Population Genetics

The fascinating domain of Population Genetics has been presented for UG students.

1. A Presentation by
Dr. N. Sannigrahi, Associate Professor,
Department of Botany,
Nistarini College, Purulia,
D,B. Road, Purulia ( W.B) India, 723101
Population Genetics :Allele frequencies, Genotype frequencies,
Hardy-Weinberg Law, Role of Natural Selection:Mutation,
Genetic drift.

2. POPULATION GENETICS

3. POPULATION GENETICS
Microevolution,orevolutiononasmallscale,isdefinedasachangeinthe
frequencyofgenevariants,alleles,inapopulationovergenerations.The
fieldofbiologythatstudiesallelefrequenciesinpopulationsandhowthey
changeovertimeiscalledPopulationGenetics.
Microevolutionissometimescontrastedwithmacroevolution,evolutionthat
involveslargechanges,suchasformationofnewgroupsorspecies,and
happensoverlongtimeperiods.However,mostbiologistsview
microevolutionandmacroevolutionasthesameprocesshappeningon
differenttimescales.Microevolutionaddsupgradually,overlongperiodsof
timetoproducemacro-evolutionarychanges.
Let'slookatthreeconceptsthatarecoretothedefinitionofmicroevolution:
populations,alleles,andallelefrequency.
Populations:Populationisagroupoforganismsofthesamespeciesthatare
foundinthesameareaandcaninterbreed.Apopulationisthesmallestunit
thatcanevolve—inotherwords,anindividualcan’tevolve.
Alleles:Analleleisaversionofagene,aheritableunitthatcontrolsa
particularfeatureofanorganism.

4. POPULATION GENETICS
InheritanceofacquiredcharactersasexploredbyMendelismostlybasedon
controlledmatingbutthenaturalpopulationshavethepleasureofrandom
mating.Here,malegameteproducedbythepopulationhasequal
opportunityortheprobabilityofmatingwithanyfemalegameteofthe
populationandvice–versa-calledrandommatingpopulationorpanmictic
population.
CharacteristicofRandomMatingPopulation:
Nocontrolledmating,
Thepatternofthetransmissionoftheindividualgenescannotbe
determined,
Thegenotypesofoneorfewindividualsarenotimportant,
Genepoolsandgenefrequenciesarethetwomainimportantattributes.
Asaconsequence,eachspeciesderivedmaybeconsideredistheoutcome
ofseveralMendelianPopulationoccupyingaseparateterritorybut
fortunatelybearthesamegenepool.Thissocalledderivedpopulationlays
theunderstandingofthedynamicsofgeneandgenotypefrequencies.

5. GENE FREQUENCY
Frequencyisthenumberofrepeatedoccurrencesofascoreorcaseina
sample.Alleleisgenerallyusedtodenoteonepairofseries,ofalternative
formsofgenethatoccuratagivenlocusinachromosome.Theproportion
ofoneallelesrelativetoallallelesatalocusinapopulationiscalledallele
frequency.So,theproportionsofdifferentallelesofagenepresentina
Mendelianpopulationareknownasgenefrequency.Thus,theproportions
ofgametesproducedbyapopulationcarryingthedifferentalleleofagene
arethefrequenciesoftheseallelesinthepopulationi.egenefrequency.This
canbeexactlyestimatedbyclassifyingtheindividualsofarandomsample
fromthepopulationintodifferentgenotypicclassesforagene.Thetotal
numberofeachoftheallelesofthegenepresentintheseindividualsisthen
computed,andtheirratiotothetotalnumberforalltheallelesofthegeneis
estimated.Theseratiosarethegenefrequenciesforthatlocus.
So,Genefrequencymeasureshowthealleleisrelativetotheotheralleles
onthesamelocus.
FrequencyofalleleA=NumberofCopiesofalleleAinthepopulation/Total
numberofcopiesofgenesinpopulation.

6. GENE FREQUENCY
LetusconsidertheflowercolorofPeaswithfollowinggenotypic
combinations-WW(purple),WW(purple),WW(purple),WW(purple)
WW(purple),WW(purple),Ww(purple),ww(white,ww(white);
Outofthetwogenecopies(W,w),Wappears13timesandwappears5
times,
Totalnumberofgenecopiesintheentirepopulation:13+5=18,
TherearetwojustallelesinthepopulationwhereW=p,w=q
P=FrequencyofW=13/18=0.72or72%&q=Frequencyofw=5/18=0.28
or28%;thefrequencyofallthealleles=(o.72+0.28)=10r100%
Thegenotypeandphenotypefrequencyisdifferentfromtheallele
frequencythatcanbecalculatedbythefollowingmethods:
Genotypefrequency:
FrequencyofWW=6/9=0.67,
FrequencyofWw=1/9=0.11
Frequencyofww=2/9=0.22

7. GENE FREQUENCY
PhenotypicFrequency:
PhenotypeofPurple:7/9=0.78
Phenotypeofwhite:2/9=0.22asweseePurpleVswhite,
Allelefrequency:
P=FrequencyofW=13/18=0.72,
Q=Frequencyofw=5/18=0.28
Now,ifwecomebackagenerationlaterfollowedbythesubsequent
generations,itwillbeobservedthatachangeintheallelefrequenciesover
thegenerationsaftergenerations.Asaconsequence,themicroevolution
willenabletofindatruthbehindthisepisode.
Inadditiontothis,thedifferentotherfactorsplayaverycrucialinthis
regardlikegeneticdrift,naturalselectionandothersthattobeaddressedin
thesubsequentslidesintheduepartofthispresentation.

8. GENE FREQUENCY

9. HARDY-WEINBERG LAW
TheHardy-Weinbergequationisamathematicalequationthatcanbeused
tocalculatethegeneticvariationofapopulationatequilibrium.In1908,G.
H.HardyandWilhelmWeinbergindependentlydescribedabasicprinciple
ofpopulationgenetics,whichisnownamedtheHardy-Weinbergequation.
TheequationisanexpressionoftheprincipleknownasHardy-Weinberg
equilibrium,whichstatesthattheamountofgeneticvariationina
populationwillremainconstantfromonegenerationtothenextinthe
absenceofdisturbingfactors.
TheHardy-WeinbergLawstatesthatgeneandgenotypefrequenciesina
Mendelianpopulationremainconstantgenerationaftergenerationifthereis
noselection,mutation,migrationorrandomdrift.Further,thefrequencies
ofthethreegenotypesforafocuswithtwoalleles,say,A&a,willremain
constantatp
2
=AA,2pq=Aa,andq2=aawherep&qarethefrequenciesofallelesAand
arespectively.

10. HARDY-WEINBERG LAW

11. HARDY-WEINBERG LAW
ToexploretheHardy-Weinbergequation,wecanexamineasimplegenetic
locusatwhichtherearetwoalleles,Aanda.TheHardy-Weinbergequation
isexpressedas:
p
2
+2pq+q
2
=1
wherepisthefrequencyofthe"A"alleleandqisthefrequencyofthe"a"
alleleinthepopulation.Intheequation,p
2
representsthefrequencyofthe
homozygousgenotypeAA,q
2
representsthefrequencyofthehomozygous
genotypeaa,and2pqrepresentsthefrequencyoftheheterozygousgenotype
Aa.Inaddition,thesumoftheallelefrequenciesforalltheallelesatthe
locusmustbe1,sop+q=1.Ifthepandqallelefrequenciesareknown,
thenthefrequenciesofthethreegenotypesmaybecalculatedusingthe
Hardy-Weinbergequation.
Astheequationdoesnotconsidermigration,mutationandgeneticdrift,
threegenotypesforagenearepresentionthedesiredfrequencies.Such
populationsarecalledHardy-WeinbergEquilibriumandp
2
,2pq&q2
calledHardy-Weinbergequilibriumfrequencies.

12. NATURAL SELECTION
Englishnaturalist,CharlesDarwinwrotethedefinitivebookoutlininghis
ideaofnaturalselection,OntheOriginofSpecies.Thebookchronicledhis
studiesinSouthAmericaandPacificislands.Publishedin1859,thebook
becameabestseller.Naturalselectionistheprocessthroughwhich
populationsoflivingorganismsadaptandchangewiththepassageofthe
time.Individualsinapopulationarenaturallyvariable,meaningthatthey
arealldifferentinsomeways.Thisvariationmeansthatsomeindividuals
havetraitsbettersuitedtotheenvironmentthanothers.Individualswith
adaptivetraits—traitsthatgivethemsomeadvantage—aremorelikelyto
surviveandreproduce.Theseindividualsthenpasstheadaptivetraitsonto
theiroffspring.Overtime,theseadvantageoustraitsbecomemorecommon
inthepopulation.Throughthisprocessofnaturalselection,favorabletraits
aretransmittedthroughgenerations.Naturalselectioncanleadtospeciation,
whereonespeciesgivesrisetoanewanddistinctlydifferentspecies.Itis
oneoftheprocessesthatdrivesevolutionandhelpstoexplainthediversity
oflifeonEarth.

13. NATURAL SELECTION
Now,ifHardy-Weinberglawisaccepted,sothegeneandgenotype
frequenciesremainedconstantgenerationaftergenerationinnatural
populationandthispopulationwillneverundergoanychangestobringthe
diversityinthetruesense.Butitisgenerallyacceptedthatthenatural
populationareconstantlyevolvingandthegenealongwiththegenotypic
frequenciesmustacknowledgethechangeasappetizersoftheevolution.
Thus,theMendelianpopulationschangeduetonumberoffactorsasstated
below-
Migration-movementtofacilitategenepool,
Mutation-suddenchangesingeneticmakeup,
Selection-Differentialratesofthereproductionofdifferentgenotypes,
Randomdrift-Randomfluctuationingenefrequenciesofpopulationdueto
chanceorsamplingerror.
Theaforesaidfactorsarecalledevolutionaryfactorsorevolutionaryforces
andtheseforcescanexpeditethethoughtoftherecipeofvariationtobring
diversityinthenaturalworld.

14. NATURAL SELECTION

15. MIGRATION
Intermatingwiththeindividualsgenerallyenhancedbythemigrationand
thisisthecontributiontothegenepoolofapopulationbysomeindividuals
ofanotherpopulation.Itmayleadtochangeingenefrequencydepending
uponthefollowingfactors-
1.Theproportionofthemigrantindividualstothetotalnumberof
individualsinthepopulationcalledmigrationratio,
2.Thedifferencebetweenthegenefrequenciesofthemigrantindividuals
andthatofthepopulationintowhichthemigrationhasoccurred
Themigrationratio(m)=M/(N+M)
WhereM=numberofmigrantindividuals,
N=numberofindividualsinthepopulationbeforemigration.
Thus.MigrationisamechanismtoaddnewgenotypesintheMendelian
population.Individualsfromsurroundingpopulationmigratetoacertain
rateintothelocalpopulationandthereforeinterbredwiththelocal
populationandbringsgenefrequency.

16. DNA-MUTATION

17. MUTATION
Themostimportantprocesstochangegenefrequencyinthepopulationis
mutation.Itisthesudden,heritablechangeatthecostoftheexposureof
mutagensinthesuddencharacteristicofanorganismiscalledmutation;and
theyaretheultimatesourceofallgeneticvariation.Onthebasisofthe
typesaffectingthecell,itmaybegenemutationormicromutationorthe
grossmutationcalledchromosomalmutation.Thedetailedmutationtypes
andthemechanismsisveryelaborativebutasfarasthecontributionof
mutationonthepopulationisconcerned,itmaybetwotypes-
Non-recurrentmutation,
Recurrentmutation.
NON-RECURRENT MUTATION
Thesamemutationwillnotoccurinafuturegeneration.Lettakenan
exampleofAAhomozygotehavingthemutationtoturnheterozygoteAa.
Nexttheprobabilityofthetransmissionofthemutantalleleatothenext
generationwilldependuponthenumberoftheprogenyproducedbythe
heterozygote,aA.Ifitdoesnotproduceanyprogeny,awillnotbe
transmittedtothenextgeneration.Butifitproducesoneprogeny,
transmissionofawillbe½,for2progeny,itwillbe¾etc.Simply,it
dependsuponthenumberofprogenybuildsupbyainthesubsequent
generation.

18. MUTATION

19. MUTATION
Thenumberofprogenyproducedbythedifferentindividualsofa
populationisneverequalevenintheabsenceofselection;actuallyit
followsaPoissondistribution.Therefore,theprobabilityoftransmissionof
ahastobecorrectedaccordingtothePoissondistribution.Afterthis
correction,ifanaveragenumberofprogenyproducedbythematinginthe
populationistwo,theprobabilityoftransmissionofabytheAaindividual
tothenextgenerationwillbe0.6321intheplaceof0.75;itwillbe0.4685
afterthreegenerationsandonly0.05after30generation.Thus,itistobe
statedthatanon-recurrentmutationinagene,themutantallelewillbelost
inoneofthesubsequentgeneration.
RECURRENT MUTATION
Whenanykindofmutationoccursataregularintervalsataverylow
frequencycalledrecurrentmutation.IfgeneAmutatingintoaattherateµ
proportionofAalleleswillchangeintotheaallele.Thus,
Thefrequencyofnewlyproducedaalleles=p0µ

20. MUTATION
Thetotalfrequencyofaalleleaftermutation=q
0
+P
0µ,
AndthefrequencyofAaftermutation=p
0-p
0µwhere,p
0andq
oarethe
frequenciesofA&aallelesrespectively,inthegenerationaftermutation.
Thusineachgeneration,thefrequencyofAallelewilldeclinebyp
uandthat
ofawillincreasebythesamevalue,wherepisthefrequencyofAallelein
thegenerationbeforemutation.Thus,thevalueofqwillincreasebyp
uin
eachgenerationi.e∆q=p
u.Itshouldbenotedthatthevalueofpwillgoon
decreasingwitheverygeneration.Ultimately.AlltheAalleleswillbe
convertedintoa,I.e.awillbecome‘fixed’inthepopulation.Whenthe
allelehasthefrequencyof1,itissaidtohavebeenfixed,sinceits
frequencycannolongerchangeexceptthroughmigrationand/ormutation.
Inmostofthecases,bothforward(A→a)andreverse(a→A)mutations
takeplace;generally,theratesofforwardmutationsaremuchhigherthan
thoseofthereversemutation.
Butattheequilibrium,nochangesaretakenintoaccounthere.

21. MUTATION-HUMAN DNA

22. SELECTION
Selectionisdefinedasdifferentialratesofreproductionofdifferent
genotypesinapopulation.Whenthedifferentialreproductionisthe
consequenceofnaturalfactorsortheenvironment,itistreatedasnatural
selection.
Whenitisbroughtaboutbytheconsciousmanmadeeffortfordesired
change,itiscalledartificialselection.
Selectionmayoperateonthedifferentissueslikehaploidphaseorgametal
selectionactsonzygotecalledzygoticselection,selectionagainstthe
recessivephenotype,selectionagainstthedominantphenotypeorthe
selectioninfavorofheterozygous.
GAMETICSELECTION
Whentheselectionactsongametesoronthehaploidphaseofthelifecycle,
itiscalledgameticselectionmostlyapplicableofthelowergroupof
organismswherehaploidphaseisdominantbutinhigherorganisms.
Certaingenotypeshavecapabilityonreproductioncalledsegregation
distortionormeioticdriveconfinedonlyoneofthetwosexes.

23. SELECTION
Therateofthereproductionofthedifferentgenotypesarerelativetothatof
thegenotypehavinghighestrate;thisrelativevalueiscalledfitnessor
selectivevalueoradaptivevaluedenotedbyW=R
i/R
h
R
I=therateofreproductionoftheconcernedgenotypeswhileR
histherate
ofthereproductionofthatgenotypewhichhasthehighestreproduction
rate.IfthereproductionrateofthegenotypesAA,Aaandaaare16,12&8
respectively,theirfitnesswillbe1.0,(16/16),0.75(12/16)&0.5(8/16)
respectively.Thefitnessmayhavethevaluebetween1&0..Nowforagene
withalleles,A&awithfrequenciesp&qrespectively,andiftheagamete
haveaselectioncoefficientofs,theirfitnesswillbe1-s.Therewillbe
gradualdeclineinthefrequencyofainthenextgenerationthatwillequal
sq.Asitwillleadtothereductionofthepopulationsize,thevalueoftheq
inthenextgenerationwillbe(q-sq)/(1-sq).Thus,ineachgeneration.The
frequencyofaallelewilldeclinebytheproportionstillitreacheszeroand
Abecomesfixedinthepopulation.

24. SELECTION
ZYGOTIC SELECTION
Thediploidorganismshavingshortlivedhaploidphase,selectionoperates
onthezygotesandthisiscalledzygoticselection.Itmaybeagainstthe
recessivephenotype,againstthedominantphenotypeorinfavorofthe
heterozygote.
Inadditiontotheaboveissues,selectionagainsttherecessivephenotypes
incaseoftherecessiveallelesisalsoamatterofconcernoftheselection.
Mendelianpopulationsretainmanyrecessivealleleswhosefitnessisless
than1,sometimesevenzero,.Thesedeleteriousrecessivereducethefitness
ofthepopulationasapartofitslostduetoselection.Thisproportionofthe
populationthatislostduetoselectioniscalledgeneticload.Thus,thelarger
thegeneticloadofapopulation,thelowerwillbeitsaveragefitness.Very
often,theselectionforheterozygotewillretainboththeallelesA&ainthe
populationandthesetwoalleleswillbemaintainedintheequilibrium.

25. GENETIC DRIFT
Geneticdriftisoneofthebasicmechanismsofevolution.Random
fluctuationingenefrequenciesofapopulationduetochanceorsampling
erroristhemajorconcernofthechangeofthegenefrequencyina
particularpopulation.Thisiscalledgeneticdriftorrandomgeneticdriftor
randomdrift.Ineachgeneration,someindividualsmay,justbychance,
leavebehindafewmoredescendants(andgenes,ofcourse!)thanother
individuals.Thegenesandothergeneticelementsofthenextgeneration
willbethoseofthe“lucky”individuals,notnecessarilythehealthieror
“better”individuals.That,inanutshell,isgeneticdrift.IthappenstoALL
populations—there’snoavoidingthevagariesofchance.Itdoesnotoccur
inthesamedirectioninallthegenerations;itsdirectionmaychangefrom
onegenerationtothenextgeneration.Geneticdriftmaycausegenevariants
todisappearcompletelyandtherebyreducegeneticvariation.Itcouldalso
causeinitiallyrareallelestobecomemuchmorefrequent,andevenfixed.
Considerapopulationofrabbitswithbrownfurandwhitefur,whitefur
beingthedominantallele.Duetogeneticdrift,onlythebrownpopulation
mightremain,withallthewhiteoneseliminated.Acouplewithbrownand
blueeyeshaschildrenwithbrownorblueeyes.

26. GENETIC DRIFT

27. GENETIC DRIFT
Geneticdriftisamechanismofevolutioninwhichallelefrequenciesofa
populationchangeovergenerationsduetochance(samplingerror).
Geneticdriftoccursinallpopulationsofnon-infinitesize,butitseffectsare
strongestinsmallpopulations.
Geneticdriftmayresultinthelossofsomealleles(includingbeneficial
ones)andthefixation,orriseto100%100%100,percentfrequency,ofother
alleles.
Geneticdriftcanhavemajoreffectswhenapopulationissharplyreducedin
sizebyanaturaldisaster(bottleneckeffect)orwhenasmallgroupsplitsoff
fromthemainpopulationtofoundacolony(foundereffect).
Unlikenaturalselection,geneticdriftdoesnotdependonanallele’s
beneficialorharmfuleffects.Instead,driftchangesallelefrequenciespurely
bychance,asrandomsubsetsofindividuals(andthegametesofthose
individuals)aresampledtoproducethenextgeneration.

28. GENETIC DRIFT
Everypopulationexperiencesgeneticdrift,butsmallpopulationsfeelits
effectsmorestrongly.Geneticdriftdoesnottakeintoaccountanallele’s
adaptivevaluetoapopulation,anditmayresultinlossofabeneficialallele
orfixation(riseto100%100%100,percentfrequency)ofaharmfulallelein
apopulation.
Thefoundereffectandthebottleneckeffectarecasesinwhichasmall
populationisformedfromalargerpopulation.These“sampled”populations
oftendonotrepresentthegeneticdiversityoftheoriginalpopulation,and
theirsmallsizemeanstheymayexperiencestrongdriftforgenerations.
Geneticdrifthasalsoafundamentalroleintheneutraltheoryofmolecular
evolutionproposedbythepopulationgeneticistMotooKimura.Inthis
theory,mostofthegeneticvariationinDNAandproteinsequencesis
explainedbyabalancebetweenmutationandgeneticdrift.Mutationslowly
createsnewallelicvariationinDNAandproteins,andgeneticdriftslowly
eliminatesthisvariability,therebyachievingasteadystate.Afundamental
predictionofgeneticdrifttheoryisthatthesubstitutionrateingenesis
constant,andequaltothemutationrate.

29. THE AWESOME ROAD OF THE MAGIC OF REALITY

30. THANK YOU VERY MUCH FOR YOUR JOURNEY
References:
Googleforimages,
DifferentWebPagesforcontent,
PrinciplesofGenetics-Basu&Hossain,
AtextbookofBotany(VolIII)Ghosh,Bhattacharya,Hait
FundamentalsofGenetics-B.D.Singh,
ATextbookofgenetics-AjoyPaul
DISCLAIMER:
Thispresentationhasbeenmadetoenrichopensourceofinformation
withoutanyfinancialinterest.ThepresenteracknowledgesGooglefor
imagesandotheropensourcesofknowledgetodevelopthisPPT.