Chapter 8 Population Genetics and Natural Selection.docx

Chapter 8 Population Genetics and Natural Selection.docx

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Chapter8PopulationGeneticsandNaturalSelection

Chapter8PopulationGeneticsandNaturalSelection

InmidOctoberof1835underabrightequatorialsun,asmallboatmovedslowlyfromtheshoreofavolcanicislandtoawaitingship.TheboatcarriedayoungnaturalistwhohadjustcompletedamonthofexploringthegroupofislandsknownastheGalapagos,whichlieontheequatorapproximately1,000kmwestoftheSouthAmericanmainland（fig.8.1）.Astheseamenrowedintotheoncomingwaves,thenaturalist,CharlesDarwin,musedoverwhathehadfoundontheisland.Hisobservationshadconfirmedexpectationsbuiltoninformationgatheredearlierontheotherislandshehadvisitedinthearchipelago.LaterDarwinrecordedhisthoughtsinhisjournalwhichhelaterpublished（Darwin1839）,'Thedistributionofthetenantsofthisarchipelagowouldnotbenearlysowonderful,if,forinstance,oneislandhadamocking-thrush,andasecondislandsomeotherquitedistinctgenus--ifoneislandhaditsgenusoflizardandasecondislandanotherdistinctgenus,ornonewhatever....Butitisthecircumstance,thatseveraloftheislandspossesstheirownspeciesofthetortoise,mocking-thrush,finches,andnumerousplants,thesespecieshavingthesamegeneralhabits,occupyinganalogoussituations,andobviouslyfillingthesameplaceinthenaturaleconomyofthisarchipelago,thatstrikesmewithwonder[emphasisadded].

FIGURE8.1OntheGalapagosIslandsCharlesDarwinencounteredmanyexamplesofreadilyobservedplantsandanimalspeciesthatdifferedphysicallyfromoneislandtoanotherisland.HereaGaapagoshawklandsonagianttortoiseforwhichtheislandsarenamed.

Darwinwonderedatthesourcesofthedifferencesamongclearlyrelatedpopulationsandattemptedtoexplaintheoriginofthesedifferences.Hewouldlaterconcludethatthesepopulationsweredescendedfromcommonancestorswhosedescendantshadchangedafterreachingeachoftheislands.TheshiptowhichtheseamenrowedwastheH.M.S.Beagle.Halfwaythroughavoyagearoundtheworld.ThemainobjectiveoftheBeagle'smission,chartingthecoastsofsouthernSouthAmericawouldhelargelyforgotten,whilethethoughtsoftheyoungCharlesDarwinwouldeventuallydevelopintooneofthemostsignificanttheoriesinthehistoryofscience.Darwin'swonderlng,carefullyorganizedandsupportedbyalifetimeofobservation,wouldbecomethetheoryofevolutionbynaturalselection,atheorythatwouldtransformtheprevailingscientificviewoflifeonearthandrebuildthefoundationsofbiology.

DarwinlefttheGalapagosIslandsconvincedthatthevariouspopulationsontheislandsweregraduallymodifiedfromtheirancestralforms.Inotherwords,Darwinconcludedthattheislandpopulationshadundergoneaprocessofevolution,aprocessthatchangespopulationsoforganismsovertime.ThoughDarwinlefttheGalapagosconvincedthattheislandpopulationshadevolved,hehadnomechanismtoexplaintheevolutionarychangesthathewasconvincedtheyhadundergone.However,aplausiblemechanismtoproduceevolutionarychangeinpopulationscametoDarwinalmostexactly3yearsafterhistakingleaveoftheGalapagosIslands.InOctoberof1838whilereadingtheessayonpopulationsbyThomasMalthus,Darwinwasconvincedthatduringcompetitionforlimitedresourees,suchasfoodorspace,amongindividualswithinpopulations,someindividualswouldhaveacompetitiveadvantage.Heproposedthatthecharacteristicsproducingthatadvantagewouldbe"preserved"andtheunfavorablecharacteristicsofotherindividualswouldbe"destroyed."Asaconsequenceofthisprocessofselectionbytheenvironment,populationswouldchangeovertime.Withthismechanismforchangeinhand,Darwinsketchedoutthefirstdraftofhistheoryofnaturalselectionin1842.However,itwouldtakehimmanyyearsandmanydraftsbeforehehonedthetheorytoitsfinalformandamassedsufficientsupportinginformation.Darwin'stheoryofnaturalselectioncanbesummarizedecsfollows:

1.Organismsbegetlikeorganisms.（Offspringappear,behave,function,andsoforthliketheirparents.）

2.Therearechancevariationsbetweenindividualsinaspecies.Somevariations（differencesamongparents）areheritable（arepassedontooffspring）.

3.Moreoffspringareproducedeachgenerationthancanbesupportedbytheenvironment.

4.Someindividuals,becauseoftheirphysicalorbehavioraltraits,haveahigherchanceofsurvivingandreproducingthanotherindividualsinthesamepopulation.Darwin（1859）proposedthatdifferentialsurvivalandreproductionofindividualswouldproducechangesinspeciespopulationsovertime.Thatis,theenvironmentactingonvariationamongindividualsinpopulationswouldresultinadaptationofthepopulationtotheenvironment.HenowhadamechanismtoexplainthedifferencesamongpopulationsthathehadobservedontheGalapagosIslands.Still,Darwinwaskeenlyawareofamajorinsufficiencyinhistheory.Thetheoryofnaturalselectiondependeduponthepassageof"advantageous"characteristicsfromonegenerationtothenext.TheproblemwasthatthemechanismsofinheritancewereunknowninDarwin'stime.Inaddition,theprevailingideaatthetime,blendinginheritance,suggestedthatraretraits,nomatterbowfavorable,wouldbeblendedoutofapopulation,preventingchangeasaconsequence.

Darwinworkedfornearlyhalfacenturytouncoverthelawsofinheritance.However,hedidnot.TodosorequiredafacilitywithmathematicsthatDarwinhadnotdeveloped.Inashortautobiography,Darwinhimself（1859）remarked,"Iattemptedmathematics,andevenwentduringthesummerof1828withaprivatetutor..,butIgotonveryslowly.Theworkwasrepugnanttome,chieflyfrommynotbeingabletoseeanymeaningintheearlystepsinalgebra.Thisimpatiencewasveryfoolish,andinafteryearsIhavedeeplyregrettedthatIdidnotproceedfarenoughatleasttounderstandsomethingofthegreatleadingprinciplesofmathematics,formenthusendowedseemtohaveanextrasense"[emphasisaddedl.

AsDarwinexploredtheGalapagosIslands,halfwayaroundtheworldincentralEuropeaschoolboynamedJohannMendelwasstudyingunderdifficultconditionsanddevelopingthefacilitywithmathematicsnecessarytocompleteDarwin'stheoryofnaturalselection.Atthirteen,JohannwashalfDarwin'sage,yethehadalreadysetacourseforalifeofstudywhichhefollowedasresolutelyasthecrewoftheBeagleontheirvoyagearoundtheworld.Attheendofhisscientificvoyage,Mendelwoulduncoverthebasicmechanismsofinheritance.

MendelwastheoldestchildofafamilythatfarmedasmalllandholdingnearBrno,atowninwhatisnowtheCzechRepublic.HewouldhavehadlittleschoolingifitwerenotforthephilanthropyofthecountessWalpurgaTruchsess-ZeilwhoruledthedistrictinwhichMendel'sfamilylived.Thecountesshadastandingordertoheradvisorsthattheyshouldidentifyallofthepromisingboysandgirlslivingwithinherdomainandsendthemtoschool,whereshepaidtheirroomandboard.Mendelhadbeenoneofthosechildren.Thecountesswasmorethanaphilanthropist,however.Shealsopaidattentiontodetails,includingthecurriculumofherschool,whichshespecifiedshouldincludethenaturalsciences.Thusfromtheoutset,Mendel'sstudiesincludedafirmgroundinginthesciences.Acountesswithforesight,intelligence,andheartandherperceptiveadvisorshaddiscoveredanintellectualtreasureandprovidedfortheblossomingofoneofbiology'sgreatgeniuses.

JohannwouldberenamedGregorMendelwhenhejoinedtheAugustinianorderofmonksthatmaintainedamonasterynearhisbirthplace.Inagardenwithinthewallsoftheabbey,MendelwoulddiscoverwhatDarwin'saround-the-worldvoyagewouldnotreveal.ThetwokeystoMendel'sdiscoverieswouldbeexcellenttraininginmathematicsandphysicsfromwhichhederivedasenseofquantitativerelationshipsandthepowerofexperimentalapproachestothestudyofthenaturalworld.

WhatdidMendeldiscover?

Brieflyhediscoveredwhatwenowcall"Mendeliangenetics,"includingtheveryfundamentalconceptofparticulateinheritance.Thatistheconceptthatcharacteristicspassfromparenttooffspringintheformofdiscretepacketsofinformationthatwenowcallgenes.Mendelalsodeterminedthatgenescomeinalternativeforms,whichwetermalleles.Forinstance,Mendelworkedwithallelessuchasroundversuswrinkledseedsandtallversusshortplants.Inaddition,hefoundthatsomeallelespreventtheexpressionofotheralleles.Wecallsuchalleles"dominant''andtheallelesthattheysuppress"recessive."Mendel'sworkalsorevealedthedistinctionbetweengenotypeandphenotypeandthedifferencebetweenhomozygousandheterozygousgenotypes.Mendel'swork,whichrevealedstillotheraspectsofthelawsofinheritance,laidasolidfoundationforthescienceofgenetics.

HowdidMendelsucceed,whilesomanyothershadfailed?

Thesourcesofhissuccesscanbetracedtohiseducationandhisownspecialgenius.Mendel'seducationattheUniversityofViennaexposedhimtosomeofthebestmindsworkinginthephysicalsciencesandtoanapproachtosciencethatemphasizedexperimentation.Hisintroductiontothephysicalsciencesincludedasolidfoundationinmathematics,includingprobabilityandstatistics.Asaconsequence.

Mendelcouldquantifytheresultsofhisexperimentalresearch.

Mendelchosetoworkwithplantswhichcouldbemaintainedintheabbeygarden.Hismostfamousandinfluentialworkwasdoneonthegardenpea,Pisumsativum,thathasmanydesirabletraits（fig.8.2）.Manydomesticvarietiesofpeas,whichshowedagreatdealofphysical