Download Lab Topic 11.2 Bottleneck Effect

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explanation?
or'tyour statistical analysis?lf not, can you suggest an
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3.Compareyourresultswrththoseofotherstudents.Howvariableare
the resuitsfor eachteam?
at Hardy4. Do your resultsmatch your predictionsfor a population
Weinbergequillbrium?
Whatwouldyouexpecttohappentothefrequenciesifyoucontinued
thls slmulationfor 25 generations?
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ls t his pr r p u l a ti o ne r trl r i n 8 ?
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Explain your response
model. Does
5. consiclereachof rhe conditions frlr the Hardy-weinberg
this model meeteachof thoseconditions?
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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
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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