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1、生物工程专业英语TheOriginofSpecies1.HowBiologistsDefineaSpeciesModernbiologygenerallydefineaspeciesasgroupofactuallyorpotentiallyinterbreedingpopulationsthatisreproductivelyisolatedfromthesuchgroups.Membersofaspeciescaninterbreedwitheachother,buttheycannotbreedwithorganismsbelongingtoanotherspecies.Oneadvan
2、tageofthestandardofreproductiveisolationisthatitisveryprecise.Notice,however,thatitcanonlybeappliedtoorganismsthatreproducesexually.Asexualreproducers,includingmostprokaryotes,manyplants,andsomeanimals,mustbeclassifiedintospeciesonthebasisofphysical(biochemicalormorphological)traits.现代生物学大体将物种定义为实际上
3、的一群或潜在的杂交群体，即从这个群体隔离繁殖的后代。种族中个体可以互相交配，但种间不能。这种繁殖隔离的一个优势是很精确。但仅适用于性繁殖的生物。非性繁殖，包括大多数原核生物，许多植物，某些动物，需要通过自然特征进行归类。2.PreventingGeneExchangeTwogeneraltypesofmechanismsoperatetoblocktheexchangeofgenesbetweenindividualsofrelatedgroups.Thefirstgeneraltypeismadeupofprezygoticisolatingmechanismsthatpreventthef
4、ormationofzygotes.Prezygoticisolationfallsintotwocategories:ecologicalandbehavioral.Inthefirstcase,tworelatedgroupmaybecomeadaptedtoslightlydifferentenvironments-perhapsvaryingsoiltypesorfoodsources.Overtime,thesegeneticdifferencesbecomesogreatthatsuccessfulcross-fertilizationcannolongertakeplace.In
5、behavioralisolation,relatedgroupsevolvedifferingbehaviorssuchasspecificmatingrituals-thatrestricttheexchangeofgenestomembersofthesamegroup.两种基因型机制阻碍了相关群体中个体的基因交换。第一种基因型由前合子机械隔离机制阻止合子的生成。前合子隔离分两类：生态学的和行为学的。第一种情况，两个相关群体可能分别适应稍微不同的环境，如土壤类型或食物来源的变化。长时间，这些遗传差异变得很大而很难异体受精。而行为学隔离，相关的群组进化成不同的行为方式，例如，结婚仪式，从而
6、限制了同群成员基因交流。Sometimesthedifferencesthatproduceprezygoticisolationinvolvemechanicalisolation.Thatis,matingisphysicallyimpossiblebetweenmembersofdifferentspeciesbecausegenitalsofmalesandfemalesarestructurallyincompatibleorbecausemoleculesonthesurfacesofspermandeggfailtobind.Afinaltypeofprezygoticmecha
7、nismistemporalisolation,inwhichtime-relatedenvironmentalcuesthattriggerreproductiveprocessesaredifferentforrelatedspecies.有时，产生前合子隔离的差异涉及了隔离机制。即，由于生殖器结构不匹配或精卵分子表面不结合而使不同种群成员间不能自然结合。最后一类前合子机制是暂时隔离，与时间有关的环境因素触发了相关物种的不同繁殖过程。Inpostzygoticisolatingmechanismsmatingoccur,buttheresultinghybridorganismisinvi
8、ableorsterile.Inaspecialcaseofhybridsterilitytermedhybridbreakdown,thesecondandsubsequentgenerationsafteracrossshowreducedreproductivesuccess.Contrastthisfactwiththeverydifferentoutcomeofcrossbreedingbetweentwogeneticallydistantmembersofthesamespecies,wheretheresultisoftenheterozygoteadvantage(hybri
9、dvigor).在合子后隔离机制中，匹配可以进行，但或不产生杂种或杂种不育。杂种不育在某些特殊情况下，是指第二代或以后几代显示出杂交繁殖能力的降低。正是同种远基因成员间的杂交产生不同的结果，而产生杂种优势。Populationsofaspeciesthatarespreadoutoverabroadgeographicalrangeareoftenarrayedinacline-agradualchangeinoneormorecharacteristicsaseachpopulationevolvesadaptationstoitsownlocalenvironment.Alongaclin
10、e,subspecieswithdistinctcharacteristicsmayarise.Often,individualsateitherendofaclinearereproductivelyisolated.地域上分布很广的种的群体中，经常存在变异群，因为适应当地的环境，每个群体逐渐产生一个或多个与众不同的特点。在变异群中，带有明显特征的亚群可能产生。通常，在两个变异群中的个体是繁殖隔离的。3.BecomingaSpecies:HowGenePoolsBecomeIsolatedErnstMayrsmodelofallopatricspeciationproposesthatspe
11、ciescanoriginateinatwo-stageprocess.Inthefirststage,populationsofexistingspeciesareseparatedbyaphysicalorgeographicalbarrier.Asaresult,overtimegeneticdifferencesleadingtopre-orpostzygoticisolationarisebetweenthetwogroups.Inthesecondstage,thedivergedpopulationsmayagaincomeintocontact.Ifthishappens,sp
12、eciationbecomescompletethroughtheactionofnaturalselection.LessonNineTheOriginofSpecies1.HowBiologistsDefineaSpeciesModernbiologygenerallydefineaspeciesasgroupofactuallyorpotentiallyinterbreedingpopulationsthatisreproductivelyisolatedfromthesuchgroups.Membersofaspeciescaninterbreedwitheachother,butth
13、eycannotbreedwithorganismsbelongingtoanotherspecies.Oneadvantageofthestandardofreproductiveisolationisthatitisveryprecise.Notice,however,thatitcanonlybeappliedtoorganismsthatreproducesexually.Asexualreproducers,includingmostprokaryotes,manyplants,andsomeanimals,mustbeclassifiedintospeciesonthebasiso
14、fphysical(biochemicalormorphological)traits.现代生物学大体将物种定义为实际上的一群或潜在的杂交群体，即从这个群体隔离繁殖的后代。种族中个体可以互相交配，但种间不能。这种繁殖隔离的一个优势是很精确。但仅适用于性繁殖的生物。非性繁殖，包括大多数原核生物，许多植物，某些动物，需要通过自然特征进行归类。2.PreventingGeneExchangeTwogeneraltypesofmechanismsoperatetoblocktheexchangeofgenesbetweenindividualsofrelatedgroups.Thefirstgener
15、altypeismadeupofprezygoticisolatingmechanismsthatpreventtheformationofzygotes.Prezygoticisolationfallsintotwocategories:ecologicalandbehavioral.Inthefirstcase,tworelatedgroupmaybecomeadaptedtoslightlydifferentenvironments-perhapsvaryingsoiltypesorfoodsources.Overtime,thesegeneticdifferencesbecomesog
16、reatthatsuccessfulcross-fertilizationcannolongertakeplace.Inbehavioralisolation,relatedgroupsevolvedifferingbehaviorssuchasspecificmatingrituals-thatrestricttheexchangeofgenestomembersofthesamegroup.两种基因型机制阻碍了相关群体中个体的基因交换。第一种基因型由前合子机械隔离机制阻止合子的生成。前合子隔离分两类：生态学的和行为学的。第一种情况，两个相关群体可能分别适应稍微不同的环境，如土壤类型或食物来
17、源的变化。长时间，这些遗传差异变得很大而很难异体受精。而行为学隔离，相关的群组进化成不同的行为方式，例如，结婚仪式，从而限制了同群成员基因交流。Sometimesthedifferencesthatproduceprezygoticisolationinvolvemechanicalisolation.Thatis,28LessonOnematingisphysicallyimpossiblebetweenmembersofdifferentspeciesbecausegenitalsofmalesandfemalesarestructurallyincompatibleorbecausemo
18、leculesonthesurfacesofspermandeggfailtobind.Afinaltypeofprezygoticmechanismistemporalisolation,inwhichtime-relatedenvironmentalcuesthattriggerreproductiveprocessesaredifferentforrelatedspecies.有时，产生前合子隔离的差异涉及了隔离机制。即，由于生殖器结构不匹配或精卵分子表面不结合而使不同种群成员间不能自然结合。最后一类前合子机制是暂时隔离，与时间有关的环境因素触发了相关物种的不同繁殖过程。Inpostzy
19、goticisolatingmechanismsmatingoccur,buttheresultinghybridorganismisinviableorsterile.Inaspecialcaseofhybridsterilitytermedhybridbreakdown,thesecondandsubsequentgenerationsafteracrossshowreducedreproductivesuccess.Contrastthisfactwiththeverydifferentoutcomeofcrossbreedingbetweentwogeneticallydistantm
20、embersofthesamespecies,wheretheresultisoftenheterozygoteadvantage(hybridvigor).在合子后隔离机制中，匹配可以进行，但或不产生杂种或杂种不育。杂种不育在某些特殊情况下，是指第二代或以后几代显示出杂交繁殖能力的降低。正是同种远基因成员间的杂交产生不同的结果，而产生杂种优势。Populationsofaspeciesthatarespreadoutoverabroadgeographicalrangeareoftenarrayedinacline-agradualchangeinoneormorecharacteristi
21、csaseachpopulationevolvesadaptationstoitsownlocalenvironment.Alongacline,subspecieswithdistinctcharacteristicsmayarise.Often,individualsateitherendofaclinearereproductivelyisolated.地域上分布很广的种的群体中，经常存在变异群，因为适应当地的环境，每个群体逐渐产生一个或多个与众不同的特点。在变异群中，带有明显特征的亚群可能产生。通常，在两个变异群中的个体是繁殖隔离的。3.BecomingaSpecies:HowGene
22、PoolsBecomeIsolatedErnstMayrsmodelofallopatricspeciationproposesthatspeciescanoriginateinatwo-stageprocess.Inthefirststage,populationsofexistingspeciesareseparatedbyaphysicalorgeographicalbarrier.Asaresult,overtimegeneticdifferencesleadingtopre-orpostzygoticisolationarisebetweenthetwogroups.Inthesec
23、ondstage,thedivergedpopulationsmayagaincomeintocontact.Ifthishappens,speciationbecomescompletethroughtheactionofnaturalselection异地物种形成模型认为，物种可能起源于两个阶段。第一阶段，已经存在的物种群体可能由于自然或地理屏障被隔离。结果，长时间后，在两个群体间遗传差异导致前合子或后合子隔离的产生。LessonOne第二阶段，这些趋异群体可能再次接触，物种发生可能通过自然选择发生。4.TheGeneticBasesofspeciationTheextentofdiffe
24、rencesbetweenpopulationsthataredivergingintoseparatespeciesorbetweenspeciesthathavealreadydivergedisrepresentedbyastatisticcalledgeneticidentity-therelativeproportionofthesamestructuralgenespresentinmembersofgroupsbeingcompared.Ingeneral,biologistsbelievethatthegeneticeventsleadingtospeciationtakepl
25、acegradually.Onceanewspecieshasarisen,ittendstodivergegeneticallyfromrelatedspeciesatamorerapidpace.Insomecases,suchastheprimateorder,majordifferencesinbodyformarenotreflectedbycorrespondingdivergencesinstructuralgenes.ThishasledBiologiststohypothesizethatsmallchangesinregulatorygenesmayaccountforma
26、nyofthelarge-scalechangesresponsibleforsepciationandtheoriginofhighertaxonomicgroups.能够趋异的群体或已经趋异的物种，他们之间差异的扩大，由一个稳定的遗传识别代表。即在可比群体成员中相同基因所占相关比例。大体而言，生物学家相信，遗传导致的物种形成是逐渐发生的。在某些情况下，身体主要差别并不能反映结构基因的趋异，生物学家因此提出，调控基因很少的改变可能导致很大规模物种改变和更高分类学上群体的产生。Onemechanismthatmayrapidlysplitpopulationsgeneticallyispoly
27、ploidizationthesuddenmultiplicationofanentirecomplementofchromosomes.Thiscanresultinsympatricspeciation,inwhichnewspeciesariseeventhoughnogeographicalisolationhastakenplace.Aphenomenonsimilartopolyploidizationinvolvingtherearrangementofchromosomeshasbeenproposedtoexplaintheevolutionaryoriginofgiantp
28、andas.Clearly,speciescanoriginateinavarietyofways.遗传学上可以快速分离群体的一个机制是多倍体，一个完全互补染色体组的突然倍增。结果导致同域物种形成，即使没有地理隔离也会产生新的物种。与多倍体相似的一个现象是染色体重排，可以解释巨大熊猫的进化起源。显然，物种可以以多种方式进化。5.ExplainingMacroevolution:Higher-orderChangesThechangesthatgeneratespeciesaresometimestermedmicroevolution;thosethatproducethemajorpheno
29、typicdifferencesthatseparategenera,classes,orders,andsoonaretermedmacroevolution.Somelinesofdescentcanbetracedbystudyingthefossilrecord.Inothercasesrelationshipsmustbeinferredbycomparisonofrelatedlivingorganisms.Whenlinesofdescentoverevolutionarytimeareconstructed,theresultisaphylogeny.遗传物种的改变有时称微进化
30、；那些产生很大差异用来区分属，纲，目的表型变化称大幅进化。LessonOne某些品系的后代可以通过化石追根溯源。而其他相关性可以通过现存活体的比较来推论。当品系后代以进化钟构建时，形成系统发育树。Therationaleforbuildingaphylogenyissimple:itassumesthatsimilaritiesinbodystructure,biochemistry,reproductivestrategies,andotherfeaturesoforganismscanbeusedtotracelinesofcommondescent.Theprocessiscomplex
31、becauseevolutionproceedsindifferentpatterns.Incasesofparallelevolution,twoormorelineagesevolvealongsimilarlines.Inconvergentevolution,verydistantlyrelatedlineagesbecomemorealikeassimilaradaptationstakeholdinresponsetodemandsoftheenvironment.Thussimilarstructuresindifferentorganismsmayreflecthomology
32、(derivationfromacommonancestor)oranalogy(independentoriginofstructuresusedforsimilarpurposes).建立系统发育树的基本理论很简单：把身体结构，生物化学，繁殖策略以及其他特征相似的生物划分进同一个品系。这个过程很复杂，因为进化经历不同模式。就平行进化而言，两个或更多的血统沿相似品系进化。在相似生存条件下，亲源关系很远的血统也可能更相似。不同生物间的相似结构可能说明它们具有同源性。Oneofthemostcommonevolutionarypatternsthatcanbeconstructedfromthe
33、fossilrecordisdivergentevolutionorradiation.Itisrepresentedbythebranchingandrebranchingofasingleline.Anothercommonfeatureofevolutionisextinction-thecompletelossofaspeciesorgroupofspecies.Massextinctionshaveoccurredatleastfivetimesintheearthshistory.从化石证据构建的最普通进化模式是趋异进化。由分支和次级分支的简单线条代替。另一个进化的共同特征是灭绝。
34、地球历史中至少发生过5次大规模的灭绝。Gapsinthefossilrecordhaveledsomepaleontologiststoproposethepunctuatedequilibriumtheoryofevolution.Thetheoryholdsthatevolutionproceedsbyspurts-radicalchangesovershort(ingeologicaltime)periodsoftime-withinterveningperiodsofequilibrium.Thetheoryiscontroversialandtendsnottobesupported
35、whenanabundantfossilrecordisavailable.化石证据上的断代使古生物学家们提出间断平衡进化理论。这个理论认为进化在短期内经历了根本突变，中间又介入均衡阶段。这个理论很有争议，一旦发现丰富的化石证据，理论就难支撑了。6.TheRoleofmicroevolutioninMacroevolutionBiologistshavenocertainanswerstoanumberofquestionsaboutlarge-scaleevolutionarychanges.Thesequestionsrangefromwhethernovelhighertaxaresul
36、tfromas-yetundescribedradicalgeneticprocesses,towhetherknownprocessessuchasgeneticdriftandsmallgenechangescanplausiblyaccountfortheevolutionofnewgenera,families,andorders.Investigatorsareexploringtheseareasusing traditionalmethodsaswellasthenewertechniquesofmolecularbiology.生物学家还无法回答关于大规模进化的许多问题。许多问
37、题是新类别的物种可能起源于还不确定的遗传过程，或者知道遗传过程，例如，遗传漂移和少量基因的改变可能是新属，科，目产生的原因。科研工作者不但利用传统方法对这些领域进行研究，还利用分子生物学新科技。TheEcologyofPopulations1.PopulationGrowthPopulations-groupsofindividualsbelongingtothesamespecies-allhavethreeverysignificantstatisticalcharacteristics:percapitabirthrateornatality;percapitadeathrateormo
38、tality;andnumberofindividualsperunitareaordensity.种群中的每个个体都属于同一个物种，具有三个非常显著的统计学特征：出生率；死亡率；种群密度。AsfirstdescribedbyMalthus,apopulationtheoreticallycangrowexponentially(geometrically)iftherearenolimitsonresourcessuchasfoodorhidingplacesandnopredation(acofpopulationgrowthisrepresentedbyanexponentialgrow
39、thcurve.Theconditionexponentialgrowthrarelyoccurinnature,however.Thefinitelevelsofresourcesinanyenvironmentsetanupperlimittopopulationsizetermedcarryingcapacity(K)thatcanbereachedbutneverlongexceeded.Alogisticgrowthcurveplotstheleveling-offofgrowthwhenpopulationsizereachesequilibriumwithavailableres
40、ources.马尔萨斯首次阐述了人口理论，他指出，如果没有类似于食物，庇护所等资源的限制，没有被捕食，群体总量的增长符合指数或几何关系。即指数生长曲线。但，这种指数生长在自然界中很少发生。环境中有限的资源限制了群体规模，即环境的容纳量，只能接近而不能超过。当群体规模接近可获得资源的最大平衡态时，逻辑生长曲线呈现负增长。Whenresourcelimitsareapproachedorexceeded,timeisrequiredforthebirthratetofallandforthedeathratetorise.Thisresponsetimeisknownasreproductivet
41、imelag.Itisonereasonforthefluctuationsinnumbersthatareseenineverypopulation.Inmanynaturalpopulations,carryingcapacity(andhencepopulationsize)fluctuatesseasonally.Ifapopulationdrasticallyexceedsthecarryingcapacityofitsenvironmenttemporarily,damagemayoccurthatpermanentlylowersenvironmentalcarryingcapa
42、city.当接近或超过资源限制时，出生率下降，死亡率升高。这段时期称繁殖滞后时期。这种群体数量的波动存在于每一个群体中。在自然群体中，群体数量随季节波动。若暂时超出环境的容纳量，可能造成对环境持久的伤害而保持很低的容纳量。Besidesenvironmentalcarryingcapacity,apopulationsagestructureandreproductivestrategyalsoaffecttherateatwhichthepopulationgrows.Agestructurereflectstherelativenumbersofyoung,middle-aged,ando
43、lderindividualsinagivepopulation.Inapopulationhavingmanymembersatornearingreproductiveage,significantgrowthmayoccur.Agestructuremayalsoberepresentedbyasurvivorshipcurve.除环境容纳量外，群体的年龄结构和繁殖方式也影响群体增长率。年龄结构反映中青老的比例关系。若生育年龄的比例大，则显著增长。存活曲线也可以反映年龄结构。Thereproducingmembersofapopulationfollowacomplexadaptivereproductivestrategythathasevolvedovermillennia.Reproductivestrategiesgenerallyfitintooneoftwocategories:thoseofr-selectedspeciesandthoseofK-selectedspecies.Inrselectionindividualsre