TPO-13 Reading 2 阅读译文

Biological Clocks生物钟Survival andsuccessful reproductionusually requirethe activitiesof animalsto becoordinated with predictableevents aroundthem.Consequeetl thetiming andrhythms ofbiologicalfunctions mustclosely matchperiodic eventslike thesolar day,the tides,thelunar cycle,and theseasons.The relationsbetween animalactivity andthese periods,particularly for the daily rhythms,have beenof suchinterest andimportance thata hugeamountof workhas beendone onthem andthe specialresearch fieldof chronobiologyhasemerged.Normally,the constantlychanging levelsof ananimafs activity-sleeping,feeding,moving,reproducing,metabolizing,and producingenzymes andhormones,forexample-are wellcoordinatedwithenvironmental rhythms,but thekey questionis whetherthe animafs scheduleis drivenby external cues,such assunrise orsunset,or isinsteaddependent somehowon internaltimers thatthemselves generatethe observedbiologicalrhythms.Almost universally,biologists acceptthe ideathat alleukaryotes acategory thatincludesmost organismsexcept bacteriaand certainalgae haveinternal clocks.Byisolating organismscompletely fromexternal periodiccues,biologists learnedthatorganisms haveinternal clocks.For instance,apparently normaldaily periodsof biologicalactivitywere maintainedfor abouta weekby thefungus Neurosporawhen itwasintentionally isolatedfrom allgeophysical timingcues whileorbiting ina spaceshuttle.The continuationof biologicalrhythms inan organismwithout external cues atteststo itshavingan internalclock.生存与繁衍通常需要动物的活动与他们所能预料到的事件同步生物的计时与交替循环的机能也就理所应当的必须与像昼夜交替，潮涨潮落，月圆月缺和四季更迭这样的周期性事件保持大体的一致动物的活动与这些周期之间的关系,特别是与昼夜交替之间的关系，因为大量的工作都是在其基础之上完成的而拥有巨大的吸引力和重要性，从而也延伸出了一个新的研究领域生物钟学科通常意义上讲，动物活动的经常性转变——例如，睡觉，喂食，活动，繁殖和产生酶与荷尔蒙，都是与环境的循环同步的，但是关键问题在于，动物的作息是否是由外界条件驱使，比如日出日落，又或者是受他们自身产生并遵循的内在生物循环，生物学家普遍接受的观点是所有的多细胞生物除了细菌与一些海藻以外的绝大多数物种都有内在钟通过将生物与外界的周期性暗示完全隔离，生物学家们发现生物确实有内在钟举个例子，通过一种叫脉抱菌的细菌清晰的证明了在绕地球公转的太空飞船中完全与所有地理事件信号隔离的情况下，所有的生物日常活动周期可以持续大概一个礼拜左右这种在没有外界信号的时候生物循环的延续性证明生物是具有内在钟的When crayfishare keptcontinuously inthe dark,even forfour tofive months,theircompound eyescontinue toadjust on a daily schedule fordaytime andnighttime vision.Horseshoe crabskept inthe darkcontinuously fora yearwere foundto maintain apersistent rhythmof brainactivity thatsimilarly adaptstheir eyesonadailyscheduleforbright orfor weaklight.Like almostall dailycycles ofanimals deprivedof environmentalcues,those measuredforthehorseshoe crabsin theseconditions werenot exactly24hours.Such arhythm whoseperiod isapproximately—but notexactly—a dayis calledcircadian.For differentindividual horseshoecrabs,themaintains its own characteristiccycle withgreat precisionfor manydays.circadian periodranged from

22.2to

25.5hours.A particularanimal typicallyIndeed,stability of the biologicalclocks periodis oneof itsmajor features,even whentheorganisms environmentis subjectedto considerablechanges infactors,such astemperature,that wouldbe expectedto affectbiological activitystrongly.Further evidencefor persistentinternal rhythmsappears whenthe usualexternal cyclesareshifted-either experimentallyor byrapid east-west travelover greatdistances.Typically,theanimafsdaily internallygenerated cycle of activitycontinues withoutchange.As aresult,its activitiesare shiftedrelative tothe externalcycleof the newenvironment.Thedisorienting effectsof thismismatch betweenexternal timecues andinternal schedulesmaypersist,like ourjet lag,for severaldays orweeks untilcertain cuessuch asthedaylight/darkness cyclereset theorganisms clockto synchronizewith thedailyrhythmofthe newenvironment.小龙虾在黑暗中持续的活动哪怕是四五个月，他们的复眼也仍然继续按日常循环的昼与夜来调节视野马蹄蟹可以在黑暗中保持一年的连续性大脑周期活动来使他们的眼睛适应日常交替的高光与弱光的周期一致如同大多数的日循环动物被剥夺了外界暗示一样，对于马蹄蟹来说在这种无光的情况下他们的时长也不一定是准时的小时这种和一天的循环周期很接近但不精确同步的循环叫做生理节奏不同24的马蹄蟹个体，生理节奏在也小时到小时之间一种特定的动物会将其

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225.5特有的精密循环长度保持很多天确实，稳定性是生物钟最重要的特性之一，即使是在生物所处的环境的诸多要素中发生了相当大的改变，例如温度，不能对生物钟产生很大影响关于生物钟持续性更进一步的证据出现在当正常的外部循环发生突变，如科研或者从东到西急速的长途旅行动物的内在以天为单位的典型周期循环活动仍然会继续而不会有什么改变但与此同时，生物的活动与却因为与新环境中的外部循环相关联而改变外部时间与内在循环的持续性不同步而产生的错乱反应，比如我们的时差综合症，要耗费几天或者几个星期直到不变信号比如白天黑夜循环重新设定生物钟并将其同步到新环境的日常循环中Animals neednatural periodicsignals likesunrise tomaintainacycle whoseperiod isprecisely24hours.■Such anexternalcuenot onlycoordinates ananimafs dailyrhythmswith particularfeatures ofthe localsolar daybut also—because doesnormallyso dayafter day-seems tokeep the internal clocksperiod close to thatof Earth!srotation.nYet despitethis synchronizationoftheperiod oftheinternalcycle,the animalstimeritself continuesto haveitsowngenetically built-in periodcloseto,but differentfrom,24hours.■Without theexternalcue,the differenceaccumulates andso theinternallyregulated activitiesofthebiological daydrift continuously,like thetides,in relationto thesolarday.-This drifthas beenstudied extensivelyin manyanimals andin biologicalactivitiesranging fromthe hatchingof fruitfly eggsto wheelrunning bysquirrels.Lighthas apredominating influencein settingthe clock.Even afifteen-minute burstof lightinotherwise sustaineddarkness canreset ananimafs circadianrhythm.Normally,internalrhythms arekept instep byregular environmentalcycles.For instance,if ahoming pigeonisto navigatewith itsSun compass,its clockmust beproperly setby cuesprovided bythedaylight/darkness cycle.动物需要例如日出这样的自然界的周期信号来保持他们的循环周期为精准的小时这类的外部信号不仅可以通过特定的标志——当地的白昼同步动物的每24日循环，而且也正是因为这些活动日复一日的保持着内在钟的周期接近地球自转但是尽管有这些内在钟的同步，动物的计时器仍然继续靠着它构筑在遗传上的区别于外部的周期，近似小时在没有外部信号时，不同的收集方式和这种内在24的调节机制作用下的生物活动保持这继续，比如潮汐，就与太阳有关系这种调节被广泛的研究在许多动物和生物活动调节从孵化的果蝇卵到松树的滚轮跑光在调节生物钟里占主导位置即使是分钟的强光在黑暗中发生也可以改变动物15的生理节奏通常来讲，内在循环会紧随环境循环的步伐举个例子，如果一个家鸽在太阳的指引下飞行，那么它的生物钟就必须严格遵守日出日落的循环。