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1、www.ylongedu.com新托福TPO13阅读原文(二):BiologicalClocksTPO13-2：BiologicalClocks Survivalandsuccessfulreproductionusuallyrequiretheactivitiesofanimalstobecoordinatedwithpredictableeventsaroundthem.Consequently,thetimingandrhythmsofbiologicalfunctionsmustcloselymatchperiodiceventslikethesolarday,theti
2、des,thelunarcycle,andtheseasons.Therelationsbetweenanimalactivityandtheseperiods,particularlyforthedailyrhythms,havebeenofsuchinterestandimportancethatahugeamountofworkhasbeendoneonthemandthespecialresearchfieldofchronobiologyhasemerged.Normally,theconstantlychanginglevelsofananimal'sactivity—
3、sleeping,feeding,moving,reproducing,metabolizing,andproducingenzymesandhormones,forexample—arewellcoordinatedwithenvironmentalrhythms,butthekeyquestioniswhethertheanimal'sscheduleisdrivenbyexternalcues,suchassunriseorsunset,orisinsteaddependentsomehowoninternaltimersthatthemselvesgeneratetheob
4、servedbiologicalrhythms.Almostuniversally,biologistsaccepttheideathatalleukaryotes(acategorythatincludesmostorganismsexceptbacteriaandcertainalgae)haveinternalclocks.Byisolatingorganismscompletelyfromexternalperiodiccues,biologistslearnedthatorganismshaveinternalclocks.Forinstance,apparentlyno
5、rmaldailyperiodsofbiologicalactivityweremaintainedforaboutaweekbythefungusNeurosporawhenitwasintentionallyisolatedfromallgeophysicaltimingcueswhileorbitinginaspaceshuttle.Thecontinuationofbiologicalrhythmsinanorganismwithoutexternalcuesatteststoitshavinganinternalclock. Whencrayfisharekeptcon
6、tinuouslyinthedark,evenforfourtofivemonths,theircompoundeyescontinuetoadjustonadailyschedulefordaytimeandnighttimevision.Horseshoecrabskeptinthedarkcontinuouslyforayearwerefoundtomaintainapersistentrhythmofbrainactivitythatsimilarlyadaptstheireyesonadailyscheduleforbrightorforweaklight.Likealm
7、ostalldailycyclesofanimalsdeprivedofenvironmentalcues,thosemeasuredforthehorseshoecrabsinthesewww.ylongedu.comconditionswerenotexactly24hours.Sucharhythmwhoseperiodisapproximately—butnotexactly—adayiscalledcircadian.Fordifferentindividu
