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

5

• 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)

6

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

7

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

8

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

9

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

10

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

11

Youshouldnowbeableto:

1. Defineanddiscussthelimitationsofthefourspeciesconcepts

2. Describeandprovideexamplesofprezygoticandpostzygoticreproductivebarriers

3. Distinguishbetweenandprovideexamplesofallopatricandsympatricspeciation

4. Explainhowpolyploidycancausereproductiveisolation

5. Definethetermhybridzoneanddescribethreeoutcomesforhybridzonesovertime

6. Describethedifferencesbetweengradualismandpunctuatedequilibriuminratesofspeciation