biologist

TPO-42

GeographicIsolationofSpecies

BiologistErnstMayrdefinedaspeciesas“anactuallyorpotentiallyinterbreedingpopulationthatdoesnot

interbreedwithothersuchpopulationswhenthereisopportunitytodoso.”Akeyeventintheoriginofmanyspeciesis

theparationofapopulationwithitsgenepool(allofthegenesinapopulationatanyonetime)fromotherpopulations

ofthesamespecies,sgenepoolisolated,aparatepopulationcan

ormationofmanyspecies,theinitialisolationofapopulationemstohave

deofevolvingnewspeciesiscalledallopatricspeciation.

ainrangemayemergeandgraduallysplita

populationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedinthisway.

Similarly,acreepingglaciermaygraduallydivideapopulation,oralandbridgesuchastheIsthmusofPanamamay

formandparatethemarinelifeintheoceanwatersoneitherside.

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Howformidablemustageographicbarrierbetokeeppopulationsapart?Itdependsontheabilityoftheorganisms

sageofwind-blowntreepollen

isalsonothinderedbysuchbarriers,andtheedsofmanyplantsmaybecarriedbackandforthonanimalsIncontrast,

mple,theGrandCanyon,inthe

southwesternUnitedStates,paratetherangeofthewhile-tailedantelopesquirrelfromthatoftheclolyrelatedHarris’

r,withashortertailthatiswhiteunderneath,thewhite-tailedantelopesquirrelinhabitsderts

northofthecanyonandwestoftheColoradoRiverinsouthernCaliforniaHams'antelopesquirrelhasamorelimited

rangeindertssouthoftheGrandCanyon.

Geographicisolationcreatesopportunitiesfornewspeciestodevelop,butitdoesnotnecessarilyleadtonew

speciesbecauspeciationoccursonlywhenthegenepoolundergoenoughchangestoestablishreproductivebarriers

elihoodofallopatricspeciationincreaswhena

populationissmallaswellasisolated,makingitmorelikelythanalargepopulationtohaveitsgenepoolchanged

mple,inlessthantwomillionyears,smallpopulationsofstrayanimalsandplantsfromtheSouth

AmericanmainlandthatmanagedtocolonizetheGalapagosIslandsgaveritoallthespeciesthatnowinhabitthe

islands.

Whenoceanicislandsarefarenoughaparttopermitpopulationstoevolveinisolation,butcloenoughtoallow

occasionaldispersionstooccur,apagosislandchainisone

oftheworld’landwasbornfromunderwatervolcanoesandwasgradually

coveredbyorganismsderivedfromstraysthatrodetheoceancurrentsandwindsfromotherislandsandcontinents.

Organismscanalsobecarriedtoislandsbyotherorganisms,suchasabirdsthattravellongdistanceswitheds

clingingtotheirfeathers.

ThespeciesontheGalapagosIslandstoday,mostofwhichoccurnowhereel,descendedfromorganismsthat

floated,flew,tance,theGalapagosislandchain

hasatoirdshavemanysimilaritiesbut

differintheirfeedinghabitsandtheirbeaktype,latedevidenceindicates

thatallthirteenfinchspecievolvedfromasinglesmallpopulationofancestralbirdsthatcolonizedoneoftheislands.

Completelyisolatedontheislandaftermigratingfromthemainland,thefounderpopulationmayhaveundergone

significantchangesinitsgenepoolandbecomeanewspecies.■Later,afewindividualsofthisnewspeciesmayhave

beenblownbystormstoaneighboringisland.■Isolatedonthiscondisland,thecondfounderpopulationcouldhave

evolvedintoacondnewspecies,whichcouldlaterrecolonizetheislandfromwhichitsfoundingpopulationemigrated.

■TodayeachGalapagosislandhasmultiplespeciesoffinches,withasmanyastenonsomeislands.■

d“key”inthepassageisclostinmeaningto

l

able

nt

d“initial”inthepassageisclostinmeaningto

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记》.

ingtoparagraph1,allopatricspeciationispossiblewhen

ationcontainsallthedifferentgenesprentinaspeciesataparticulartime

ationbecomesisolatedduetotheprenceofageographicbarrier

cmixingbeginstooccurinpreviouslyparatepopulationsofaspecies

esissuccessfulincrossingageographicbarrier

aragraph2relatedtoparagraph1?

aph2pointsoutanumberofwaysinwhichthephenomenonofgeographicisolationmentionedinparagraph1

canoccur

aph2identifiesdiscoveriesthatledtotheconclusionprentedinparagraph1thatgeographicisolationhas

playedaroleintheoriginofmanyspecies

aph2providevidencesupportingthestatementinparagraph1thatapopulationcanfollowitsown

evolutionarycouronceitsgenepoolbecomesisolated

aph2explainswhytheterm“allopatric”wasadoptedtodescribethemethodofspeciationdescribedin

paragraph1

graph3,theauthorcontrastsavarietyoforganismstoillustratewhichofthefollowingpoints?

phicbarriersarelesslikelytokeepapartpopulationsofplantsthanpopulationsofanimals

phicbarriersaremorelikelytokeepapartpopulationsoflargeorganismsthanpopulationsofsmallorganisms

mbersofaspeciesareabletocrossgeographicbarriers,whileothermembersofthesamespeciesarenot

ectivenessofgeographicbarriersinkeepingorganismsapartdependsonanorganism’sabilitytomoveacross

barriers

aph3supportstheideathatwhite-tailedantelopesquirrelsandHams'antelopesquirrelshavewhichofthe

followingincommon?

ethetwosmallestrodentsnowfoundinthesouthwesternUnitedStates.

vewhitecoloringunderneaththeirtails

nnotcrosstheGrandCanyon

nnotsurviveindertconditions

d“undergoes"inthepassageisclostinmeaningto

ences

ts

fthentencesbelowbestexpresstheesntialinformationinthehighlightedntenceinthepassage?

Incorrectchoiceschangethemeaninginimportantwaysorleaveoutesntialinformation

phicisolationissometimesbutnotalwaysthereasonforthecreationofreproductivebarriersbetweenaparent

populationandthepopulationsdescendedfromit

cchangesinageographicalisolatedpopulationdonotnecessarilymakethepopulationlookdifferentenough

fromitsparentpopulationtobeconsideredanewspecies

phicalisolationallowstheparatedpopulationstoevolveindependentlyofeachotherandsocanleadtothe

formationofnewspecies

phicisolationcanleadtonewspeciesonlyifthegenepooloftheisolatedpopulationchangenoughto

preventitfromreproducingwiththeparentpopulation

ingtoparagraph4,whydoesthesizeofapopulationaffectthelikelihoodofallopatricspeciation?

esmallerpopulationsaremorelikelythanlargeronestobecomegeographicallyisolated

ethegenepoolofasmallisolatedpopulationismorelikelytoundergosubstantialchangethanisthegene

poolofalargerpopulation

eanisolatedpopulationcanbecomeanewspecieswithsubstantiallylesschangetoitsgenepoolthanwould

berequiredbyalargerpopulation

esmallerpopulationsaremorelikelytobemadeupofstrayanimalsorplantsthanlargerpopulationsare

d“managed”inthepassageisclostinmeaningto

le

rced

d

ed

aph5supportstheideathattheGalapagosislandchainwasabletobecome“oneoftheworld'sgreatest

showcasofevolution”primarilybecauof

hnessofthevolcanicsoilofeachoftheislandsinthechain

tanceoftheindividualislandsfromeachotherandfromthemainland

ativitylongtimeittookfortheislandstobecomecoveredbyorganisms

doorlaboratoriesthatscientistshavebuiltontheislandstostudyevolution

ingtoparagraph6,whatistrueaboutthethirteenspeciesofGalapagosfinches?

rteenspeciesarenowfoundonmostoftheGalapagosIslands

rteenspeciesaredescendedfromthesamepopulationofancestralbirds

rteenspecievolvedontheislandthatwasoriginallycolonizedbyfinchesfromthemainland.

rteenspeciesoccuronlyinsmall,completelyisolatedpopulations.

thefoursquares[█]thatindicatewherethefollowingntencecouldbeaddedtothepassage

Thisprocessofspeciationandcolonizationcouldhavebeenrepeatedoverandoveragain,graduallyinvolvingall

theislandsinthechain.

Wherewouldthentencebestfit?Clickonasquaretoaddthentencetothepassage.

ions:Antethesummary

bylectingthntencesdonot

belonginthesummarybecautheyexpressideasthatarenotprentedinthepassageorareminorideasinthepassage.

Thequestionisworth2points.

Thegeographicisolationofapopulationcanresultintheriofanewspecies.

●

●

●

AnswerChoices

○Isolationcanresultwhenageographicbarrierformsandsplitsapopulationorwhenafeworganismssomehowget

carriedacrossanexistinggeographicbarrierandformanewpopulation

○Speciationismorelikelywhenanisolatedpopulationissmallbecausignificantgeneticchangesaremorelikelyto

occurinasmallpopulationthaninalargeone

○BecauofthegeographicisolationoftheGalapagosIslands,thespeciesthatnowinhabitthemhavegenepoolsthat

havenotchangedverymuchsincetheislandswerefirstpopulated.

○Fishpopulationsaremoreeasilyisolatedbygeographicbarriersthanarepopulationsofmostotherorganismsbecau

fishcannotmoveacrossareaswherethereisnowater.

○TheGalapagosIslandsarewellsituatedforspeciationbecautheyprovideopportunitiesforpopulationisolation

whilealsomakingoccasionaldispersionsbetweenislandspossible.

○EvidenceindicatesthatthefirstorganismstoreachtheGalapagosIslandswereprobablyasmallpopulationoffinches

that,inlessthantwomillionyearsofisolation,evolvedintothirteenspecies.

答案：1-5:BCBAD6-10:CADBA11-14:BBCABE

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参考译文：物种的地理隔离

生物学家厄恩斯特·迈尔将物种定义为“一个可以互相交配或可能互相交配的群体，但这个群体不能与其它生

物交配，即便有交配的机会”。许多物种起源时的一个关键事件是有着自己的基因库的种群（任何时候这个种群

的所有基因）与该物种其他种群分离。由于基因库被隔离，各自的种群可以按照自身的进程来进化。在许多物种

的形成过程中，最初的种群隔离似乎来自一种地理上的屏障，这种进化新物种的模式被称为异域物种形成。

许多因素可以在地域上隔离一个种群。某些山脉的出现会逐渐分离出一个只能在低洼的湖泊中生活的物种，

某些鱼类可能就这样被隔离出来了。类似地，冰川的上升可能会慢慢地将同一个种群分开，或者像巴拿马地峡这

样的大陆桥会将海洋生物隔离在大陆桥的两边水域。

多么强大的地理屏障才能将种群分开呢？它取决于生物跨越障碍的能力。鸟类和狼可以轻易越过山川河流。

那些通过风来传播花粉的树木也不会受到这些壁垒的阻碍，并且很多植物的种子会被动物搬来搬去。相对来说，

一个很深的峡谷或一条宽阔的河流可能是小型啮齿类动物的有效屏障。例如，美国西南部的大峡谷将白尾羚羊松

鼠的生活范围以及与其密切相关的哈里斯羚羊松鼠的生活范围隔开。白尾羚羊松鼠体型较小，尾巴很短且下面是

白色的，栖息于大峡谷以北、科罗拉多河以西的南加利福尼沙漠地区。哈里斯羚羊松鼠的生活范围比较有限，位

于大峡谷以南的沙漠地区。

地理隔离为新物种的进化创造了机会，但这并不一定会形成新物种。因为物种形成只有在基因库发生足够大

的变化，被隔离的种群与亲本种群之间出现繁殖障碍（即不能交配繁殖）时才发生。种群的数量比较小且被隔离

开来时，比数量大的种群更容易发生基因库的实质变化，也就增加了异域物种形成的可能性。例如，从南美大陆

流浪而来，成功移居到加拉帕戈斯群岛的动物和植物小种群，在不到二百万年的时间就进化出了现在岛上存在的

所有物种。

当海洋岛屿互相之间的距离比较远，使得种群得以独立进化，但又不至于太远从而使得物种可以在偶然的条

件下传播过去时，它们就是有效的户外进化实验室。加拉帕戈斯群岛是世界上最伟大的进化展示窗之一。它的每

个岛屿都来自海底火山喷发，岛上逐渐住满了洋流和风从其他岛屿和陆地带来的生物。这些生物也可能是被其他

生物带到岛屿上来的，如长途飞来的海鸟羽毛上可能沾有其他植物的种子。

今天生活在加拉帕戈斯群岛的物种，是那些由从南美大陆飘过来、飞过来、或者被风吹过来的物种演变而来

的，其中大多数物种是岛上独有的，在其他地方见不到。例如，加拉帕戈斯群岛有十三种联系紧密的鸟都叫加拉

帕戈斯雀类。这些鸟类有许多相似之处，但他们的饮食习惯和嘴型（与它们所吃的东西有关）存在不同。已经收

集到的证据表明，这十三种雀鸟都是从一个小种群的鸟类祖先进化而来的，这个鸟类的祖先在其中的一个岛栖息

繁殖。从大陆迁移来后，它们在岛上完全被隔离开来，最初的种群基因库发生了很大的变化，成为一个新的物种。

后来，新物种的小部分可能被风暴吹到了邻近的一个岛。它们在这个岛上再次被隔离开来，这第二个初始种群可

能又进化出另一个新物种，而这个新物种之后又可能重新回到它的初始种群来自的那个岛。今天每个加拉帕戈斯

岛都有多种雀鸟，有些岛上多达十种。

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ExplainingDinosaurExtinction

Dinosaursrapidlybecameextinctabout65millionyearsagoaspartofamasxtinctionknownastheK-Tevent,

becauitisassociatedwithageologicalsignatureknownastheK-Tboundary,usuallyathinbandofdimentation

foundinvariouspartsoftheworld(KisthetraditionalabbreviationfortheCretaceous,derivedfromtheGermanname

Kreidezeit).█Manyexplanationshavebeenpropodforwhydinosaursbecameextinct.█Forexample,somehave

blameddinosaurextinctiononthedevelopmentoffloweringplants,whichweresuppodlymoredifficulttodigestand

couldhavecaudconstipationorindigestion—exceptthatfloweringplantsfirstevolvedintheEarlyCretaceous,about

60millionyearsbeforethedinosaursdiedout.█Infact,veralscientistshavesuggestedthattheduckbilldinosaursand

horneddinosaurs,withtheircomplexbatteryofgrindingteeth,evolvedtoexploitthisnewresourceofrapidlygrowing

floweringplants█Othershaveblamedextinctiononcompetitionfromthemammals,whichallegedlyateallthedinosaur

eggs—exceptthatmammalsanddinosaursappearedatthesametimeintheLateTriassic,about190millionyearsago,

andthereisnoreasontobelievethatmammalssuddenlyacquiredatastefordinosaureggsafter120millionyearsof

planations(suchastheonestatingthatdinosaursalldiedofdias)failbecauthereisnoway

toscientificallytestthem,andtheycannotmovebeyondtherealmofspeculationandguesswork.

Thisfocusonexplainingdinosaurextinctionmissanimportantpoint:theextinctionattheendoftheCretaceous

doutmanykindsofplanktoninthe

oceanandmncludedavarietyof

clamsandsnails,andespeciallytheammonites,agroupofshelledsquidlikecreaturesthatdominatedtheMesozoicas

-Teventmarkedtheendofthemarinereptiles,suchasthe

mosasaursandtheplesiosaurs,whichwerethelargestcreaturesthathadeverlivedintheasandwhichruledtheas

,therewasalsoacrisisamongthelandplants,inadditiontothedisappearanceof

ventthatcanexplainthedestructionofthebaofthefoodchain(planktonintheocean,plantson

land)canbetterexplainwhathappenedtoorganismsatthetopofthefoodchain,rast,any

explataceouxtinctionswereaglobal

phenomenon,anddinosaurswerejustapartofabiggerpicture.

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Accordingtoonetheory,theAgeofDinosaurndedsuddenly65millionyearsagowhenagiantrockfromspace

tedtobetentofifteenkilometersindiameter,thisbolide(eitheracometoranasteroid)was

travelingatcosmicspeedsof20-70kilometerspercond,or45,000-156,ugemass

ebolidestruckthinergywas

releadandgeneratedahugeshockwavethatleveledeverythingforthousandsofkilometersaroundtheimpactand

idestruckanareaoftheYucatanPeninsulaofMexicoknown

asChicxulub,excavaactdisplaced

hugevolumesofawater,ile,thebolideitlfexcavated100

cubickilometersofrockanddebrisfromthesite,itfellback

immediately,terial,alongwiththesmokefromthe

fires,shroudedEarth,ingtocomputerizedclimatemodels,globaltemperatures

felltonearthefreezingpoint,photosynthesishalted,ebottomofthe

foodchaindestroyed,dinosaurscouldnotsurvive.

graph1,whydoestheauthorincludeadiscussionofwhenfloweringplantvolved?

explainwhysomescientistsbelievethatthedevelopmentoffloweringplantsledtodinosaurextinction

doubtonthetheorythatthedevelopmentoffloweringplantscauddinosaurstobecomeextinct

estthatdinosaurswereabletosurviveforaslongastheydidbecauoftheavailabilityoffloweringplants

asizethatduckbilldinosaursandhorneddinosaurswerethefirstdinosaurstobecomeextinct

d“allegedly”inthepassageisclostinmeaningto

ably

lly

edly

singly

ingtoparagraph1theextinctionofthedinosaursisunlikelytohavebeentheresultofcompetitionfrom

mammalsbecau

swouldnothavebeencapableofeatingdinosaureggs

sdidnotappearinanysignificantnumbersuntilaftertheLateTriassic

sanddinosaursdidnot,infact,competeforanyofthesameresources

sanddinosaurslivedtogetherforroughly120millionyearsbeforetheextinction

ingtoparagraph2,whatisproblematicaboutsomescientists'focusondinosaurextinction?

ursbecameextinctsolongagothatnotheoryabouttheirdisappearancecanbeprovenscientifically.

urswerenottheonlyorganismsthatwentextinctattheendoftheCretaceousperiod.

rineorganismswentextinctduringtheCretaceousthandiddinosaurspecies.

reimportanttounderstandhowplanktonandothermarineorganismscametothriveduringtheCretaceous

period.

ingtoparagraph2,eachofthefollowingbecameextinctduringtheK-TeventEXCEPT

peciesofwhales

reptiles

sspeciesofclams

eciesoflandplants

kestheextinctionof“theammonites”especiallysignificant?

reamongthelargestcreaturesthateverlived.

istedatthelowestlevelofthefoodchain.

dbeenabletosurviveintheMesozoicas.

dsurvivedmanypreviousmasxtinctions.

d“halted”inthepassageisclostinmeaningto

d

cted

ed

d“strictly"inthepassageisclostinmeaningto

ively

lly

y

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d“crisis”inthepassageisclostinmeaningto

bance

alsituation

sparagraph3relatetoparagraph2?

aph3providesanalternativeexplanationtotheoneprovidedinparagraph2.

aph3providesanexplanationthatsatisfiestheconditionstforthinparagraph2.

aph3providesthefactstosupportthetheoryprentedinparagraph2.

aph3prentsatheorythatcallsintoquestionthepositiondescribedinparagraph2.

aph3answersallofthefollowingquestionsEXCEPT:

thebolidefalltoEarth?

twasthebolidetraveling?

thebolidecapableofgeneratingashockwave?

thebolidecauflooddamagetotheCaribbean?

aph3stronglysuggeststhatifthebolideimpacttheoryiscorrect,themajorityoftheextinctionsassociated

withtheK-Teventresultedfrom

htbeingblockedformonthsbydustandsmokeinEarth'satmosphere

readfloodingthatfollowedthedisplacementofhugevolumesofawater

elingofthelandscapebytheshockwavethatwasgeneratedwhenthebolidestruckEarth

einglobaltemperaturescaudbythefiresthatburnedmuchofthelandscape

thefoursquares[█]thatindicatewherethefollowingntencecouldbeaddedtothepassage

Someexplanationsemplausibleuntilthefactsareconsidered.

Wherewouldthentencebestfit?Clickonasquaretoaddthentencetothepassage.

ions:Antethesummary

bylectingthntencesdonot

belonginthesummarybecautheyexpressideasthatarenotprentedinthepassageorareminorideasinthepassage.

Thequestionisworth2points.

Overtheyears,scientistshavepropodanumberoftheoriesastowhydinosaurssuddenlybecameextinctabout65

millionyearsago.

●

●

●

AnswerChoices

○Manyexplanationsfordinosaurextinctionhavebeenpropod,butmostofthemareeithercalledintoquestionby

knownfactsoraremerelyunsupportedhypothes.

○Focusingondinosaursmissthepointthattheextinction,ataboutthesametime,oftheshelledsquidlikecreatures

thatdominatedtheMesozoicaswasfarmorescientificallysignificant.

○Computerizedclimatemodelsofglobaltemperaturefluctuationssupportthetheorythatahugerockfromspacehitthe

YucatanPeninsulainMexicoabout65millionyearsago.

○AlthoughmammalsanddinosaursappearedataboutthesametimeintheLateTriassic,theK-Tevent,whichmarked

theendofthedinosaurs,apparentlyhadrelativelylittleimpactonmammals.

○Anysatisfactoryexplanationofthemasxtinctionofdinosaursmusttakeintoaccountthefactthatthedisappearance

ofthedinosaurswaspartofaglobalmasxtinction.

○AhugebolidestrikingEarthwouldhavecreatedconditionsinwhichmostplantswouldhavedied,thuxplainingthe

masxtinctionoforganisms—includingdinosaurs—furtherupthefoodchain.

答案：1-5:BCDBA;6-10:DBACB11-14:AABAEF

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参考译文：解释恐龙灭绝现象

大约在6500万年前，恐龙迅速地灭绝，成为K-T大规模灭绝事件的一部分。之所以称作K-T是因为它与地

质学上的标签K-T边界有关。K-T边界通常是一个薄薄的沉积带，在世界各处都有存在（K在惯例上是白垩纪

Cretaceous的缩写，源自德国名字kreidezeit）。关于恐龙为何绝种有很多解释。例如，有些将恐龙的灭绝归咎于

开花植物的发展，这种植物据称更难消化，可引起便秘和消化不良——但是开花植物最初是在早白垩世纪进化的，

也就是在恐龙灭绝前约60万年就出现了。实际上，一些科学家提出，鸭嘴恐龙和有角恐龙已经进化出一口复杂

的磨牙，能够消化吸收这些迅速增长的开花植物。也有科学家将恐龙的灭绝归咎于其他哺乳动物的竞争，据说是

它们吃掉了所有的恐龙蛋——但是，哺乳动物和恐龙都生活在在约190万年前的晚三叠世，没理由说哺乳动物在

和恐龙共同生活了120万年之后，忽然爱吃恐龙蛋了。还有些解释（比如有一种解释认为恐龙死于疾病）也说不

通，因为有没有办法进行科学验证，而且这些解释也无非都是臆想和猜测。

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对恐龙灭绝的解释都错过了一个重要信息：白垩纪末的灭绝是一个全球性的事件，这次事件杀死的生物来自

整个食物链。有许多海洋中的浮游生物都灭绝了，而在食物链的底端以这些浮游生物为食的许多海洋生物也都灭

绝了。这些生物包括各种蛤蜊和蜗牛，尤其是菊石，一种类似鱿鱼的带壳生物，它曾经在中生代海洋中大规模存

在，并且在以前的很多大规模灭绝中幸存了下来。K-T事件标志着海洋爬行动物（如沧龙类与蛇颈龙，曾经生活

在海洋，并且早在鲸鱼出现之前统治海洋很久的最大生物）的结束。在陆地上，除了恐龙灭绝以外，陆地植物也

面临着生存危机。因此，任何能解释食物链底端生物（海洋中的浮游生物、陆地上的植物）毁灭的原因，都能更

好地解释发生在食物链顶端的生物，如恐龙的灭绝。相比之下，那些只关注恐龙本身的解释就没有解释到点子上，

白垩纪灭绝是一个全球性的现象，恐龙的灭绝只是其中的一部分。

根据某一个理论，恐龙时代在6500万年前突然结束，是因为当时一个巨大的星体从太空撞到了地球上。这

颗火流星（彗星或小行星）的直径估计在10000~15000米左右，以20~70千米每秒或45000~156000英里每小时

的速度旋转。这个体积庞大、速度飞快的星体带着大量的能量。当星体撞击地球时，这些能量被释放，产生了巨

大的冲击波，受到影响的数千公里的土地瞬间夷为平地，大部分景观突然起火。星体击中了墨西哥尤卡坦半岛的

希克苏鲁伯地区，凿出一个直径15~20千米、深度至少有170千米的大坑。撞击造成大量海水移位，在加勒比海

地区引起了很多洪灾。与此同时，火球自身也在撞击地凿出了100立方公里的岩石和碎片，使得当地的高度增加

了100千米。大部分的碎片立刻就掉下来了，但有一些变成了灰尘仍然在大气中漂浮了数月之久。这些灰尘与火

灾产生的烟雾一起笼罩着地球，创造出一种“核冬天”的感觉。根据计算机模拟的气候模型，全球气温下降到接近

冰点，光合作用停止，陆地和海洋中的大部分植物都死掉了。随着食物链的底端被破坏，恐龙也就无法生存了。

CallistoandGanymede

From1996to1999,theGalileospacecraftpasdthroughtheJoviansystem,providingmuchinformationabout

Jupiter'to,theoutermostofJupiter'sfourlargestsatellites,orbitstheplanetinventeendaysata

rownMoon,Callistorotatesinthesameperiodasitrevolves,so

ntimesurfacetemperatureisonlyabout-140°C,sowatericeis

tohasadiameterof4.820kilometers,s

isonlyone-thirdasgreat,llsusthatCallistohas

farlessoftherockymetallicmaterialsfoundintheinnerplanetsandmustinsteadbeanicybodythroughmuchofits

interior.

Callistohasnotfullydifferentiated,omerscan

tellthatitlacksadencorefromthedetailsofitsgravitationalpullontheGalileospacecraftduringveralveryclo

ctsurpridscientists,cheasier

foranicybodytodifferentiatethanforarockyone,ittleheating

willsoftentheiceandgettheprocessstarted,allowingtherockandmetaltosinktothecenterandtheslushyicetofloat

listoemstohavefrozensolidbeforetheprocessofdifferentiationwascomplete.

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LikeourMoon'shighlands,vivalofthecraterstells

kingoficesofarfromtheSun,itis

importantnottojudgeitsbehaviorfromthatofthemuchwarmericeweknowonEarth;atthetemperaturesoftheouter

solarsystem,iceonthesurfaceisnearlyashardasrock,allistodoesnotdeformorflow

toisuniqueamongtheplanet-sizedobjectsofthesolarsysteminitsabnceof

ellitewasborndeadandhasremainedgeologicallydeadformore

thanfourbillionyears.

Ganymede,anotherofJupiter'ssatellitesandthelargestinoursolarsystem,isalsocratered,butlesssothan

Callisto.█Aboutone-quarterofitssurfaceemstobeasoldandheavilycratered;therestformedmorerecently,aswe

cantellbythesparcoveringofimpactcratersaswellastherelativefreshnessofthecraters.█Ganymedeisa

differentiatedworld,liketheterrestrialplanets.█Measurementsofitsgravityfieldtellusthattherockandmetalsankto

formacoreaboutthesizeofourMoon,withamantleandcrustoficefloatingaboveit.█Inaddition,theGalileo

spacecraftdiscoveredthatGanymedehasamagneticfield,deisnota

deadworld,butratherapitssurface

maybeasyoungashalfabillionyears.

aturesformedwhenthecrustcracked,

ivemountainrangeswereformedfromcompressionof

thecrust,foplacesolderimpact

reevenindicationsoflarge-scalecrustalmovementsthataresimilartothe

platetectonicsofEarth.

WhyisGanymededifferentfromCallisto?Possiblythesmalldifferenceinsizeandinternalheatingbetweenthe

elikelythegravityofJupiteristoblameforGanymede's

deiscloenoughtoJupiterthattidalforcesfromthegiantplanetmayhave

episodicallyheateditsinteriorandtriggeredmajorconvulsionsonitscrust.

ingtoparagraph1,whichofthefollowingstatementsaboutCallistoistrue?

esatelliteclosttoJupiter'ssurface

facetemperatureisconstantatalltimesoftheday.

hesamemassanddiameterastheplanetMercury

letesonerotationeveryventeendays.

ingtoparagraph1,howdoscientistsknowthatCallistoismadeuplargelyofice?

eofitsinteriorwastakenbytheGalileospacecraftinthelate1990s

oolowadensitytocontainmuchrockymetallicmaterial

oontimesurfacetemperatureofonlyabout.140°onitnevermelts

hebodiesintheJoviansystemareicy,becautheyaresofarfromtheSun.

stheauthorprovidetheinformationthat“Itismucheasierforanicybodytodifferentiatethanforarocky

one”?

orttheclaimthatallofthebigicymoonsaredifferentiated

estthatCallistomaybearockybodyratherthananicyone

ainwhyscientistxpectedCallistotobedifferentiated

tetheclaimthatCallistocouldnotdifferentiatebecauitwasfrozensolid

hefollowingquestionsareansweredinparagraph2EXCEPT:

Callistofrozensolidbeforedifferentiationwascomplete?

lowstheprocessofdifferentiationtogetstarted?

teasierforanicybodytodifferentiatethanarockyone?

stronomersknowthatCallistolacksadencore?

fthentencesbelowbestexpresstheesntialinformationinthehighlightedntenceinthepassage?

Incorrectchoiceschangethemeaninginimportantwaysorleaveoutesntialinformation.

ldnotbeassumedthatsurfaceicehasthesamecharacteristicswhereveritisfound

eiceandrockaremuchmoresimilarintemperatureintheoutersolarsystemthantheyareonEarth.

therawaysurfaceiceisfromtheSun,themoreitstemperaturediffersfromthatofthewarmericeonEarth

oldoftheoutersolarsystem,surfaceiceissoharditbehavesmorelikerockthanlikethewarmericeonEarth

ingtoparagraph3,howisCallistodifferentfromallotherplanet-sizedobjectsinthesolarsystem?

ormandretainimpactcratersonitsicysurface

ceglaciersthatdonotflowordeform.

everhadtheinteriorforcesrequiredforgeologicalevolution.

rethanfourbillionyearsold.

d“signature”inthepassageisclostinmeaningto

ycau

ult

fyingmark

-product

ingtoparagraph4,eachofthefollowingprovidevidenceaboutGanymede'sinteriorEXCEPT

de'slargesize

ementsofGanymede'sgravityfield

tthatGanymedehasamagneticfield

tthatGanymedecontinuestobegeologicallyactive

d“Extensive”inthepassageisclotinmeaningto

l

Youenjoytheconvenienceofhavingallvocabularyquestionslistedasaparatepartin《新托福TPO阅读词汇速查速

记》.

aph5supportachofthefollowingstatementsaboutGanymedeEXCEPT:

eronithasalwaysbeenfrozensolid.

videncethatpartofitscrustoncebrokeopen

sthasbeensubjecttoforcesthathavecreatedmountainsandvalleys.

itsoldercratershavebeensplitapartbymorerecentgeologicalactivity

ingtoparagraph6,thedifferencesinhowCallistoandGanymedeevokedaremostprobablydueto

differencesintheir

dinternalheating

cefromJupiter

alandphysicalcomposition

thefoursquares[█]thatindicatewherethefollowingntencecouldbeaddedtothepassage

ThedifferencebetweenGanymedeandCallisto,however,extendmuchfurtherbelowthesurfaceofthe

satellites.

Wherewouldthentencebestfit?Clickonasquaretoaddthentencetothepassage.(2)

13.【Directions】Selectfromthevenntencesbelow,thetwontencesthatcorrectlycharacterizeendogenousrivers

chntenceyoulectintotheappropriate

estionisworth3points.

CallistoGanymede

53

71

4

AnswerChoice

corethatconsistsmainlyofrockandmetal.

aveformedasrecentlyashalfabillionyearsago

eriorisnotfullysolid

lutionhasprobablybeenstronglyinfluencedbyJupiter'sgravity

yskeepsthesamefacetowardJupiterasitorbitstheplanet

stiscoveredbyslow-movingglaciers

iresurfaceisheavilycratered

答案：1-5：DBCAD6-10:CCABA11-13:BBCallisto:5,7;Ganymede:3,1,4

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参考译文：木卫四和木卫三

从1996年到1999年，伽利略号宇宙飞船围绕木星飞行，提供了很多关于木星的卫星的信息。木卫四，木星

的四个最大的卫星中最外层的一个，围绕木星转一圈需要17天，轨道半径200万公里。像地球的卫星月球一样，

木卫四在同一周期公转和自转，所以它总是与木星保持固定的相对位置。它中午的表面温度大约只有零下140°C，

所以表面常年都是冰。木卫四的直径为4820千米，几乎与水星（的直径）相同，但它的质量仅为水星的三分之

一，这也意味着它的密度应该也只有水星的三分之一。这就告诉我们，木卫四内核的岩石金属材料比较少，内核

的大部分应该是冰。

木卫四还没有完全分化，也就意味着还没分离成几层不同密度的材料。在几次近地的飞行中，通过伽利略号

宇宙飞船的重力探测，天文学家可以知道木卫四缺乏一个高密度的内核。这使科学家们感到吃惊，因为他们认为

所有大的冰卫星都会有分层现象。一个冰质天体要比一个岩石天体更容易分层，因为冰的融化温度很低。只要有

一点热量，冰就开始软化，分化便开始了。这使得岩石和金属向中心下沉，融化的冰会浮到表面。然而木卫四似

乎在分化过程完成之前就已经冻成固体了。

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就像地球的卫星月球的高地一样，木卫四表面到处都是撞击坑。这些撞击坑的存在告诉我们，一个冰体可以

形成并保留其表面的撞击坑。在考虑距离太阳如此之远的冰体时，不要根据我们在地球上所了解的温度更高的冰

体来判断其行为是很重要的；在外太阳系的温度下，表面的冰几乎就像岩石一样坚硬，且性质也相似。木卫四上

的冰不像地球上的冰川那样会变形或流动。作为太阳系一个行星大小的天体，木卫四是唯一一个无法依靠内部力

量推动地质演变的星体。卫星一形成就是死的，它保持地质不变的时间超过了4亿年。

木卫三是木星的另一个卫星，也是太阳系中最大的卫星，其表面也是坑坑洼洼的，但不如木卫四那么多坑。

木卫三有大约四分之一的表面似乎是很老了，布满了坑洞；其余的四分之三形成的时间更近一些，这一点我们可

以从稀疏分布的撞击坑和相对较新的坑判断出来。像类地行星一样，木卫三已经分化了。通过测量它的引力场，

我们可以知道下沉的岩石和金属形成了一个月球大小般的核，而冰形成地幔和地壳浮在其上面。此外，伽利略号

宇宙飞船发现木卫三有一个磁场，意味着它的内部已经部分熔融了。木卫三并非一个死的星体，其内部的热能会

持续地引发地质活动。它的表面大部分的年龄可能是五亿年左右。

较年轻的地形是地质和火山活动形成的。当地壳破裂时，一些地理特征便形成了，内部涌出的洪水淹没了大

部分的地壳。巨大的山脉是由地壳挤压形成的，形成了长长的山脊和间隔一到两千米的平行山谷。在一些地方，

更早的撞击坑被拉扯开来。甚至有迹象表明木卫三存在类似于板块构造的大型地壳运动。

为什么木卫三和木卫四有这么多不同？可能体积上和内部热量上的小差异导致了他们在进化中的差距。但更

可能的原因是，木星的引力造成了木卫三持续的地质活动。木卫三离木星较近，这个巨行星的潮汐力偶尔会使得

木卫三的内部温度升高，引发重大的地壳运动。