1
SpeciationBIOL221
Ch.24
• Speciation
• Originofnewspecies,isatthe
focalpointofevolutionarytheory
• Microevolution
• consistsofadaptationsthat
evolvewithinapopulation
• confinedtoonegenepool
• Macroevolution
• referstoevolutionarychange
abovethespecieslevel
• accumulationof
microevolutionarychanges
Speciation
TheBiologicalSpeciesConcept• biologicalspeciesconcept
• speciesisagroupofpopulations
• Whosemembershavethepotentialtointerbreed
• Produceviable,fertileoffspring
• Donotbreedsuccessfullywithotherpopulations
• Geneflowbetweenpopulations
• holdsthephenotypeofapopulationtogether
(a)Similaritybetweendifferentspecies
(b)Diversitywithinaspecies
2
Fig.24-3
EXPERIMENT
RESULTS
ExampleofagenetreeforpopulationpairA-B
Allele PopulationGeneflowevent 1 B
B
B
B
5
6
7
2
3
4
A
A
A
Allele1ismorecloselyrelatedtoalleles2,3,and4thanto
alleles5,6,and7.Inference:Geneflowoccurred.
Alleles5,6,and7aremorecloselyrelatedtooneanotherthanto
allelesinpopulationA.Inference:Nogeneflowoccurred.
Pairof
populationsw ithdetected
geneflow
Estimatedm inimum
numberofgenefloweventstoaccountfor
geneticpatternsDistancebetween
populations(km )
A-B
K-L
A-C
B-C
F-G
G-I
C-E
5
3
2–3
2
2
2
1–2
340
720
1,390
1,190
1,110
760
1,310
ReproductiveIsolation• Reproductiveisolation
• Existenceofbiologicalfactors(barriers)
• Impedetwospeciesfromproducingviable,fertileoffspring
• Prezygoticvs.postzygotic
• Hybrids
• Offspringofcrossesbetweendifferentspecies
Mule
62chromosomes 64chromosomes
63chromosomes
Donkey Horse
• Prezygoticbarriers
• blockfertilizationfromoccurringby:
• Impedingdifferentspeciesfromattemptingtomate
• Preventingthesuccessfulcompletionofmating
• Hinderingfertilizationifmatingissuccessful
ReproductiveIsolation
Prezygoticbarriers
HabitatIsolation
Individualsof
differentspecies
TemporalIsolation BehavioralIsolation
Matingattempt
MechanicalIsolation GameticIsolation
Fertilization
ReducedHybridViability ReducedHybridFertility
Postzygoticbarriers
HybridBreakdown
Viable,fertile
offspring
(a)
(b)
(d)
(c) (e) (f) (g) (h) (i)
(j)
(l)
(k)
3
• Habitatisolation:
• Twospecies
encountereach
otherrarely,ornot
atall
• becausethey
occupydifferent
habitats
• eventhoughnot
isolatedby
physicalbarriers
ReproductiveIsolation
• Temporalisolation:
• Speciesthatbreedatdifferenttimesoftheday,differentseasons,ordifferentyears
• cannotmixtheirgametes
ReproductiveIsolation
• Behavioralisolation:
• Courtshipritualsandotherbehaviorsuniquetoaspeciesareeffectivebarriers
ReproductiveIsolation
PLAY
4
• Mechanicalisolation:
• Morphologicaldifferencescanpreventsuccessfulmating
ReproductiveIsolation
(f)
Bradybaena withshellsspiralinginopposite
directions
• Gameticisolation:
• Spermofonespeciesmaynotbeabletofertilizeeggsofanotherspecies
ReproductiveIsolation
• Postzygoticbarriers
• preventthehybridzygotefromdevelopingintoaviable,fertileadult:
• Reducedhybridviability
• Reducedhybridfertility
• Hybridbreakdown
ReproductiveIsolation
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• Reducedhybridviability:
• Genesofthedifferentparentspeciesmayinteractandimpairthehybrid’sdevelopment
• Cytologic
• Zygoticmortality
ReproductiveIsolation
13.17 19 XX10.16
9.128.11
1 2.4 3.14 5.18 6 7.15
9.10
1 2.19
11.1213.1715.18
3.8 4.16 5.14 6.7
XX
• Reducedhybridfertility:
• Evenifhybridsarevigorous,theymaybesterile
ReproductiveIsolation
(i)
Donkey
(j)
Horse
(k)
Mule(sterilehybrid)
• Hybridbreakdown:
• Somefirst-generationhybridsarefertile
• butmatedwithanotherspeciesorwitheitherparentspecies
• offspringofthenextgenerationarefeebleorsterile
ReproductiveIsolation
(l)
Hybridcultivatedriceplantswithstuntedoffspring(center)
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OtherDefinitionsofSpecies• Thebiologicalspeciesconcept
• cannotbeappliedtofossilsorasexualorganisms
• includingallprokaryotes
• Otherspeciesconcepts
• emphasizetheunitywithinaspecies
• ratherthantheseparatenessofdifferentspecies
• Morphologicalspeciesconcept
• definesaspeciesbystructuralfeatures
• Itappliestosexualandasexualspeciesbutreliesonsubjectivecriteria
• Ecologicalspeciesconcept
• viewsaspeciesintermsofitsecologicalniche
• sexualandasexualspecies
• Phylogeneticspeciesconcept
• definesaspeciesassmallestgroupofindividualsonaphylogenetictree
• sexualandasexualspecies
• butitcanbedifficulttodeterminethedegreeofdifferencerequiredforseparatespecies
OtherDefinitionsofSpecies
Speciationwithorwithoutgeographicseparation
• Speciationcanoccurintwoways:
• Allopatricspeciation
• Sympatricspeciation
(a)Allopatricspeciation(b)Sympatricspeciation
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Allopatric(“OtherCountry”)Speciation• Allopatricspeciation
• geneflowisinterruptedorreduced
• whenapopulationisdividedinto
geographicallyisolatedsubpopulations
• Abarrierformssplittingapopulationintwo
• Orisolatingtwonearbyseparatepopulations
• Whatconstitutesabarrier
• dependsontheabilityofapopulationtodisperse
• Separatepopulationsmayevolveindependently
• throughmutation,naturalselection,andgeneticdrift
EvidenceofAllopatricSpeciation• Regionswithmanygeographicbarriers
• typicallyhavemorespecies
• thandoregionswithfewer
barriers
• Reproductiveisolationbetween
populations
• Generallyincreases
• asthedistancebetween
themincreases
Geographicdistance(km)
Degreeofreprodu
ctiveiso
latio
n
00
50 100 150 250200 300
0.5
1.0
1.5
2.0
Sympatric(“SameCountry”)Speciation• Sympatricspeciation
• speciationtakesplaceingeographicallyoverlappingpopulations
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Polyploidy• Polyploidy
• Isthepresenceofextrasetsofchromosomes
• duetoaccidentsduringcelldivision
• Autopolyploid
• Anindividualwithmorethantwochromosomesets
• derivedfromonespecies
2n =6 4n =12Failureofcelldivisionafterchromosome
duplicationgivesrisetotetraploid
tissue.
2nGametesproduced
arediploid..
4nOffspringwith
tetraploidkaryotypesmaybeviableand
fertile.
• Allopolyploid
• aspecieswithmultiplesetsofchromosomesderivedfromdifferent
species
Polyploidy
SpeciesA2n =6
Normalgamete
n =3
Meioticerror
SpeciesB2n =4
Unreducedgametewith4
chromosomesHybridwith7
chromosomes
Unreducedgametewith7
chromosomes
Normalgamete
n =3
Viablefertilehybrid
(allopolyploid)2n =10
SexualSelection• Sexualselection
• candrivesympatricspeciation
• Sexualselectionformatesofdifferentcolors
• alsolikelycontributedtothespeciationincichlidfishinLakeVictoria
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PatternsWithinHybridZones• hybridzone
• Regioninwhichmembersofdifferentspeciesmate
• andproducehybrids
• Canoccurinasinglebandwhereadjacentspeciesmeet
• Hybridsoftenhavereducedfitness
• comparedwithparentspecies
• Thedistributionofhybridzonescanbemorecomplex
• ifparentspeciesarefoundinmultiplehabitatswithinthesameregion
EUROPE
Fire-belliedtoadrangeHybridzone
Yellow-belliedtoadrangeYellow-belliedtoad,
Bombinavariegata
Fire-belliedtoad,Bombinabombina
Allelefrequency(logscale
)
Distancefromhybridzonecenter(km)40 30 20 2010 100
0.01
0.1
0.5
0.9
0.99
Geneflow
Population(fiveindividuals
areshown)
Barriertogeneflow
Isolatedpopulationdiverges
Hybridzone
Hybrid
Possibleoutcomes:
Reinforcement
OR
OR
Fusion
Stability
HybridZonesoverTime• Whencloselyrelatedspeciesmeetina
hybridzone
• therearethreepossibleoutcomes:
1. Strengtheningofreproductive
barriers
2. Weakeningofreproductivebarriers
3. Continuedformationofhybridindividuals
Pundamilianyererei Pundamiliapundamilia
Pundamilia“turbidwater,”hybridoffspringfromalocation
withturbidwater
Reinforcement:StrengtheningReproductiveBarriers• Reinforcementofbarriers
• occurswhenhybridsarelessfitthantheparent
species
• Overtime,therateofhybridizationdecreases
• Wherereinforcementoccurs
• reproductivebarriersshouldbestrongerfor
sympatricthanallopatricspecies
• Ifhybridsareasfitasparents
• therecanbesubstantialgeneflowbetweenspecies
• Ifgeneflowisgreatenough
• theparentspeciescanfuseintoasinglespecies
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PatternsintheFossilRecord• Fossilrecord
• Includesexamplesofspeciesthatappearsuddenly
• persistessentiallyunchangedforsometime
• andthenapparentlydisappear
• punctuatedequilibrium- NilesEldredgeandStephenJayGould
• describeperiodsofapparentstasispunctuatedbysuddenchange
• contrastswithamodelofgradualchangeinaspecies’ existence
• gradualism
(a)Punctuatedpattern
(b)Gradualpattern
Time
SpeciationRates• Thepunctuatedpattern
• inthefossilrecordandevidencefromlabstudies
• suggeststhatspeciationcanbe
rapid
• Theintervalbetweenspeciationeventscanrangefrom
• 4,000years
• somecichlids
• to40,000,000years
• somebeetles
• withanaverageof6,500,000years
StudyingtheGeneticsofSpeciation• Theexplosionofgenomics
• isenablingresearchers
toidentifyspecificgenes
• involvedinsome
casesofspeciation
• Dependingonthespeciesin
question
• speciationmightrequire
thechangeofonlya
singleallele
• ormanyalleles
Large number of base differences between humans
and yeast
Small number of base differences between humans
and pigs
Number of DNA nucleotide base differences in the cytochrome coxidase gene66
3631
2017
13
human pig ducksnake tuna moth yeast
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Youshouldnowbeableto:
1. Defineanddiscussthelimitationsofthefourspeciesconcepts
2. Describeandprovideexamplesofprezygoticandpostzygoticreproductivebarriers
3. Distinguishbetweenandprovideexamplesofallopatricandsympatricspeciation
4. Explainhowpolyploidycancausereproductiveisolation
5. Definethetermhybridzoneanddescribethreeoutcomesforhybridzonesovertime
6. Describethedifferencesbetweengradualismandpunctuatedequilibriuminratesofspeciation