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1、www.ylongedu.com新托福TPO13阅读原文(二):BiologicalClocksTPO13-2：BiologicalClocks Survivalandsuccessfulreproductionusuallyrequiretheactivitiesofanimalstobecoordinatedwithpredictableeventsaroundthem.Consequently,thetimingandrhythmsofbiologicalfunctionsmustcloselymatchperi
2、odiceventslikethesolarday,thetides,thelunarcycle,andtheseasons.Therelationsbetweenanimalactivityandtheseperiods,particularlyforthedailyrhythms,havebeenofsuchinterestandimportancethatahugeamountofworkhasbeendoneonthemandthespecialresearchfieldofchronobiologyhaseme
3、rged.Normally,theconstantlychanginglevelsofananimal'sactivity—sleeping,feeding,moving,reproducing,metabolizing,andproducingenzymesandhormones,forexample—arewellcoordinatedwithenvironmentalrhythms,butthekeyquestioniswhethertheanimal'sscheduleisdrivenbyexternalcues
4、,suchassunriseorsunset,orisinsteaddependentsomehowoninternaltimersthatthemselvesgeneratetheobservedbiologicalrhythms.Almostuniversally,biologistsaccepttheideathatalleukaryotes(acategorythatincludesmostorganismsexceptbacteriaandcertainalgae)haveinternalclocks.Byis
5、olatingorganismscompletelyfromexternalperiodiccues,biologistslearnedthatorganismshaveinternalclocks.Forinstance,apparentlynormaldailyperiodsofbiologicalactivityweremaintainedforaboutaweekbythefungusNeurosporawhenitwasintentionallyisolatedfromallgeophysicaltimingc
6、ueswhileorbitinginaspaceshuttle.Thecontinuationofbiologicalrhythmsinanorganismwithoutexternalcuesatteststoitshavinganinternalclock. Whencrayfisharekeptcontinuouslyinthedark,evenforfourtofivemonths,theircompoundeyescontinuetoadjustonadailyschedulefordaytimeandnig
7、httimevision.Horseshoecrabskeptinthedarkcontinuouslyforayearwerefoundtomaintainapersistentrhythmofbrainactivitythatsimilarlyadaptstheireyesonadailyscheduleforbrightorforweaklight.Likealmostalldailycyclesofanimalsdeprivedofenvironmentalcues,thosemeasuredforthehors
8、eshoecrabsinthesewww.ylongedu.comconditionswerenotexactly24hours.Sucharhythmwhoseperiodisapproximately—butnotexactly—adayiscalledcircadian.Fordifferentindividu
