave their own offspring. The cyclewould go on and on, and the elephantpopulation would explode. Since theworld isn’t neck to neck with ele-phants, Darwin realized that theremust be something in nature thatlimits populations.

What does this have to dowith tortoises and rheas?

From his observations, Darwinproposed a process he called natural

selection. As the environmentchanges, a species may evolve bychanging its physical features overtime, making it better adapted to its new habitat. Animals with traitsbest suited for their habitat—such as being especially able at gatheringfood or resisting predators—oftenwill produce more young in the nextgeneration than those less suited tothe environment. These offspring willnormally inherit the same traits as

their parents, or in rare cases,develop new ones. Over time,these better-adapted traits,

or adaptations, will replace the oldtraits, and so the physical features ofthe population will change.

How have Darwin’s ideas influenced your work?

I’m interested in all aspects of life,and my expertise is in fossils calledtrilobites. These traces of extinct organ-isms hint at what life was like millionsof years ago and how organisms havechanged since then. Darwin openedthe door to this field of study. 9

SADDLEBACK TORTOISESdwell on dry islands thathave various tall plants.

VOYAGE OF THE BEAGLE (1831–1836)The ship visited the Galápagos in 1835.

Checkit Out:

During his voyage aboard the Beagle,Charles Darwin collected numerouspreviously unknown plants and ani-mals. Many of these species can be seen in an exhibition at theAmerican Museum of Natural Historydocumenting this great scientist’s lifeand work. With more than 30 millionobjects in its collection, the Museumhas been researching and celebratingthe natural world for more than 135years. Its 200 scientists travel aroundthe world on 100 field expeditionseach year, studying everything fromfossils to lizards to the universe.

To learn more, visit:www.scholastic.com/adapt

To learn more, ask your teacher,or visit www.amnh.org.

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Canary Is.Madeira Is.

Plymouth, England

Cape Verde Is.

Saint Helena

Tierra del Fuego

Galápagos Is.

Tahiti Mauritius

Cocos Is.

New Zealand

Nuts& Bolts

DOME-SHAPED TORTOISESlive on moist islands filled

with grass and low vegetation.