Biology 1B - Lecture 21: Recombination, sexual reproduction


Uploaded by UCBerkeley on 07.03.2011

Transcript:
SO,BEFOREWEGETSTARTEDTHISMORNING,IWANT
TOLETYOUINONASECRET.ITHASTODOWITH
STRATEGY.SO,IFINDMYSELFSLIGHTLYAHEADOFWHERE I
WASWHENIGAVETHISSETOFLECTURESLASTYEAR.AND,
ONEWAYTOSLOWMEDOWN,THEREFORE,TOHAVELESS
MATERIALCOVERED,ISTOASKLOTSOFQUESTIONSDURING
LECTURES.OKAY?SOTHINKABOUTTHAT.BECAUSEICAN
JUSTTALKONANDONANDONANDONANDCOVERMOREAND
MORESTUFFANDBUT,PERHAPSYOUWOULDLIKEMETOCOVER
LESSSTUFF,INWHICHCASEASKLOTSOFQUESTIONS.
SO,INTHELASTLECTUREWETALKEDABOUTFITNESS.
THERELATIVECONTRIBUTIONOFGENESTOTHENEXT
GENERATIONINBROADTERMS.WETALKEDABOUTDIFFERENT
FORMSOFNATURALSELECTION.STABILIZINGSELECTION,
FAVORINGINTERMEDIATES,WHICHHASTHEEFFECTOF
MAINTAININGGENETICDIVERSITYINPOPULATIONS. WE
TALKEDABOUTDISRUPTIVESELECTION.SELECTIONAGAINST
THEINTERMEDIATESWHICHCANHAVETHEEFFECTOF SPLITTINGPOPULATIONS.PERHAPS,EVENSPECIATION
IFIT
ISCONNECTEDTONON-RANDOMMATING.ANDTHENWETALKED
ABOUTDIRECTIONALSELECTIONWHICH,SOLONGASTHE
TRAITANDTHESELECTIONISHERITABLEWILLSHIFTTHE
PHENOTYPEANDTHEGENOTYPESFROMONEGENERATION TOTHE
NEXT,THEREFOREPROMOTINGADAPTATION.ANDWESAW THE
THISPROCESS,PARTICULARLYDIRECTIONALSELECTION OR
ANYFORMOFSELECTIONCANBEVERYRAPID.JUSTOVER
AFEWGENERATIONS.SO,ITLENDSITSELFTO
EXPERIMENTALSTUDYFILLEDMANIPULATIONSLIKETHE
GUPPIESORLABBASEDSTUDIESWITHSHORTGENERATION
TIMESLIKEMICROBESORDROSOPHILAORYEASTANDSO ON.
ANDAGREATDEALOFTHATSORTOFWORKGOESONAND
SUPPORTSTHEGENERALPREDICTIONSTHATAREOUTLINED
THERE.SO,ARETHEREANYQUESTIONSFROMTHATINTHE
SPIRITOFSLOWINGMEDOWN?WELL.COMEONUSEYOUR
IMAGINATIONS.ANYTHING.OKAY.YOUASKEDFORIT.
AGAINIFYOUCAN'THEARMEFROMTHEBACKORMYWRITING GETSTOOSMALLORIFISTARTSPEAKINGINTONGUESJUST
YELLOUT.IAMHAPPYWITHTHATANDSAYSTOPANDI
WILLDOSO.
ANYQUESTIONS?YES.AQUESTION,YEAH.
OKAY.SO,WHATISTHEDIFFERENCEBETWEEN
DIRECTIONALANDDISRUPTIVESELECTION?SO,IFTHIS IS
ADISTRIBUTIONOFPHENOTYPESINTHEPOPULATION, SAY,
COLORORNUMBEROFSPOTSONAFISHORHEIGHT.THISIS
THEFREQUENCY,DIRECTIONALSELECTIONFAVORSOR PERHAPS
ONEENDOFTHATPHENOTYPEDISTRIBUTIONOVERTHEOTHER.
SOLONGASTHISISAGENETICBASISTOTHISVARIATION
INTHETRAITTHENEXTGENERATIONWILLBEMORECOLORFUL
ORBIGGERSPOTS,WHATEVER,THANTHEPROCEEDINGONE.
DISRUPTIVESELECTIONFAVORSBOTHTAILSOFTHE
DISTRIBUTIONOVERTHEMIDDLE.SO,ITCOULDHAVETHE
EFFECTOFSPLITTINGTHEPHENOTYPEDISTRIBUTION
FAVORINGPALEONESANDVERYCOLORFULONESOVERTHE
INTERMEDIATES.OKAY? ANYOTHERQUESTIONS?YES?
SO,INTHEPOCKETMICETHEYARETALKINGABOUT
OVER-EXPRESSION.OKAY.SONOWWEAREGETTINGINTO
SORTOFTHEFINEDETAIL.WHATIWANTYOUTOGETIS
THEPRINCIPLESTHATYOUCANHAVEDIFFERENTGENETIC
CAUSATIONOFTHESAMEPHENOTYPE.SO,INTHATCASE IT
WASTHEDARKMICE.ANDYOUCANGETTHATBYMUTATIONS
INTHECODINGSEQUENCEOFONEGENEMC1R.HEREWEHAVE
AMUTATIONINTHAT,CAUSESINCREASEMELANIN
PRODUCTION.SOTHATWASACODINGSEQUENCECHANGE OR
MUTATION.THEOTHERGENEWASAGENECALLEDAGOUTI.
THATSUPPRESSESTHEACTIONOFMC1R.SOIFAGOUTIIS
UNDER-EXPRESSED,THEREISNOTENOUGHOFITTO
TURNMCR1OFFANDYOUGETMOREDARKCOLOR.IFYOU
DECREASEAGOUTI,SOTHATISREGULATORYMUTATION, IT
HASTHESAMEEFFECTOFPRODUCINGTHESEDARKCOLORED
MICE.OKAY?DOESTHATANSWERYOURQUESTION?OKAY.
ANYOTHERS?YES.WELL,QUESTIONIS.DOESTHE DISRUPTIVESELECTIONHAVEANYTHINGTODOWITHGENETIC
DRIFT?NO,ITISASEPARATEPROCESS.SO,THATISDUE
TODIFFERENCEINFITNESS.WHEREAS,GENETICDRIFT IS
ALLABOUTRANDOMCHANGE.SO,ONLYSMALLPOPULATIONS
BOUNCINGAROUNDWITHALLELEFREQUENCIESDUETOTHE
SAMPLINGEFFECT.NOW,ALLELEPOPULATIONS,BOTH THINGS
AREGOINGON.YOUMIGHTHAVESELECTIONOPERATING ON
DIFFERENTTRAITS.YOUMIGHTHAVESAMPLINGEFFECT
GOINGONASWELL.ANDONEEFFECTWESAWTHATWASTHAT
IFA,IFTHISIS,THISALLELEFREQUENCYONTHETIME
ANDINTRODUCINGANEWBENEFICIALMUTATIONPARTICULARLY
IFITTHERECESSIVEITSTAYSATLOWFREQUENCYFORA
LONGTIMEBEFOREWEHAVEENOUGHINDIVIDUALSEXPRESSING
THEIRTRAIT.THATHOMOZYGOTESHAVETHEADVANTAGE AND
THEREFOREAREBETTERREPRESENTEDONEGENERATION TOTHE
NEXT.SO,DURINGTHISPROCESSWEHAVEGOTTHIS
ADVANTAGEOUSALLELEATVERYLOWFREQUENCY.OFCOURSE
YOUHAVEALSOGOTGENETICDRIFTGOINGON.QUITEOFTEN YOUWILLLOSEIT.SOTHOSEPLUSSESAREINTERACTING
IN
REALPOPULATIONS.YOUHAVEALREADYGOTMIGRATION.
ALLOFTHESETHINGSGOINGONATONCE.OKAY.ANY
OTHERQUESTIONS?YES.THATISGOOD.YOUARE
GETTINGINTOTHESPIRITOFIT.EXCELLENT.RIGHT. SO
THEQUESTIONISWHYDOESMIGRATIONINCREASEGENETIC
VARIATION?ITDOESSOBECAUSEITOPPOSESTHELOSS OF
VARIATIONDUETOGENETICDRIFT.SO,REMEMBERIFYOU
TAKETWOPOPULATIONSANDIFWEDOITFOREACHONE
ACHARTOFALLELEFREQUENCYOVERTIMEISTHETIMEON
THISAXIS.SO,LOTSOFGENERATIONS.WECANSEETHAT
INONEPOPULATIONBYCHANCE,THISALLELEGOESTO
AFREQUENCYOF1.P,BYCHANCE,MAYBEGOESTO
AFREQUENCYOFZERO.SO,NOWYOUHAVEGOTTHIS
POPULATIONISALL,SAYA1.ANDTHISONENOWISALL
A2.SOYOUHAVELOSTTHEVARIATIONWITHINTHE
POPULATIONBECAUSEOFTHESAMPLINGPROCESS.ONE WAY
TOGETITBACKISBYMUTATION.THATISAVERYSLOW PROCESS.ITISAVERYRAREPROCESSPERGENE.THE
OTHERWAYYOUCANDOITISTOMOVESOMEOFTHESEA1
ALLELESOVERTOHEREANDVICEVERSA.A2ALLELESOVER
HERE.ANDTHATPUTSAVARIATIONBACKWITHIN
POPULATIONSANDDECREASESTHEDIFFERENCESBETWEEN
POPULATIONS.SOTHEMIGRATIONISAVERYPOWERFUL
FORCEWHEREGENEFLOWSSPECIFICALLYINMAINTAINING THE
GENETICCOHESIONAMONGPOPULATIONSINOPPOSING THE
EFFECTSOFGENETICDRIFT.OKAY?DOESTHATANSWER
YOURQUESTION,WHEREVERYOUARE?LET'SGOONANDLOOK
ATALITTLEBITMOREDIRECTIONALSELECTION.ANDTHIS
TIMEINTHECONTEXTOFGENOMICS.SOTRADITIONALLY
NATURALLYSELECTIONISBEINGSTUDIEDBYTHESORT OF
EXPERIMENTSIOUTLINEDINTHELASTLECTURE.EITHER
COMPARATIVEDATAFROMTHEFIELD,LABEXPERIMENTS, OR
LOOKINGATTHEWHOLEPHENOTYPESTHATWENOWHAVEA NEW
WINDOWFORTHISSORTOFGENOMICEFFECTSOREFFECTS OF
NATURALSELECTIONONGENOMICVARIATION.BECAUSE WE
CANNOWSEQUENCEWHOLEGENOMESORATLEASTEXPRESS
PARTOFTHEGENOMEFORLOTSOFINDIVIDUALSANDLOOK AT
THEGENETICVARIATIONACROSSTHEWHOLEGENOME,WHICH
ISREALLYCOOL.SO,WHATMIGHTWEEXPECTTOSEE?
THISRELATESTOSLIDE2INTODAY'SHANDOUT.SO,
IMAGINEWEHAVEACHROMOSOME.ITISESSENTIALLYTHE
TWOARMSOFTHECHROMOSOMEANDWEHAVEGOTACOUPLE OF
DIFFERENTGENESHERE.LOCIONTHESECHROMOSOMES.
THISONELOOKSLIKE[INAUDIBLE]SELECTION. SOITIS
VARIABLE.THESEMUTATIONSCOMINGIN,DRIFTTAKING
THEMOUT.BUTYOUBUILDUPAPOOLOFGENETIC
VARIATIONFORTHATGENETHROUGHTHEJOINTEFFECTS OF
PASTMIGRATION,MUTATIONANDDRIFTBUTNOSELECTION.
OKAY.SOTHATISWHYIHAVEA1,A2,A3.YADA,YADA,
YADA.DIFFERENTALLELESATTHATGENE.HEREIS
ADIFFERENTGENE.WEGETANEWMUTATION.SOWEGET
SORTOFPOSITIVEORDIRECTIONALSELECTION FAVORING
THISNEWALLELE.WEWILLCALLITTHEBGENE.SONOW YOUHAVEJUSTGOTTHATALLELEPRESENTINTHE
POPULATION.WEHAVESORTOFBEENTHROUGHTHIS
PROCESS.LET'SCALLTHISBINSTEAD,ITDOESN'T
MATTER.KEEPITCONSISTENT.WEHAVEHADPREVIOUSLY
YOUHADAPOPULATIONWITHB1,B2,BTOTHEN.HOW
MANYDIFFERENTALLELES.ANDTHENASTHISNEWMUTATION
ARISES,IFITSURVIVESITSWEEPSTOAFREQUENCYOF 1.
THEREFORE,REPLACINGALLOFTHEPRE-EXISTING
VARIATION.THEONLYALLELETHEREISTHEONETHATIS
FAVOREDBYSELECTION.THISISCALLEDASELECTIVE
SWEEP.ANEWMUTATIONARISINGFAVOREDBYSELECTION
GOINGTOAFREQUENCYOF1.THEREFOREREPLACINGALL OF
THEPRE-EXISTINGVARIATION.SO,IFYOUCANDOTHE
SCANALONGTHECHROMOSOMEANDLOOKATTHELEVELSOF
VARIATIONYOUSEELOTSVARIATIONGOINGONHEREAT EACH
INDIVIDUALGENE,ANDWHAMYOUWILLHITAREGIONTHAT
HASNOVARIATION.IFTHISHASOCCURREDRECENTLY. THE
RECENTSELECTIVESWEEPORRECENTEPISODEOFSELECTION WILLCAUSEAPATTERNINTHEGENOMETHATHASVERYLARGE
VARIATIONCOMPAREDTOTHEBACKGROUND,ADJACENT
REGIONS.IFWEDOALITTLEGRAPHALONGHEREOF
VARIATION,BYTHEWAYAMIWRITINGLARGEENOUGHFOR
YOUTOSEEUPATTHEBACK?NODYES.SHAKEHEADIF
NO.IDON'TSEEANYHEADSMOVINGATALL.THANKYOU.
SO,IFTHISISVARIATION,ASYOUARESCANNINGTHE
GENOME,ITBOUNCESAROUNDANDYOUGETALMOSTNO
VARIATIONINIT.SO,WECANDOTHESESCANSMUCHMORE
READILYNOW.INFACT,WITHTHERECENTSELECTIVESWEEP
ITISN'TJUSTTHEEXACTMUTATIONTHATHASLOW
VARIATION,ITISTHEPARTOFTHECHROMOSOMEAROUND IT.
BECAUSEOFGENETICLINKAGE.RECOMBINATIONHASN'T YET
BROKENUPSOTHISMUTATIONHASASTRONGADVANTAGE. IT
WILLTAKEACHUNKOFTHECHROMOSOMEALONGWITHIT
UNTILITGETSBROKENUPBYGENETICRECOMBINATION BY
CROSSINGOVERANDSOFORTH.SO,YOUWILLSEEAREGION
OFTHECHROMOSOMETHATHASLESSVARIATIONTHANYOU WOULDEXPECT.THATISWHATTHISCARTOONINFIGURE
2
TRIESTOSHOWYOUWITHA,ATTHETOPPANEL
PRE-EXISTINGVARIATION.EACHDOTTHEREISADIFFERENT
SNIPORMUTATION.THENTHEONEWITHTHECIRCLEARISES
ISONTHEPOSITIVESELECTION,THATISDIRECTIONAL
SELECTIONTHATISFAVORED.ANDZOOMSALONGTO
AFREQUENCYOF1DRAGGINGTHATPARTOFTHECHROMOSOME
WITHIT.SO,YOUCANSEETHEPATHHERETHATIS
LABELEDALOTOFVARIATIONISTHATPARTOFTHE
CHROMOSOMETHATCONTAINSTHISADVANTAGEOUSALLELE.
THEOTHERBITSOFTHECHROMOSOMEANDOTHERCHROMOSOMES
INTHEGENOMEIFTHEYARENOTSUBJECTTOTHISJUST
HAVETHENORMALBACKGROUNDLEVELOFVARIATION.SO, WE
CANDOTHEYES?
ITISJUSTTHEPARTOFTHEGENOMETHATHASTHAT
ADVANTAGEOUSMUTATION.THERESTOFTHEGENOMEIS JUST
BOUNCINGALONGWITHDRIFTANDMIGRATIONANDMUTATION.
ITISJUSTTHATPARTOFTHEGENOME.SOTHISEFFECT APPLIESONLYTOTHEPARTOFTHEGENOMETHATISUNDER
RECENTSELECTION.THERESTOFTHEGENOMEIS
UNAFFECTEDBECAUSEOFRECOMBINATION.BITSOFTHE
GENOMEAREBEHAVINGINDEPENDENTLYTHROUGHINHERITANCE
BECAUSEOFRECOMBINATIONCROSSINGOVERANDSOON.
OKAY?OKAY,SO,WECANSEEANEXAMPLEOFTHATONTHE
THIRDSLIDE,TODOWITHDOGS.SO,DOGS,BREEDSOF
DOGS,HAVEBEENUNDERINTENSIVEARTIFICIALSELECTION
FROMSOMEWOLF-LIKEANCESTORINTHEPAST.SO,THAT
YOUHAVEALLTHESECRAZYBREEDSOFDOGSDELIBERATELY
BREDFORTHEIRSPECIFICTRAITS.ANDONEEXAMPLEHERE
HASTODOWITHSMALLDOGSVERSUSLARGEDOGS.
ANCESTRALCONDITIONISLARGE,LIKEAWOLFORWOLF-LIKE
ANCESTOR.THENWEBREEDALLTHESELITTLEYAPPY
THINGS.TERRIERS.IHAVEAJACKRUSSELLTERRIER.
ANDIWANTTOUNDERSTANDWHYTHISISSUCHACRAZYDAMN
ANIMAL.ITISDIABOLICAL.HASANYONEHEARDOFJACK
RUSSELLTERRIERS?YOUHAVEN'TLIVED.ANYWAY,SO, THISGROUPOFGENETICISTSWANTEDTOUNDERSTANDONE
OF
THEGENESTHATHAVEBEENUNDERSELECTIONASWEHAVE
BREDFORTHESESMALLYAPPYCREATURES.TURNSOUTTHEY
FOUNDONETHATDIDAGENOMESCANLOOKINGATVARIATION
ACROSSTHEWHOLEGENOMEANDFOUNDONEPARTOFTHE
GENOMETHATHADALMOSTNOVARIATION.SOAGRAPHLIKE
THISISSHOWNATTHETOPLEFT.DON'TWORRYABOUTTHE
OTHERONETHEYAREJUSTDIFFERENTVARIETIESOFTHAT.
TURNSOUTITISAGROWTHFACTORGENE.SO,SMALLYAPPY
DOGS,MOSTBREEDSOFSMALLYAPPYDOGS,HAVETHE
SIGNATUREOFRECENTPOSITIVESELECTIONFORTHIS GROWTH
FACTORGENE.WHICHKINDOFMAKESSENSE.THEYARE
SMALLSOAGENETHATCONTROLSGROWTHRATEMIGHTBE
UNDERSELECTION.SOMETHINGSIMILAR,MINDYOUTHAT
STILLDOESN'TEXPLAINWHYMYCRAZYDOGISSMALLBUT
DOESN'TYETEXPLAINWHYITISCRAZY,THATHASGOTTO
BEOTHERGENES. YES?GOODQUESTION.HOWLONGDOES
THISLAST?SO,TWOTHINGSWILLHAPPEN;ONEIS, RECOMBINATIONWILLGOONSOYOUMIGHTSTARTWITHTHE
INITIALPHASE,AMUCHLARGERCHROMOSOMESEGMENT AND
THATWILLGETBROKENDOWNBYRECOMBINATION.THEOTHER
THINGTHATWILLHAPPENISTHATMUTATIONSWILLOCCUR IN
THISAREAANDSTARTTORESTORETHEVARIATION.BUT
THATTAKESALONGTIME.IFTHEPRESSUREISSTILL
THERE?THATISAGOODPOINT.IFTHEPRESSUREIS
STILLTHEREMAINTAININGTHISASANADVANTAGEOFOTHER
ALLELESOTHERALLELESTHATCOMEINBYMUTATIONWILL BE
RAPIDLYLOSTBECAUSETHEYNEEDPROPORTIONATELY LESS
OFFSPRING.GOODQUESTION.YES?WHICHONE?OKAY.
THATISJUSTAMEASUREOFWHATISCALLED
HETEROZYGOSITYWHICHISAMEASUREOFDIVERSITY. OKAY.
INRESPONSETOTHENUMBEROFALLELESPRESENTINTHEIR
FREQUENCIES.OKAY?YES.SO,ACROSSTHEWHOLE
GENOMEITREFLECTSTHEACTIONSOFGENETICDRIFT,
MUTATION,MIGRATION,BUTNOTSELECTION.BUT,THE PART
OFTHECHROMOSOMETHATISUNDERRECENTSELECTION SHOWS
THISSIGNATURE.THISISTHEPARTTHATISUNDERRECENT
SELECTION,NEWMUTATIONS,RECENTSELECTION,WHAM ALL
OFTHEVARIATIONTHATWASTHEREISNOWGONEBECAUSE
YOUHAVEONLYGOTTHEONEADVANTAGEOUSALLELELEFT.
OKAY?THISISGOOD.KEEPTHEQUESTIONSCOMING.
YES?YES.YES.OKAY.SOANOTHERRECENTEXAMPLE,
ANDWEAREGOINGTOCOMEBACKTOHOWTHISCONNECTS
WITHHUMANEVOLUTIONATTHEENDOFTHECOURSE.BUT
ONEEXAMPLEIS,AVERYRECENTSTUDYPUBLISHEDLAST
YEARWASTOCOMPARETIBETANVERSUSHANPOPULATIONS AND
CHINESEAGAINSTETHNICTIBETANS.THESETWOGROUPS ARE
VERYCLOSELYRELATEDRELATIVETOHUMANGENETICS IN
GENERAL.AND,AGROUPOFCHINESEINVESTIGATEDTHIS
TOGETHERWITHRASMUSNIELSEN,INTHEDEPARTMENT HERE
COMPAREDESSENTIALLYTHEWHOLETRANSCRIPTION, THE
WHOLEEXPRESSEDPARTOFTHEGENOMEACROSSBIG
POPULATIONSEEMSOFTIBETANS,HAN,CHINESEAND
ANOTHERGROUPASWELL.WHATTHEYFOUNDWHENTHEY COMPAREDTHETWOWASAVERYRECENT,THATISWITHIN
THE
LAST2,000YEARS,INCREASEDINTHEFREQUENCY OF
APARTICULARGENE,EPAISONE. ANDTHISGENEISKNOWN
TOCAUSETOLERANCETOHYPOXIA.ITPREDICTSTOSOME
EXTENTATHLETICPERFORMANCEINTHEGENERALPOPULATION
ANDBYCOMPARINGTHEWHOLEGENOMESORSOMETHINGLIKE
IT,THEWHOLETRANSCRIPTIONSATLEAST,BETWEENTHESE
TWOPOPULATIONSTHEYSAWTHISRAPIDCHANGEIN
FREQUENCY,OFINCREASEOFFREQUENCYOFTHISPARTICULAR
ALLELEATTHISHYPOXIATOLERANCEGENE.THEREIS
AWHOLEBUNCHOFOTHERSTUFFTHATRELATESTOMETABOLIC
EFFICIENCYASWELL.ANDOFCOURSETHETIBETANSLIVE
ATVERYHIGHELEVATION.HAN,CHINESEPOPULATION WAS
OFTENLESSTHAN2,000NATIVES.THETIBETANVILLAGES
WASSEQUENCEDABOUT4,000NATIVES.SOHUMANS,HUMAN
GENOMESCANSSHOWINGBYCOMPARINGDIFFERENT
POPULATIONSFROMTHEVARIATIONSWITHINANDBETWEEN
POPULATIONSAREALSODIVIDINGSIGNATURESOF,REVEALING SIGNATURES,OFRECENTSELECTIONACTINGONHUMAN
POPULATIONS.HUMANSEVOLVE,WEWILLCOMEBACKTO THAT
THING?YES?HYPOXIA?HYPOXIAISEFFICIENCYOF
OXYGENUPTAKEINTRANSPORTATHIGHELEVATION.OKAY?
SO,MAKESSENSE.THESEPEOPLELIVEINVERYHIGH
ELEVATION.THEPARTIALPRESSUREOFOXYGENISLESS.
THEYHAVETOBEMOREEFFICIENTATSCAVENGINGAND
TRANSPORTINGOXYGEN.ANDSOTHISNOWGIVESUSAGENE
THATPREVIOUSLYWASUNKNOWNASHAVINGANAFFECTAT
LEASTINOTHERTHANATHLETICPERFORMANCETHATPEOPLE
CANSTUDYFURTHERTOUNDERSTANDTHEEVOLUTIONOF
HIGHERALTITUDETOLERANCEINHUMANPOPULATION. SO,
MOREOFTHISSTUFFWILLCOMEOVERTIME.OKAY.SO
LET'SMOVEON.IWANTTOSHIFTTOPICSALITTLENOW
AWAYFROMSELECTIONONSINGLETRAITSORSINGLESPECIES
TOTHEQUESTIONOFCOEVOLUTION.
SO,LET'SGETBACK
AGAINTODARWINANDHISTHINKING.HEWASVERY
IMPRESSEDBYTHE,BYINTERACTIONSAMONGSPECIES AS
AMAJORFORCEINEVOLUTION.SO-CALLEDBIOTIC
PRESSURESINEVOLUTION.ONESPECIFICFORMOFTHAT IS
COEVOLUTIONWHERETWOINTERACTINGSPECIESORAT LEAST
TWOINTERACTINGSPECIESDRIVEEVOLUTIONOFEACH OTHER.
SOWEHAVESPECIES1,SPECIES2.THISONEIS
AFFECTINGTHEEVOLUTIONOFTHATONEANDTHERESPONSE
TOTHEEVOLUTIONHEREAFFECTSTHEEVOLUTIONTHERE.
OKAY?SO,THATSELECTIONOPERATINGASSPECIES1
CAUSESSELECTIONONTHISONE.THISONECHANGESIN
RESPONSETOTHATSELECTION.WHICHFEEDSBACKAND
AFFECTSTHEEVOLUTIONOFTHISSPECIES.SO
COEVOLUTION.RECIPROCALINTERACTION.SO,WHEN YOU
THINKOFVARIOUSPROCESSESINECOLOGY,INPARTICULAR,
THATMIGHTAFFECTTHISORMIGHTBEEXAMPLESOFTHIS.
ONEISMUTUALISMS.ASSORTOF,THISISAPOSITIVE
INTERACTIONBETWEENSPECIES.SOTHEESTABLISHMENT IN
PARTICULAR,DEPENDENTMUTUALISMWHEREONESPECIES
REQUIRESSOMEPROCESSORFUNCTIONFROMTHEOTHER SPECIES.THATEFFECTSTHEFIRSTSPECIES.SOAN
EXAMPLEIWILLSHOWYOUONASLIDEIS,SLIDE4ONTHE
TOPRIGHTISANTFUNGUSINTERACTION.SOTHINGSLIKE
THESELEAFCUTTERANTSHERE.IFYOUGOTOTHENEAR
TROPICSYOUWILLSEELONGLINESOFTHESELITTLEANTS
DRAGGINGLEAVESALONGANDTHEYARETAKINGTHEMTO
GARDENSWHERETHEREISAVERYSPECIFICFUNGUSTHAT THE
ANTSTRANSPORTTOTHEIRNESTSWHICHCANTHENHELP
BREAKDOWNTHOSELEAVESANDPROVIDEESSENTIAL
NUTRIENTSTOTHEANTS.SOTHEANTSHAVEANIMPORTANT
EFFECTONTHEFUNGUSINTHATTHEYFARMTHEM,
LITERALLY.THEYNURTURETHECOLONIES,THEYTRANSPORT
THEFUNGUSCOLONIESFROMONENESTTOANEWNESTAND SO
FORTH.ANDINRETURNTHEFUNGUSHELPSBREAKDOWNTHE
CELLULOSEINTHELEAVESTOPROVIDEESSENTIALNUTRIENTS
TOTHEANTS.THESAMEISTRUEOFMICROBESINTHEGUTS
OFINSECTSTHATHELPBREAKDOWNTHESPECIAL,THAT
HELPSTHEMMETABOLIZEANDEVENPROVIDEESSENTIAL AMINO
ACIDSTOPARTICULARTYPESOFINSECTS.SOTHESESORT
OFMUTUALISMSAREPROBABLYVERYCOMMONINNATURE AS
THEEVOLUTIONOFONESPECIESWILLAFFECTTHESECOND
SPECIESWHICHINTURNWILLAFFECTTHEEVOLUTIONOF THE
FIRSTSPECIES.IFWELOOKONTHENEGATIVESIDEWE
HAVEVARIOUSSORTSOFANTAGONISMS. SOONESPECIES
DETRIMENTALLYAFFECTSOTHERSPECIESANDTHISGIVES
RISETOSORTOFARMSRACES.SO,EVOLUTIONARYARMS
RACES.SOCANANYONETHINKOFWHATMIGHTBE
ANTAGONISTICINTERACTIONS.GIVEMESOMEEXAMPLES?
WHATISTHAT?(INAUDIBLE)PREDATORPREY
INTERACTIONS,MOREGENERALLY.THISISPRETTYEASY FOR
YOU,ISHOULDHAVEWRITTENACOUPLEDOWN.WHATISTHE
OTHERONE?YEAH?PATHOGENHOSTINTERACTIONSIN
GENERAL.SOWEHAVEALREADYHADONEEXAMPLEOFTHAT,
WHICHISHIVEVOLUTION.OKAY.WHEREINGENERAL
ANTIBIOTICRESISTANCEANDSOFORTH.BUTPARTICULARLY
HIVORTHEOTHERPATHOGENSOFHUMANSEVOLVEVERY QUICKLYTOEVADETHEHOSTIMMUNEDEFENSES.SOTHIS
IS
THEIMMUNESYSTEM. ANDTHISISTHEANTIBODIESOVER
SURFACEPROTEINSTHATELICITTHATIMMUNEREACTION OR
ANTIGENS,RATHER.
SOTHEIMMUNESYSTEMISGEAREDTORECOGNIZE
PARTICULARANTIGENSORSERVEASPROTEINSONTHE
PATHOGENS.PATHOGENSHAVEAMAZINGGENETICMECHANISMS
FORRAPIDLYDIVERSIFYINGTHEIRSURFACEPROTEINS, TO
ESCAPERECOGNITIONBYTHEIMMUNESYSTEMWHICHRESPONDS
EVOLUTIONARILYTOBEABLETORECOGNIZEMOREANTIGEN
SYSTEMS.SOITISANARMSRACEBETWEENTHEPATHOGENS
ANDTHEIRHOSTSTRYINGTOEVADETHESEPATHOGENS, THESE
PATHOGENSTRYTOEVADETHESEIMMUNESYSTEMTHEIMMUNE
SYSTEMTRIESTOKEEPUPTORECOGNIZETHEBROADERRANGE
OFPATHOGENSTHATISPOSSIBLEANDPROTECTUSAGAINST
THEM.ANOTHEREXAMPLESHOWNHEREATTHEBOTTOMRIGHT
OFSLIDE4ISAKINDOFCOOLONE.ITISALOCALONE
BETWEENGARTERSNAKESANDNEWTS.THENEWTSAREHIGHLY TOXIC.APARTICULARTOXINTHATEFFECTSIONTRANSPORT
ACROSSMEMBRANES.SODON'TEATNEWTS.ITISREALLY
BADFORYOU.IFGARTERSNAKES,PARTICULARLYTHELOCAL
GARTERSNAKES,AROUNDHERECANEATTHESESNAKESWITH
IMPUNITY,TURNSOUTTHEGARTERSNAKESHAVEAMUTATION
INTHEIRIONTRANSPORTCHANNELSTHATRENDERSTHEM
RESISTANTTOTHETOXININTHENEWTS.LOCALGARTER
SNAKESWHERENEWTSDON'TOCCUR,YOUDON'TSEETHIS
MUTATION.SOTHENEWTSAREINTENSIFYINGTHEIRTOXINS.
THEGARTERSNAKESAREDEVELOPINGMECHANISMSTORESIST
THOSETOXINSANDITISWELLUNDERSTOODATAMOLECULAR
LEVELBUTALSOINTERMSOFTHEVARIATIONACROSSTHE
POPULATIONSOFNEWTSANDTHEPOPULATIONS,ACROSS THE
POPULATIONSOFGARTERSNAKES.SOTHESESORTSOF
DYNAMICSCANHAVEANINTERESTINGEFFECTINEVOLUTION
CALLEDFREQUENCYDEPENDENTSELECTION.
THATIS,THAT
THERELATIVEFITNESSOFAPARTICULARGENOTYPEOR
PHENOTYPEDEPENDSHOWCOMMONITISINTHEPOPULATION. SO,PARTICULARLYRARETYPEADVANTAGE.SO,IFYOU
DO
ALITTLEGRAPHHERE,IWILLDOTWOGRAPHS,ONEIS
FITNESSOFAPARTICULARALLELEAGAINSTTHEFREQUENCY
OFTHATALLELE.SO,WEWILLCALLTHISZEROPLUS1
MINUS1.UPABOVEHERETHEALLELEHASANADVANTAGE
ANDDOWNBELOWTHELINEITISDISADVANTAGED.SOYOU
CANIMAGINEIFTHESIMPLESTCASE,SOMETHINGLIKE THAT.
SO,THATTHEALLELEIS(INAUDIBLE)BYSELECTIONWHEN
ITISRAREANDITISDISADVANTAGEOUSWHENITIS
COMMON.SO,IMAGINEFOLLOWINGTHATALLELEFREQUENCY
THROUGHTIME.STARTSRARE.ITWILLINCREASEIN
FREQUENCY.BECOMESCOMMONANDWILLDECREASEIN
FREQUENCY,ANDSOON.WHENITBECOMESCOMMONIT
LEAVESRELATIVELYLESSCOPIESOFITSELFFORTHENEXT
GENERATION.ITISDISADVANTAGEOUSWHENITISRARE,
FAVOREDBYASELECTION.THISISAVERYPOWERFULFORCE
AGAINFORMAINTAININGGENETICVARIATIONINPOPULATIONS
EVENIFTHEREISGENETICDRIFT.SO,COMINGBACKTO THESE.SOARMSRACES,APARASITETHATHASARARE
COMBINATIONOFANTIGENSORSURFACEPROTEINSISLESS
LIKELYTOBERECOGNIZEDBYTHEIMMUNESYSTEM.ITHAS
HIGHERSURVIVALWHICHISANADVANTAGEANDITBECOMES
MORECOMMON.THEREISAGREATERCHANCETHATTHE
IMMUNEWASONLYRECOGNIZEDANDCANMAPANEFFECTIVE
RESPONSE.SOTHATTHENFEEDSBACKTODECREASEOF
FREQUENCYTHAT,WHENITISCOMMON.INCREASEWHEN IT
ISRARE.SOYOUMAINTAINALOTOFGENETICVARIATION
INTHEANTIGENSANDCONVERSELYTOTHEIMMUNESYSTEM
DURINGTHISFREQUENCYDEPENDENTSELECTION.WETALKED
ABOUTSELECTIONOFSELECTIONBEFOREDIRECTIONAL
SELECTIONWHATEVER,WEASSUMEDTHATITDOESN'TMATTER
WHATTHEFREQUENCYOFTHEALLELE,ITSTILLHADTHE
SAMEADVANTAGE.YOUCANSEEINTHESEINTERACTING
SYSTEMS,THESECOEVOLUTIONARYSYSTEMS,YOUCAN IMAGINE
GIVINGRISETOTHISSORTOFSITUATIONQUITECOMMONLY.
OKAY?YES?YES.SO,QUESTION,DOESITALSOAPPLY TOPREDATORPREYDIRECTIONS?IMAGINEABIRDEATING
APARTICULARTYPEOFBUTTERFLY.BIRDSUSEVISUAL
RECOGNITIONOFPREYSOAPARTICULARPHENOTYPEFOR THAT
BUTTERFLYTHATBECOMESVERYCOMMONINTHEPOPULATION
WILLSUFFERAHIGHERPREDATIONRATETHANARARE
PHENOTYPE.SOTHERAREPHENOTYPEINCREASEIN
FREQUENCY.ADIFFERENTSPOTTINGOFTHEWINGSOR
WHATEVER.INCREASEINFREQUENCY,THEPREDATORS
RECOGNIZETHAT.THATGOESDOWNAFREQUENCY.SOTHESE
AREVERYPOWERFULEVOLUTIONARYFORCES.ANDMANY OF
THEMCANOPERATETHISWAY.OKAY.ALLRIGHT.SO,
SEX.ACTUALLYISHOULDJUSTLEAVEITTHERE.IWANTED
YOUTOTHINKABOUTTHETOPICFORASECOND.
SO,
WHATEVERYOUTHINK,THATISARIDICULOUSQUESTION TO
ASK,THEREISAMOREFUNDAMENTALQUESTIONBEHIND IT.
WHATISTHEADVANTAGEOFSEXUALREPRODUCTION?SORRY?
GENERATESDIVERSITY.THATACTUALLYISTHECORE
ARGUMENT.BUT,SEXISALSOTOSOMEEXTENT DISADVANTAGEOUS.SO,WHYNOTHAVESEX?ANDBYTHIS
IMEANSEXUALREPRODUCTION.SOITREDUCES
REPRODUCTIVEEFFICIENCY. SODIDYOUSAYSOMETHING?
NO?OKAY.SOIMAGINETWOTYPESOFPOPULATIONS.ONE
WITHMALESANDFEMALES.ANDONEJUSTWITHFEMALES.
SO,IMAGINEEACHFEMALECANPRODUCETWOOFFSPRING
THROUGHGENERATION.THISFEMALEWILLPRODUCEON
AVERAGE1MALE,1FEMALE,DUETOSEGREGATIONOFSEX
CHROMOSOMES,FOREXAMPLE.THISFEMALEWILLPRODUCE 2
FEMALES.THISFEMALEWILLGOONTOPRODUCEASON,AND
ADAUGHTER.THESEWILLGOONTOREPRODUCEBUNCHES OF
DAUGHTERS.THISPOPULATIONWILLGROWTWICEASFAST,
THERIGHTOFGROWTHISTWICEASFASTASTHIS
POPULATION.SO,JUSTFROMTHEDEMOGRAPHICPOINT OF
VIEW,MALESAREACOMPLETEWASTEOFTIME.IAMNOT
TALKINGABOUTPERSONALITYTRAITSORANYTHINGELSE JUST
INGENERALFROMADEMOGRAPHICPOINTOFVIEW,WHY
BOTHER.WHYHAVEMALES?THEREAREOTHER DISADVANTAGESTOSEX.YOUHAVEGOTTOFINDAMATE.
THATISKINDOFADRAGSOMETIMES.MATINGISRISKY.
INSOMEORGANISMSTHEACTOFMATINGISCONSPICUOUS AND
CANLASTFORHOURS,IFNOTDAYS,PUTTINGYOUAT
ADISADVANTAGE.THEOTHERTHINGABOUTSEXISTHAT IT
BREAKSUPGOODGENECOMBINATIONS.SOITISAPAIN.
BREAKSUPGOODGENECOMBINATIONS.
SOIFYOUFLIPOVER
TOSLIDE7,YOUWILLSEEWHATIAMGETTINGAT.ANDIT
COMESBACKTOYOURCOMMENTTHATSEXISGOOD,SEXUAL
REPRODUCTIONISGOOD,BECAUSEITGENERATESGENETIC
DIVERSITY.YOUTHINKABOUTWHATGOESONINTHE
CONTEXTOFSEXUALREPRODUCTION,YOUHAVEGOTRANDOM
MATINGACROSSTHEPOPULATION,SODIFFERENTWAYS OF
COMBININGTHEGENOTYPESACROSSAPOPULATION.YOU HAVE
GOTINDEPENDENTASSORTMENTOFLOCI,MENDEL'S
PRINCIPLE.ANDYOUHAVEGOTRECOMBINATION.YOUHAVE
ALOTOFMECHANISMSFORGENETICSHUFFLING.SOTHAT,
THEDISADVANTAGEISITBREAKSUPGOODGENE COMBINATIONS.BUTTHEADVANTAGES,ITGENERATED
LOTS
OFDIVERSITYESPECIALLYYOURIMMEDIATEOFFSPRING.
EACHOFYOU,YOUARENOT,WHENBABIESAREBORNAND
GRANDMOTHERSCOMEALONGAND,ITHASGOTDAD'SNOSE AND
MOM'SFINGERSANDTHISTHATANDTHEOTHER.
GRANDPARENTSLOVETOTHATSORTOFTHING.SWAPTHE
CHARACTERISTICS.THEPOINTIS,YOUAREARECOMBINANT
COMPAREDTOYOURPARENTS.YOUHAVEGOTSHUFFLEDGENES
DERIVEDFROMEACHOFYOURPARENTS.ANDACROSSALARGE
NUMBEROFOFFSPRINGTHATSORTOFSPREADSYOURRISK.
IFAPARTICULARGENOTYPECOMBINATIONMIGHTBEAT AN
ADVANTAGETOTHENEXTGENERATIONYOUHAVEGOTABETTER
CHANCEOFHAVINGANOFFSPRINGTHANIFTHEYAREALL
GENETICALLYIDENTICAL.IFYOUDON'TGOTHROUGH
MEIOSISTOPRODUCEOFFSPRING. SO,WHATARESOMEOF
THEPROPOSED,SOTHESEARESOMEDISADVANTAGES.WHAT
ARESOMEPROPOSEDADVANTAGESOFSEXUALREPRODUCTION?
IWILLJUSTYOUONE.ANDTHESEHAVETOBEVERY POWERFULTOOFFSETTHISHUGEDEMOGRAPHICCOSTOF
MALES.THEREHASGOTTOBEAPOINTTOMALES,I
BELIEVETHATSINCERELY.SOLET'STRYANDFIGUREOUT
WHATITIS.YES.DIDYOUHAVEAQUESTION?YES.SO
ITCOULDPRODUCE,WELLYOUARESAYINGSPECIES.THAT
ISAVERYLONG-TERMEFFECT.SOSOMETHINGTHATHAS AN
IMMEDIATEADVANTAGEWITHIN THATISTHEGENERAL
THING.SO,AGAINTHISCOMBINATIONOFRECOMBINATION
ANDANASSORTMENTISPROVIDINGNEWCOMBINATIONS OF
GENES. PERHAPSBRINGINGTOGETHERMUTATIONSTHATHAVE
ARISENINDIFFERENTFAMILIESTHATAREBENEFICIAL AND
PUTTINGTHEMINTOTHESAMEGENOME.SOCOMBINING
INDEPENDENTADAPTIVEMUTATIONSINASORTOFPSEUDOGENE
TYPE.THATISVERYPOWERFUL.SOTHATWASNUMBER2ON
THISLIST.NUMBER1ISREDUCINGACCUMULATIONOF
MUTATIONSANDITDOESTHATBYEXACTLYTHEOPPOSITE
EFFECTOFTAKINGDISADVANTAGEOUSMUTATIONSINTHE SAME
POPULATION.PACKTHEMINTOTHESAMEGENOMEAND GETTINGRIDOFTHEM.BECAUSETHEYAREMORE
DISADVANTAGEOUSTOGETHERTHANWHENTHEYARE
SEPARATELY.SOTHEYGETWEEDEDOUTOFTHEGENEPOOL
MUCHMOREEFFICIENTLYWITHSEXUALREPRODUCTION. SO
YOUBOTHBRINGTOGETHERPOSITIVECOMBINATIONSOF GENES
ANDWEEDINGOUTDISADVANTAGEOUSALLELES.ANDTHE
THIRDONE,THEVERYPOWERFULONEISHASTODOWITH
THISCOEVOLUTIONOFGENERATINGGENETICALLYDIVERSE
OFFSPRING.
SOTHATISANADVANTAGEIFTHEENVIRONMENT
FLUCTUATESFROMONEGENERATIONTOTHENEXT.SO,
SOMETHINGTHATISAGOODGENOTYPEINTHISGENERATION
ISPERHAPSAPOORGENOTYPEINTHENEXTGENERATION. IF
YOUSHUFFLETHEGENESINYOUROFFSPRINGITISLIKE BET
HEDGING,SORTOFPROTECTINGYOUROPTIONS,YOUR
EVOLUTIONARYOPTIONSAMONGYOUROFFSPRING.THAT IS
PARTICULARLYTHECASEIFYOUHAVEGOTVERYRAPID
FLUCTUATIONSINFITNESSASYOUMIGHTGETWITHTHESE
PROCESSES.THEARMSRACEAGAINSTPATHOGENS,PREDATORS ANDSOON.SO,WHATARESOMEOFTHEALTERNATIVESTO
SEX?SOMETHINGCALLEDPARTHENOGENESIS,ITIS
BASICALLYTHIS,SORRY,IAMOUTOFTHICKCHALK.BET
YOUCAN'TSEETHIS.ITISPARTHENOGENESIS,LITERALLY
VIRGINBIRTHFROMTHEGREEK.ANDTHATDOESOCCUR. IT
OCCURSINALARGENUMBEROFINVERTEBRATESANDIT
OCCURSINPLANTS,DANDELIONSFOREXAMPLE.
PARTHENOGENETIC,THATISTHEYCANCLONETHEMSELVES
WITHGENETICALLYIDENTICALLINESOFFEMALEPLANTS.
ONEPERCENTOFLIZARDSPECIESAREPARTHENOGENETIC,
COUPLEOFEXAMPLESSHOWNHERE.THESEARE,THISIS
AWOODTAILLIZARD.13DIFFERENTSPECIESINTHE
ARIZONA,NEWMEXICODESERTSTHATCONSISTSONLYOF
FEMALES.THATCLONETHEMSELVESANDPRODUCE
GENETICALLYIDENTICALDAUGHTERS.THESAMEWITH THESE
LITTLEGUYS.THESEAREGEKKOSFROMTHEAUSTRALIAN
DESERTTHATISTUDIEDFORSEVERALYEARS,WHICHHAVE
SOMEPOPULATIONSTHATARENORMALSEXUAL,MALESAND FEMALES.BUTACROSSMOSTOFTHEWESTERNDESERTOF
AUSTRALIACONSISTSOFALLFEMALEPOPULATIONSTHAT
CLONETHEMSELVESGENERATIONPASTGENERATION.YES?
RIGHT.SOHOWDOYOUGOFROMBETWEENTHISAND
THIS?SOWEKNOWINVERTEBRATES,THISISTHEANCESTOR
STATE.YOUSTARTOUTWITHSEXUALPOPULATIONS.THERE
ARETWOWAYSTODOIT.ONEISAMUTATIONINTHEGENES
THATREGULATEMEIOSIS.SOTHATISTHECASEIN
SOMETHINGLIKEDAPHNIAORWATERFLEAS.TOHAVE
AREALLYCOOLTRICKINDAPHNIA,YOUCANHAVETHIS
STATEORTHATSTATE.SOYOUWILLFINDDAPHNIAONYOUR
SLIDE.WHENTIMESAREGOOD,THEENVIRONMENTIS
PREDICTABLEANDTHEYCHURNALONGBYPRODUCINGHUGE
POPULATIONSBYAPARTHENOGENESIS.THENTHEYSWITCH
GENETICALLY.THEGENETICCONTROLOFMEIOSISGOES FROM
ESSENTIALLYAMITOTICSTATE.THISISJUSTLIKE
MITOSISTOPRODUCEGAMETES.THEYFLIPBACKANDTURN
MEIOSISONWHENTIMESGETTOUGH.SOWHERETHEPONDS OFTHESEWATERFLEASLIVEANDDRYUPTHEYSWITCHBACK
TOTHISSTATETOGENERATEALOTOFGENETICDIVERSITY
INOFFSPRING.WHENTIMESAREGOOD,THEHIGHEST
FITNESSGENOTYPESREPRODUCETHISWAY.ITISCALLED
CYCLICPARTHENOGENESIS,ITISAVERYCLEVERTRICK. IN
THEVERTEBRATESTHESESORTOFSITUATIONSARISEBY
HYBRIDIZATION.TWODIFFERENTSEXUALSPECIESCROSS IN
NATURE.THATHASALLSORTSOFDIFFERENTTYPESOF
OUTCOMES.ONEPOSSIBILITYISTOMESSUPTHEGENETIC
CONTROLOFMEIOSISSOTHATYOUGOFROMSEXUALTO
APARTHENOGENETICSTATE.ANDINSOMECIRCUMSTANCES
THATCANBEAHUGEADVANTAGE.BECAUSEYOUCANPRODUCE
HEAPSANDHEAPSOFOFFSPRING.SO,ASANEXAMPLE,LOOK
ATSLIDE8,WHICHISWHATIDIDYEARSAGOONTHESE
LITTLEGUYS.THESEAREGEKKOSFROMTHEAUSTRALIAN
DESERT.WEFIGUREDOUTTHATTHEDESERTSINCENTRAL
WESTAUSTRALIAWEREMOSTLYPARTHENOGENETICFORMS
ARISENBYHYBRIDIZATIONBETWEENTWOSEXUALSPECIES TO
GIVERISETOTHESE.ANDTHATSPREADVERYRAPIDLYOVER
THELASTFEW,SINCETHELASTICEAGEANDTHELASTTEN
THOUSANDYEARSORSOTOCOVERANAREAABOUTHALFTHE
SIZEOFTHECONTINENTALU.S.DISPLACINGSEXUAL
POPULATIONS.SOINTHATCASETHEYGOTRIDOFTHOSE
WASTEFULMALES.CHURNINGOUTLOTSANDLOTSOF
FEMALES.BUTTHEYAREGENETICALLYIDENTICAL.SO THEY
AREMORESUSCEPTIBLETODISEASE.SOONEDISEASEWE
MEASUREDWASTHESEBLOODSUCKINGMITESTHATACCUMULATE
ONTHESELIZARDSORWITHINTHEEYESANDSOFORTH.IT
INCREASESWATERLOSS,DECREASESVISION,PREY-FINDING
ABILITYANDSOONANDTURNSOUTTHEPARTHENOGENES,
THESEFORMSHAVEHUNDREDSOFTHESEMITESONTHEM. THE
MITESAREADAPTEDTOTHESECLONALGENOTYPICALLY
IDENTICALLINEAGESOVERTIME.BUTTHESEXUAL
POPULATIONSTHATOCCURWITHTHEMINMANYPLACESHAVE
ALMOSTNOMITES.ANDTHESEXUALPOPULATIONSAREABLE
TOGENETICALLYRESISTTHEMITES.THEPARTHENOGENES ARENOT,BECAUSEONCEMITESAREADAPTEDTOTHEMOTHER
THEYAUTOMATICALLYADAPTEDTOTHEIRDAUGHTERSAND HER
GRANDDAUGHTERSANDTHEIRGREAT-GRANDDAUGHTERS. SOSEX
ISGOOD.MALESAREIMPORTANT.IWILLSEEYOUON
WEDNESDAY.