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A - 3 explanation? or'tyour statistical analysis?lf not, can you suggest an n t TT rn II n EI 3.Compareyourresultswrththoseofotherstudents.Howvariableare the resuitsfor eachteam? at Hardy4. Do your resultsmatch your predictionsfor a population Weinbergequillbrium? Whatwouldyouexpecttohappentothefrequenciesifyoucontinued thls slmulationfor 25 generations? E E ls t his pr r p u l a ti o ne r trl r i n 8 ? € E- -E Explain your response model. Does 5. consiclereachof rhe conditions frlr the Hardy-weinberg this model meeteachof thoseconditions? -,at - E X E R C IS E II.2 Simulation of Evolutionary Change Using the Bead Model fl -i model (randommatUnder the conditionsspeciliedby the Hardy-Weinberg the ing in a largepopulation,no mutation, no migration,and no selection)' ln occur. not should evolution and g"'',"ti. t .[r"r-r.i"r should not change, the detennine and condltions the of the classwill modify ihi. "rih "^...ir", You will simulate in iubsequentgenerati.o,ns. .ft ., or-,genetlcfrecluencies not met. th" euol.,iionarychangesthat occur when thesecondi.tionsare two of the worklng in teams of Lwo or three stuclents,you will simulate determlnethe e"perimintal scenariospresen[edand, usrngthe beadmodel' rnclude scenarios The generations. several .fr'"r-rg.,i,-,geneticfrecluinciesover flow; gene called also populations, oi individualsbetweentwo Lh" -rgruri,rn the elfectsof smailpopulationsrze,calledgeneticdrift; and examplesof narural selection' The effectsof mutation,uli" l.,ng., to simulatewith the bead model,to y:: wrll usecomputersimulationto consider thesein ExerciseI1.3. Ail teamswill beginby simulatingthecifectof generic drift, specifi.uffy, ,n. bottieneckefrect(Experimenra.t). por rhe s"cor-rd simuration,il;'."n chooseto investigate migration,one o[ two examplesof naturaiselection, or the foundereffect_anotherexampleof genetic drilt. The procedurefo.rinvestigatingeachof the conditions will follow rhe general proceduresdescribecl as follows.Befcrre beginnrngone oi the simularron experiments, be surey.u understandthe pioceduiesto be usecr. procedure 1. Sampling with Replacement Unlessotherwiseinstructed,the genepool sizewill be 100 beads.Eachnew generationwill be formed by ranclomiychoosing 50 diploid individuals represenred by pairs of beads-Afrer.removingeachpaii of u.ua, r..pr. senringthe_genotlpe of one indivicluar)from th"ebag,replacetn. pni, f.ior. removing,thenext set,samplLngwitlt replaceru:nf. Conrlnueyour simulatrons for se'eralgenerarions. Foi eximple, ii the ,tarrrr-rg populationhas50 beads eachof A and a.(allelicirequencyof 0.5), then ii ,h. n"r, g.";;r;;;;" might produce the lbllowing results: Number of individuals:14AA,24Aa. I2aa Numberof alleles(beads):2BA+ 24A.24a+ 24a Totalnumber of alleies:I0O Frequency ofA:28+ 24=ft Frequency ofu:24 + ', '-= = Oi:. 48 =u'id 1oo In this example,the lrequencyshoulclcontinueto approximate 0.5 rorA anda 2. Reestablishing a population with New Ailelic Frequencies In somecases,the number of individuarswiil clecrease as a resurto1-the simulation.In thosecases, returnthe populatronto 100,but reestabrish the populationwith new a[elic lrequ.,-r.i.r.For example, if yo., elrminateby selectionall homozygousrecessive (aa)indi'iduals rn y..ru.'rr-.,iotion, then the resultingfrequencies would be: Number ol individuals.I 4AA, 24Aa,jaa Numberof alleles(beads):2BA+ 24A,24a Totainumber of alleles:76 Frequency oI A:28 + 24 52 0.68 76 F requency ofa:24= ft = O:Z To reestablish a populationof 100,then,the numberof beadsshould reflect thesenew frequencies.Adjustthenuntber.fheads so that A is now 6g/100 and a is 32/100 Then continuethe next round of the simuration. a C 4 Lab Topic 1 l: PopularionGenerics:The Hard Theorem 281 lf this informationis not clearto you, ask for assistance beforebeginningyour simulations. Experiment A. Simulation of Genetic Drift Materials plasticor paperbagcontaining 100beads,50 eachof two colors additional beads as needed Introduction Geneticdrift is the changein allelicfrequenciesin smallpopularionsasa resulr of chancealone. In a small population, comblnation. oTgn-"t", may nor be random, owing to samplingerror. (lf you toss a coin 500 times,you expectabouta 50 : 50 ratio of headsro tails;bur if you rossrhe coin only l0 times,the ratio may der,rategreatlyin a smallsampleowing to chancealone.) Genetic fixation, the loss of all but one possiblealleleai a genelocus in a population,is a common resultof geneticdrift in smallnaturalpopulations. Gene[icdrift is a significantevolutionaryforce in situationsknown as the bottleneckeffectand rhe foundereffect. All teams will investigatethe bottleneckeffect.you may choosethe founder effect(pp.284-285)for your second simulation. 7. BottleneckEffect A bottleneck occurs when a population undergoesa drastic reduction in size as a result of chance events (not dilferential selection), such as a volcamc erurDtion or hurricane. (Bad luck, not bad genes!)In Figure 11.3, the beads pass through a bottleneck, which results in an unpredictable combination of Fi gure 11.3. The bottleneck effect. The genepool can drift by chancewhen the populatron is drasticallyreducedby factors that act unselectivelyBad luck, not bad geneslThe resultingpopulation will haveunpredictablecombinatlons of genes.What has happenedto the amount of variation? 2fr. ll LlliJ]l Original --________--__population Bottlenecking event Surviving population 282 Lab Topic 11: PopulationGenetics:The Hardy-WeinbergTheorem beadsthat passto the other side.Thesebeadswould consrirurethe beginning of the next generation. Hypothesis As your hypothesis,either proposea hypothesisthat addressesrhe botrleneck effectspecificallyor statethe Hardy-Weinbergtheorem. Prediction Either predict equilibrium valuesas a result of Hardy-Weinbergor predict the type of changethat you expecrro occur i.na small population (il7then). Procedure 1. To investigatethe bottleneckeffect,establisha startingpopulatroncontaining 50 individuals (how many beads?)wirh a frequencyof 0.5 for eachallele(Generation0). 2. Withoutreplacement, randomlyseiectfive individuals(I0% of the population), two allelesat a time. This representsa drasticreductionin populationsize.On a separatesheetof paper,recordthe genotypesand the number of A and a allelesfor the new population. 3. Count the numbersof eachgenorypeand the numbersof eachallele. Using thesenumbers,determinethe genotypicfrequenciesfor AA, Aa, andaa and the new a1le1ic frecluencies forA (p) and a (q) for the surviving five indii'idua1s.Theseareyour observed frequencies.Enterthese freuuencies in TableI I .4, CenerationI +. Usingthe newobserved alleliclrequencies. calculate rhecxprctcd geno). typic frequencies )fq.q' Record these lrequencies in Table11.4. lp2. Generation1. 5. Reestablish the populationto 50 individualsusing rhe new allelic lrequencies on p. 280). Repeat lreferto the examplein the Procedure steps2, 3, and 4. Recordyour resultsin the appropriategenerarion in Table11.4. 6. Reestablish the genepool with new lrequenciesaftereachgenerationuntil one of the alielesbecomesfixedin the populationfor severalgenerarions. 7 . Summarizeyour resultsin the Discussionsectron. Results 1. How many generationsdid you simulate? 2. using the graph paperar the end of the Lab Topic,sketcha graph of the changeinp andqoverrime.you shouldhu,r.t*o lines,onef-.uih ull"l.. 3. Did one allelego ro fixation in that time period? Which allele? Remember,geneticfixation occurswhen the genepoor is composedof only one ailele.The orhershavebeeneliminated.Did the other aileleever appearto be going to fixarion? Table ll.4 Changesln Allelic and GenotypicFrequenciesfor Simularionsof the BottleneckEffect, an Exampleof GeneticDrift. First, record frequenciesbasedon the observednumbersrn your experiment.Then, using the observedallelic frequencies,calculatethe expected genotypicfrequencies Genotypic Frequency Observed Generation 0 I 2 3 + 5 6 7 B 9 t0 AA Aa aa Allelic Frequency Observed A (p) a (q) 0.5 0.5 Genotypic Frequency Expected pt 2pq q' 0.25 0.50 0.25 284 Lab Topic 1 l: PopulationGenetics:The Hardy-WeinbergTheorem go to fixation?If nonedid, 4. Did any of the expectedgenotypicfrequencies not? why 5. Compare results with other teams. Did the same allele go to fixation for all teams? If not, how many became fixed for A and how many for a? Discussion 1. Compare the pattern of change forp and q. Is there a consistenttrend or do the changessuggeslchance events?Look at the gr aphs of ot her teams before decidins. 2. Explain your observationsof geneticfixation for the replicatesimulationscompletedby the class.What would you expectif you simulated the bottleneckeffect100 times? 3. How might your resultshave differedif you had startedwith different allelicirequencies. for example. f = 0 2and q = 0.8? +. Sinceonly chanceevents-that is, the effectof small populationsizefor the changein genefrequencies. wouldyou sayIhal areresponsihle Explain. evolutionhasoccurred? On completionof this simulation,chooseone or two of the remainingscenariosto investigate.All scenariosshould be completedby at leastone team in the laboratory. 2. FounderEffect When a small group of individualsbecomesseparatedfrom the largerparin this smallgenepool may be dilent population,the alleliclrequencies ferentfrom thoseof the originalpopulationasa resultof chancea1one. This