Chapter6Thewaysofchange:driftandselectionGeneticsofPopulationsWearemovinguptopopulationlevel!Wewillbefocusingonasinglelocuswith2allelesinawholepopulation!Hangontothisdefinitionofevolution(“Changeingeneorallelefrequenciesovertime.”)1.Hardy-Weinberg-howdoweuseit?2.GeneticDrift(Drosophilaexamplebottlenecksandfounderevents)3.Landscapegenetics(GeneFlow,isatendofchapterbutIammovingforward)4.DiggingaroundinthefieldofPopulationGenetics(wewillseewhatwegetthrough)

Ifyouareafruitfly(orDrosophila)geneticist,afarmercrossingtwochickensortrackinganastymutationinhumansthatcausesaparticularcondition,PunnetSquaresmightbeok.Wealreadywentovercomplications,butnowwearegoingtostartthinkingbeyondindividualsandtheirgenotypesandphenotypes.

Butwhathappenswhenyougettoawholepopulation?Thingsgettoocomplicated!GeneticistsextendedMendel’smodelfromparentsandtheiroffspringtoentirepopulationsacrossgenerationsusingmathematicalmodels.Theyimagined(mathematically)asituationwherethepopulation...•  Wasreallybig•  Wasmatingrandomly•  Wasnotexperienceselectionatthelocusofinterest(meaning

thetwoalleleswereequally“successful”)

Herewehave5pairsofhomologouschromosomesin5differentindividuals(A-E)(imaginewepulltheseindividual’schromosome#1outoftheirbodiesandputthemsidebyside).

1.  Whichindividualsareheterozygousatthegumballlocus?2.  Howmanydifferentallelesarethereatthegumballlocusinthis

population(of5individuals)?3.  Whichlocushastheleastamountofgeneticvariation?4.  Whichlocushasthegreatestheterozygosity?(Whichlocushasthe

largestproportionofheterozygousindividuals.)

IndividualA IndividualB IndividualDIndividualC IndividualE

Herewehave5pairsofhomologouschromosomesin5differentindividuals(A-E)(imaginewepulltheseindividualschromosome#1outoftheirbodyandputthemsidebyside).

5.Inthispopulation,whataretheallelefrequenciesofthetwoallelesatthem+mlocus?Freqoftheyellowallele(inotherwordshowmanyyellowm+msareinthispopulationatthislocus/totalallelesinpopulation)=Freqofthepurpleallele=

IndividualA IndividualB IndividualDIndividualC IndividualE

Herewehave5pairsofhomologouschromosomesin5differentindividuals(A-E)(imaginewepulltheseindividualschromosome#1outoftheirbodyandputthemsidebyside).

6.Whatarethegenotypefrequenciesatthism+mlocus?FreqofYYindividualsis=FreqofYPindividualsis=FreqofPPindividualsis=

IndividualA IndividualB IndividualDIndividualC IndividualE

Nowletsturntoabiggerpopulationandfocusonlyonelocusatatime…..

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7.Whatisthefrequencyofthe“L”alleleinthispopulation?(Imaginedroppingalltheselettersintothesamebucket!Whatisthefrequencyofthe“l”alleleinthispopulation?Whatarethegenotypefrequencies?ActualfreqofLLindividualsis….ActualfreqofLlindividualsis...Actualfreqofllindividualsis...

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LLLL LL

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7.Whatisthefrequencyofthe“L”alleleinthispopulation?(Imaginedroppingalltheselettersintothesamebucket!Whatisthefrequencyofthe“l”alleleinthispopulation?Whatarethegenotypefrequencies?ActualfreqofLLindividualsis…..25ActualfreqofLlindividualsis....50Actualfreqofllindividualsis....25

Nextwewillask..IsthispopulationinHardyWeinbergEquilibrium?NoticehowIusethecalculationsbelowtoanswerthisquestion.Youanswerthisbycalculatingwhatyouwouldexpectthegenotypefrequenciestobe.Imaginemixingaroundallthoseallelesinthebucketandpullingallelesout.WhatisthechanceofpullingoutanLallele?(.5right?)WhatisthechanceofpullingoutanotherLallele?(.5right?)(imaginethepopulationsizeisreallylargesoyoudon’thavetoworryaboutreplacement)Gobacktoyourmiddleschoolmathclassonprobability...ifyouareinterestedinthechanceofpullingouttwo“L”alleles,whatdoyoudo?Youmultiplythechanceofpullingoutonewiththechanceofpullingouttheother.Soyourchanceofgettingan“LL”individualis.5x.5right?=.25(p2)Letsdothesameforyourllindividual.Whatisthechanceofan“ll”individual?Itwillalsobe.5x.5right?=.25(q2)

LL Ll

ll

Nowletsworkonthoseheterozygotes!Thechanceofgettingan“L”is.5,thechanceofan“l”is.5too.Sothiswould=.25againforan“Ll”individualBUTthereareactuallyTWOwaysofgettingaheterozygote!BiologistsdonotcareabouttheORDERofthetwoalleles.BothLlandlLareinthecategoryheterozygotesoinadditiontotheLlwayweneedtoaddtheotherwaywhichislL.WhenwedothiswefindthechanceofgettinganlLindividualisalso.25Soweaddthesetwoways(.25+.25)toget.5=(2pq)Sohowdotheactualandexpectedgenotypefrequenciescompare?IsthispopulationinHW?(Howclosearetheactualandexpectedfrequencies?)Well,yeahitissoitiskindofaboringexample!IfitwasNOTwewouldwanttodigdeeperandthinkaboutwhatmightbegoingoninthispopulationtopushitoutofHWEquilibrium.ThereareanumberoffactorsthatmightpushpopulationsoutofHW-youwilllearnabouttheseinthecomingslides.

Nextwewillask..IsthispopulationinHardyWeinbergEquilibrium?Dotheexpectedorpredictedgenotypefrequenciesmatchtheactual?ActualfreqofLLindividualsis…..25ActualfreqofLlindividualsis....50Actualfreqofllindividualsis....25ExpectedfreqofLLis(.5)(.5)=.25=(p2)ExpectedfreqofLlis(.25+.25)=.5=(2pq)Expectedfreqofll(.5)(.5)=.25=(q2)

Nowletschangeourpopulation!

Ll

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LLLL Ll

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WhatarethegenotypefrequenciesNOW?ActualfreqofLLindividualsis….2/20=.10ActualfreqofLlindividualsis...16/20=.80Actualfreqofllindividualsis...2/20=.10Whatisthefrequencyofthe“L”alleleinthispopulation?Isthatthesameasbefore?

IsthispopulationinHardyWeinbergEquilibrium?Whatarethegenotypefrequencies?ActualfreqofLLindividualsis….2/20=.10ActualfreqofLlindividualsis...16/20=.80Actualfreqofllindividualsis...2/20=.10ExpectedfreqofLLis(.5)(.5)=.25=(p2)ExpectedfreqofLlis(.25+.25)=.5=(2pq)Expectedfreqofll(.5)(.5)=.25=(q2)