參考譯文-第9課

1、LessonNiheiginofSpeciesHowBiologistsDefineaSpeciesModernbiologistsgenerallydefineaspeciesasgroupofactuallypotentiallyinterbreedingpopulationsthatisreproductivelyisolatedfromothersuchgroups.Membersofaspeciescaninterbreedw

2、itheachotherbuttheycannotbreedwithganismsbelongingtoanotherspecies.Oneadvantageofthestardofreproductiveisolationisthatitisveryprecise.Noticehoweverthatitcanonlybeappliedtoganismsthatreproducesexually.Asexualreproducersin

3、cludingmostprokaryotesmanyplantssomeanimalsmustbeclassifiedintospeciesonthebasisofphysical(biochemicalmphological)traits.現(xiàn)代生物學大體將物種定義為實際上的一群或潛在的雜交群體，即從這個群體隔離繁殖的后代。種族中個體可以互相交配，但種間不能。這種繁殖隔離的一個優(yōu)勢是很精確。但僅適用于有性繁殖的生物。無性繁殖，包括大多數(shù)

4、原核生物，許多植物，某些動物，需要通過自然特征進行歸類。PreventingGeneExchangeTwogeneraltypesofmechanismsoperatetoblocktheexchangeofgenesbetweenindividualsofrelatedgroups.Thefirstgeneraltypeismadeupofprezygoticisolatingmechanismsthatpreventthefmati

5、onofzygotes.Prezygoticisolationfallsintotwocategies:ecologicalbehavial.Inthefirstcasetworelatedgroupmaybecomeadaptedtoslightlydifferentenvironmentsperhapsvaryingsoiltypesfoodsources.Overtimethesegeicdifferencesbecomesogr

6、eatthatsuccessfulcrossfertilizationcannolongertakeplace.Inbehavialisolationrelatedgroupsevolvedifferingbehavissuchasspecificmatingritualsthatrestricttheexchangeofgenestomembersofthesamegroup.兩種基因型機制阻礙了相關群體中個體的基因交換。第一種基因型

7、由前合子機械隔離機制阻止合子的生成。前合子隔離分兩類：生態(tài)學的和行為學的。第一種情況，兩個相關群體可能分別適應稍微不同的環(huán)境，如土壤類型或食物來源的變化。長時間，這些遺傳差異isolationarisebetweenthetwogroups.Inthesecondstagethedivergedpopulationsmayagaincomeintocontact.Ifthishappensspeciationbecomescomplet

8、ethroughtheactionofnaturalion.異地物種形成模型認為，物種可能起源于兩個階段。第一階段，已經存在的物種群體可能由于自然或地理屏障被隔離。結果，長時間后，在兩個群體間遺傳差異導致前合子或后合子隔離的產生。第二階段，這些趨異群體可能再次接觸，物種發(fā)生可能通過自然選擇發(fā)生。TheGeicBasesofspeciationTheextentofdifferencesbetweenpopulationsthatared

9、ivergingintoseparatespeciesbetweenspeciesthathavealreadydivergedisrepresentedbyastatisticcalledgeicidentitytherelativeproptionofthesamestructuralgenespresentinmembersofgroupsbeingcompared.Ingeneralbiologistsbelievethatth

10、egeiceventsleadingtospeciationtakeplacegradually.Onceanewspecieshasarisenittendstodivergegeicallyfromrelatedspeciesatamerapidpace.Insomecasessuchastheprimatedermajdifferencesinbodyfmarenotreflectedbycrespondingdivergence

11、sinstructuralgenes.ThishasledBiologiststohypothesizethatsmallchangesinregulatygenesmayaccountfmanyofthelargescalechangesresponsiblefsepciationtheiginofhighertaxonomicgroups.能夠趨異的群體或已經趨異的物種，他們之間差異的擴大，由一個穩(wěn)定的遺傳識別代表。即在可比群體成員

12、中相同基因所占相關比例。大體而言，生物學家相信，遺傳導致的物種形成是逐漸發(fā)生的。在某些情況下，身體主要差別并不能反映結構基因的趨異，生物學家因此提出，調控基因很少的改變可能導致很大規(guī)模物種改變和更高分類學上群體的產生。Onemechanismthatmayrapidlysplitpopulationsgeicallyispolyploidization—thesuddenmultiplicationofanentirecomplemen

13、tofchromosomes.Thiscanresultinsympatricspeciationinwhichnewspeciesariseeventhoughnogeographicalisolationhastakenplace.Aphenomenonsimilartopolyploidizationinvolvingtherearrangementofchromosomeshasbeenproposedtoexplainthee