Other Mechanisms of Isolation Changes in Chromosome Number Changes in Chromosome Number – Polyploidization can create barriers to gene flow that are not.

Other Mechanisms of Isolation

Other Mechanisms of Isolation

• Changes in Chromosome NumberChanges in Chromosome Number– Polyploidization can create barriers to gene flow Polyploidization can create barriers to gene flow

that are not geographicthat are not geographic– Very important mechanism of isolation in plantsVery important mechanism of isolation in plants• 43-70% of extant plant species may have arisen by 43-70% of extant plant species may have arisen by

polyploidizationpolyploidization

– Much less common in animals but occasionally Much less common in animals but occasionally important in barrier creationimportant in barrier creation

– Sometimes smaller scale chromosomal changes Sometimes smaller scale chromosomal changes can cause speciation as wellcan cause speciation as well

• Changes in Chromosome NumberChanges in Chromosome Number– Polyploidization can create barriers to gene flow Polyploidization can create barriers to gene flow

that are not geographicthat are not geographic– Very important mechanism of isolation in plantsVery important mechanism of isolation in plants• 43-70% of extant plant species may have arisen by 43-70% of extant plant species may have arisen by

polyploidizationpolyploidization

– Much less common in animals but occasionally Much less common in animals but occasionally important in barrier creationimportant in barrier creation

– Sometimes smaller scale chromosomal changes Sometimes smaller scale chromosomal changes can cause speciation as wellcan cause speciation as well

What Can Cause Divergence?What Can Cause Divergence?

Mutation? - maybe, but we know it’s incredibly slow

Drift? - SURE, especially when populations are small

Inbreeding? - Yes for genotypes, but only causes gene frequency change when coupled with drift or selection

Selection? - Of course, especially if environment differs between two populations

Migration? - NO WAY! Gene flow causes populations to converge, not diverge. It will negate other differentiating forces (drift and selection). Populations must be isolated from one another to undergo independent evolution.

Mutation? - maybe, but we know it’s incredibly slow

Drift? - SURE, especially when populations are small

Inbreeding? - Yes for genotypes, but only causes gene frequency change when coupled with drift or selection

Selection? - Of course, especially if environment differs between two populations

Migration? - NO WAY! Gene flow causes populations to converge, not diverge. It will negate other differentiating forces (drift and selection). Populations must be isolated from one another to undergo independent evolution.

NOTE: isolation by itself is not enough to cause differentiation, other factors must be acting to cause populations to diverge

NOTE: isolation by itself is not enough to cause differentiation, other factors must be acting to cause populations to diverge

What causes divergence?What causes divergence?

Mechanisms of DivergenceMechanisms of Divergence

• After a barrier has been formed, genetic After a barrier has been formed, genetic drift and natural selection act on drift and natural selection act on mutations to cause divergence in mutations to cause divergence in isolated populationsisolated populations• Drift is most important in small Drift is most important in small

populationspopulations– Most species are formed from few Most species are formed from few

individualsindividuals– Drift may rapidly cause divergence Drift may rapidly cause divergence

because of bottleneck effectsbecause of bottleneck effects

• After a barrier has been formed, genetic After a barrier has been formed, genetic drift and natural selection act on drift and natural selection act on mutations to cause divergence in mutations to cause divergence in isolated populationsisolated populations• Drift is most important in small Drift is most important in small

populationspopulations– Most species are formed from few Most species are formed from few

individualsindividuals– Drift may rapidly cause divergence Drift may rapidly cause divergence

because of bottleneck effectsbecause of bottleneck effects

The Role of DriftThe Role of Drift

• This is very difficult to determine This is very difficult to determine because it is tough to tease apart the because it is tough to tease apart the magnitude of drift’s impactmagnitude of drift’s impact

• However, it is definitely a strong force However, it is definitely a strong force in founder effect speciation and other in founder effect speciation and other situations where Nsituations where Nee is small is small

• This is very difficult to determine This is very difficult to determine because it is tough to tease apart the because it is tough to tease apart the magnitude of drift’s impactmagnitude of drift’s impact

• However, it is definitely a strong force However, it is definitely a strong force in founder effect speciation and other in founder effect speciation and other situations where Nsituations where Nee is small is small

The Role of SelectionThe Role of Selection

• Widely held view that vicariant Widely held view that vicariant speciation is caused by Natural speciation is caused by Natural Selection resulting in pre- & post Selection resulting in pre- & post zygotic incompatibilityzygotic incompatibility

• This occurs while the taxa are allopatricThis occurs while the taxa are allopatric

• Thus, both pre- and post zygotic Thus, both pre- and post zygotic isolating mechanisms are by-products isolating mechanisms are by-products of divergent selection that occurs of divergent selection that occurs during allopatryduring allopatry

• Widely held view that vicariant Widely held view that vicariant speciation is caused by Natural speciation is caused by Natural Selection resulting in pre- & post Selection resulting in pre- & post zygotic incompatibilityzygotic incompatibility

• This occurs while the taxa are allopatricThis occurs while the taxa are allopatric

• Thus, both pre- and post zygotic Thus, both pre- and post zygotic isolating mechanisms are by-products isolating mechanisms are by-products of divergent selection that occurs of divergent selection that occurs during allopatryduring allopatry

Categories of SelectionCategories of Selection

• Ecological SelectionEcological Selection – side effect of – side effect of adaptive divergenceadaptive divergence

• Sexual SelectionSexual Selection – variation in – variation in reproductive success due to variation reproductive success due to variation in the ability to acquire mates in the ability to acquire mates

• Ecological SelectionEcological Selection – side effect of – side effect of adaptive divergenceadaptive divergence

• Sexual SelectionSexual Selection – variation in – variation in reproductive success due to variation reproductive success due to variation in the ability to acquire mates in the ability to acquire mates

Ecological SelectionEcological Selection

• Almost no evidence but probably is Almost no evidence but probably is significantsignificant

• Requires the genes underlying the Requires the genes underlying the adaptive differences either have adaptive differences either have pleiotropic effects resulting in pleiotropic effects resulting in reproductive isolation or be closely reproductive isolation or be closely linked to those genes responsible for linked to those genes responsible for isolationisolation

• Almost no evidence but probably is Almost no evidence but probably is significantsignificant

• Requires the genes underlying the Requires the genes underlying the adaptive differences either have adaptive differences either have pleiotropic effects resulting in pleiotropic effects resulting in reproductive isolation or be closely reproductive isolation or be closely linked to those genes responsible for linked to those genes responsible for isolationisolation

Ecological SelectionEcological Selection

• Little known about genes that control Little known about genes that control pre & post zygotic isolationpre & post zygotic isolation• Sperm lysin genes of marine inverts.: Sperm lysin genes of marine inverts.:

prezygotic isolation results from prezygotic isolation results from constant change in female egg proteins constant change in female egg proteins which is ‘chased’ by evolution of male which is ‘chased’ by evolution of male lysin proteinslysin proteins• Ground finch (Ground finch (GeospizaGeospiza) – different ) – different

beak sizes will court different species beak sizes will court different species with wrong headwith wrong head

• Little known about genes that control Little known about genes that control pre & post zygotic isolationpre & post zygotic isolation• Sperm lysin genes of marine inverts.: Sperm lysin genes of marine inverts.:

prezygotic isolation results from prezygotic isolation results from constant change in female egg proteins constant change in female egg proteins which is ‘chased’ by evolution of male which is ‘chased’ by evolution of male lysin proteinslysin proteins• Ground finch (Ground finch (GeospizaGeospiza) – different ) – different

beak sizes will court different species beak sizes will court different species with wrong headwith wrong head

Evidence for Ecological Selection “Parallel

Speciation”

Evidence for Ecological Selection “Parallel

Speciation”• Independent evolution of the “same” Independent evolution of the “same”

reproductively isolated forms in reproductively isolated forms in different localities suggests that different localities suggests that ecological selection can result in ecological selection can result in speciationspeciation

• Sticklebacks Sticklebacks

• Independent evolution of the “same” Independent evolution of the “same” reproductively isolated forms in reproductively isolated forms in different localities suggests that different localities suggests that ecological selection can result in ecological selection can result in speciationspeciation

• Sticklebacks Sticklebacks

Parallel Speciation Sticklebacks

Parallel Speciation Sticklebacks

Sexual SelectionSexual Selection

• Arises from competition among individuals Arises from competition among individuals of one sex (usually males) for access to the of one sex (usually males) for access to the other sex, or from the preferences of one sexother sex, or from the preferences of one sex

• The males secondary sex characteristics The males secondary sex characteristics frequently play a role in both contextsfrequently play a role in both contexts

• Females choose among conspecific males Females choose among conspecific males on the basis of the state of one or more on the basis of the state of one or more features in the courtship displayfeatures in the courtship display

• Generally the female prefers the male with Generally the female prefers the male with more elaborate stimuli or traitsmore elaborate stimuli or traits

• Arises from competition among individuals Arises from competition among individuals of one sex (usually males) for access to the of one sex (usually males) for access to the other sex, or from the preferences of one sexother sex, or from the preferences of one sex

• The males secondary sex characteristics The males secondary sex characteristics frequently play a role in both contextsfrequently play a role in both contexts

• Females choose among conspecific males Females choose among conspecific males on the basis of the state of one or more on the basis of the state of one or more features in the courtship displayfeatures in the courtship display

• Generally the female prefers the male with Generally the female prefers the male with more elaborate stimuli or traitsmore elaborate stimuli or traits

• Females have a Females have a fixed maximum fixed maximum fecundityfecundity

• Males fecundity Males fecundity essentially based essentially based on number of on number of mates onlymates only

• So, variance in So, variance in male fitness is male fitness is always > female always > female fitnessfitness

• Females have a Females have a fixed maximum fixed maximum fecundityfecundity

• Males fecundity Males fecundity essentially based essentially based on number of on number of mates onlymates only

• So, variance in So, variance in male fitness is male fitness is always > female always > female fitnessfitness

Sexual Selection – WHY?Sexual Selection – WHY?


• Why females have preference is Why females have preference is controversialcontroversial

1.1. Runaway selection: self reinforcingRunaway selection: self reinforcing

2.2. Preference is adaptive: Male traits are correlated Preference is adaptive: Male traits are correlated with fitnesswith fitness

3.3. Females prefer traits that will produce more Females prefer traits that will produce more competitive sonscompetitive sons

4.4. Perceive louder, brighter, etc. against Perceive louder, brighter, etc. against background noisebackground noise

5.5. Females have sensory biases towards certain Females have sensory biases towards certain specific stimulispecific stimuli

• Why females have preference is Why females have preference is controversialcontroversial

1.1. Runaway selection: self reinforcingRunaway selection: self reinforcing

2.2. Preference is adaptive: Male traits are correlated Preference is adaptive: Male traits are correlated with fitnesswith fitness

3.3. Females prefer traits that will produce more Females prefer traits that will produce more competitive sonscompetitive sons

4.4. Perceive louder, brighter, etc. against Perceive louder, brighter, etc. against background noisebackground noise

5.5. Females have sensory biases towards certain Females have sensory biases towards certain specific stimulispecific stimuli

• The weight of evidence is that sexual The weight of evidence is that sexual selection is a major source of pre selection is a major source of pre zygotic isolationzygotic isolation• Sexually selected characters act as a Sexually selected characters act as a

barrier to interbreeding (e.g. barrier to interbreeding (e.g. PhysalaemusPhysalaemus call preferred only call preferred only intraspecifically)intraspecifically)• High species richness of many taxa in High species richness of many taxa in

which sexual selection appears to be which sexual selection appears to be diverse and intense diverse and intense

• The weight of evidence is that sexual The weight of evidence is that sexual selection is a major source of pre selection is a major source of pre zygotic isolationzygotic isolation• Sexually selected characters act as a Sexually selected characters act as a

barrier to interbreeding (e.g. barrier to interbreeding (e.g. PhysalaemusPhysalaemus call preferred only call preferred only intraspecifically)intraspecifically)• High species richness of many taxa in High species richness of many taxa in

which sexual selection appears to be which sexual selection appears to be diverse and intense diverse and intense


• Many species that are genetically similar and Many species that are genetically similar and have diversified rapidlyhave diversified rapidly

• These show females which look very similar These show females which look very similar yet males which look VERY different!!yet males which look VERY different!!

• Suggests strong female mate choice Suggests strong female mate choice powering speciation (e.g. sea lions!)powering speciation (e.g. sea lions!)

• Study by Barroclaugh – found the clade with Study by Barroclaugh – found the clade with sexual dimorphism contains more species sexual dimorphism contains more species

• Many species that are genetically similar and Many species that are genetically similar and have diversified rapidlyhave diversified rapidly

• These show females which look very similar These show females which look very similar yet males which look VERY different!!yet males which look VERY different!!

• Suggests strong female mate choice Suggests strong female mate choice powering speciation (e.g. sea lions!)powering speciation (e.g. sea lions!)

• Study by Barroclaugh – found the clade with Study by Barroclaugh – found the clade with sexual dimorphism contains more species sexual dimorphism contains more species



• Hawaiian Hawaiian DrosophilaDrosophila have elaborate have elaborate courtship behaviors and secondary courtship behaviors and secondary sexual characteristicssexual characteristics

• Males meet in leks and fight to attract Males meet in leks and fight to attract femalesfemales

• Males have bizarre head shapes to help Males have bizarre head shapes to help them win fightsthem win fights

• Hawaiian Hawaiian DrosophilaDrosophila have elaborate have elaborate courtship behaviors and secondary courtship behaviors and secondary sexual characteristicssexual characteristics

• Males meet in leks and fight to attract Males meet in leks and fight to attract femalesfemales

• Males have bizarre head shapes to help Males have bizarre head shapes to help them win fightsthem win fights

Sexual SelectionSexual SelectionD. heteroneuraD. heteroneura have hammerhead and butt headshave hammerhead and butt headsduring fightsduring fights

D. silvestrisD. silvestris have heads just like females and have heads just like females and grapple during fightsgrapple during fights

Evolutionary scenarioEvolutionary scenario• Ancestor of both species had normal headsAncestor of both species had normal heads• Mutation occurred to cause head-butting Mutation occurred to cause head-butting

behaviorbehavior• Mutants won more contestsMutants won more contests• Mutation increased to fixation in a sub-populationMutation increased to fixation in a sub-population• Additional mutations caused change in head Additional mutations caused change in head

shape for better fightingshape for better fighting

D. heteroneuraD. heteroneura have hammerhead and butt headshave hammerhead and butt headsduring fightsduring fights

D. silvestrisD. silvestris have heads just like females and have heads just like females and grapple during fightsgrapple during fights

Evolutionary scenarioEvolutionary scenario• Ancestor of both species had normal headsAncestor of both species had normal heads• Mutation occurred to cause head-butting Mutation occurred to cause head-butting

behaviorbehavior• Mutants won more contestsMutants won more contests• Mutation increased to fixation in a sub-populationMutation increased to fixation in a sub-population• Additional mutations caused change in head Additional mutations caused change in head

shape for better fightingshape for better fighting

Peripatric SpeciationPeripatric Speciation•The process by The process by which drift effects which drift effects initial changes in initial changes in allele frequenciesallele frequencies

•Then selection acts Then selection acts to further diverge to further diverge colony from parent colony from parent populationpopulation

•The process by The process by which drift effects which drift effects initial changes in initial changes in allele frequenciesallele frequencies

•Then selection acts Then selection acts to further diverge to further diverge colony from parent colony from parent populationpopulation

Peripatric SpeciationPeripatric Speciation

How does this differ from Vicariant How does this differ from Vicariant Allopatry?Allopatry?

• Typically, most allopatric events are Typically, most allopatric events are thought of as a major split leaving two thought of as a major split leaving two large populations on either side, large populations on either side, whereas a Peripatric division typically whereas a Peripatric division typically results in the nearly intact parent pop. results in the nearly intact parent pop. and a small founder population (thus, and a small founder population (thus, drift is very powerful in the founder) drift is very powerful in the founder)

How does this differ from Vicariant How does this differ from Vicariant Allopatry?Allopatry?

• Typically, most allopatric events are Typically, most allopatric events are thought of as a major split leaving two thought of as a major split leaving two large populations on either side, large populations on either side, whereas a Peripatric division typically whereas a Peripatric division typically results in the nearly intact parent pop. results in the nearly intact parent pop. and a small founder population (thus, and a small founder population (thus, drift is very powerful in the founder) drift is very powerful in the founder)

• Genetic change can be very rapid in localized Genetic change can be very rapid in localized populations founded by few individuals cut populations founded by few individuals cut off from gene flowoff from gene flow

• Differences in ecological selection can play a Differences in ecological selection can play a role because the environment of a small area role because the environment of a small area is often more homogenous than the larger is often more homogenous than the larger areaarea

• So... conflicting pressures that act on a So... conflicting pressures that act on a widespread species may be less numerous widespread species may be less numerous and more directed at certain adaptationsand more directed at certain adaptations

• Genetic change can be very rapid in localized Genetic change can be very rapid in localized populations founded by few individuals cut populations founded by few individuals cut off from gene flowoff from gene flow

• Differences in ecological selection can play a Differences in ecological selection can play a role because the environment of a small area role because the environment of a small area is often more homogenous than the larger is often more homogenous than the larger areaarea

• So... conflicting pressures that act on a So... conflicting pressures that act on a widespread species may be less numerous widespread species may be less numerous and more directed at certain adaptationsand more directed at certain adaptations

Peripatric Speciation: Remember...


• Allele frequencies at certain loci will be Allele frequencies at certain loci will be different from the parent population due to different from the parent population due to drift and founder effectsdrift and founder effects

• Changes in allele freq. could create Changes in allele freq. could create genotypes that have higher fitness that genotypes that have higher fitness that Natural Selection may select for or againstNatural Selection may select for or against

• Many species have arisen by peripatric Many species have arisen by peripatric speciationspeciation

• eg. eg. Peromysus maniculatusPeromysus maniculatus

• Allele frequencies at certain loci will be Allele frequencies at certain loci will be different from the parent population due to different from the parent population due to drift and founder effectsdrift and founder effects

• Changes in allele freq. could create Changes in allele freq. could create genotypes that have higher fitness that genotypes that have higher fitness that Natural Selection may select for or againstNatural Selection may select for or against

• Many species have arisen by peripatric Many species have arisen by peripatric speciationspeciation

• eg. eg. Peromysus maniculatusPeromysus maniculatus



Peripatric Example:

Peromysus maniculatus

Peripatric Example:

Peromysus maniculatus

ParapatricSpeciationParapatricSpeciation

Evolution of reproductive Evolution of reproductive isolation between populations isolation between populations that are continuously that are continuously distributed in space, so that distributed in space, so that there is substantial movement there is substantial movement of individuals and genes of individuals and genes between thembetween them

Evolution of reproductive Evolution of reproductive isolation between populations isolation between populations that are continuously that are continuously distributed in space, so that distributed in space, so that there is substantial movement there is substantial movement of individuals and genes of individuals and genes between thembetween them

Parapatric SpeciationParapatric Speciation

• NOTE:NOTE: The observation that two sister The observation that two sister species have parapatric (or sympatric) species have parapatric (or sympatric) ranges does not mean that they have ranges does not mean that they have speciated parapatrically or speciated parapatrically or sympatrically! sympatrically! this may instead be this may instead be allopatry followed by secondary allopatry followed by secondary contact of ranges once distinctcontact of ranges once distinct

• NOTE:NOTE: The observation that two sister The observation that two sister species have parapatric (or sympatric) species have parapatric (or sympatric) ranges does not mean that they have ranges does not mean that they have speciated parapatrically or speciated parapatrically or sympatrically! sympatrically! this may instead be this may instead be allopatry followed by secondary allopatry followed by secondary contact of ranges once distinctcontact of ranges once distinct

Parapatric SpeciationParapatric Speciation

• Divergent selection GREATER than Divergent selection GREATER than gene flow over timegene flow over time

• Generally a hybrid zone may be formedGenerally a hybrid zone may be formed

• The degree of selection and gene flow The degree of selection and gene flow depend on the size of the character depend on the size of the character clinesclines

• Divergent selection GREATER than Divergent selection GREATER than gene flow over timegene flow over time

• Generally a hybrid zone may be formedGenerally a hybrid zone may be formed

• The degree of selection and gene flow The degree of selection and gene flow depend on the size of the character depend on the size of the character clinesclines

Secondary ContactSecondary Contact

• If populations come back into contact If populations come back into contact after steps 1 and 2, have opportunity to after steps 1 and 2, have opportunity to interbreedinterbreed

• Hybridization is common in plants and Hybridization is common in plants and birdsbirds– Will hybrids be viable and fertile?Will hybrids be viable and fertile?– Will hybrids have characteristics of parent Will hybrids have characteristics of parent

species or new characteristics?species or new characteristics?– Depends on outcome of speciation eventDepends on outcome of speciation event

• If populations come back into contact If populations come back into contact after steps 1 and 2, have opportunity to after steps 1 and 2, have opportunity to interbreedinterbreed

• Hybridization is common in plants and Hybridization is common in plants and birdsbirds– Will hybrids be viable and fertile?Will hybrids be viable and fertile?– Will hybrids have characteristics of parent Will hybrids have characteristics of parent

species or new characteristics?species or new characteristics?– Depends on outcome of speciation eventDepends on outcome of speciation event


• Theodosius Dobzhansky formulated the Theodosius Dobzhansky formulated the reinforcement hypothesis about the third reinforcement hypothesis about the third stage of speciationstage of speciation– If populations sufficiently diverged in allopatry, If populations sufficiently diverged in allopatry,

hybrids should have reduced fitnesshybrids should have reduced fitness– Should be strong selection for assortive matingShould be strong selection for assortive mating– Selection that reduces fitness of hybrids is Selection that reduces fitness of hybrids is

ReinforcementReinforcement• Completes reproductive isolationCompletes reproductive isolation

• Theodosius Dobzhansky formulated the Theodosius Dobzhansky formulated the reinforcement hypothesis about the third reinforcement hypothesis about the third stage of speciationstage of speciation– If populations sufficiently diverged in allopatry, If populations sufficiently diverged in allopatry,

hybrids should have reduced fitnesshybrids should have reduced fitness– Should be strong selection for assortive matingShould be strong selection for assortive mating– Selection that reduces fitness of hybrids is Selection that reduces fitness of hybrids is

ReinforcementReinforcement• Completes reproductive isolationCompletes reproductive isolation


• Reinforcement HypothesisReinforcement Hypothesis– Predicts that pre-mating isolation will Predicts that pre-mating isolation will

evolve in species in secondary contactevolve in species in secondary contact– Prezygotic isolating mechanisms prevent Prezygotic isolating mechanisms prevent

fertilizationfertilization• Mate choice, time of breeding, genetic Mate choice, time of breeding, genetic

incompatibilityincompatibility– Postzygotic isolationPostzygotic isolation• Offspring are sterile Offspring are sterile • Reinforcement not necessaryReinforcement not necessary

• Reinforcement HypothesisReinforcement Hypothesis– Predicts that pre-mating isolation will Predicts that pre-mating isolation will

evolve in species in secondary contactevolve in species in secondary contact– Prezygotic isolating mechanisms prevent Prezygotic isolating mechanisms prevent

fertilizationfertilization• Mate choice, time of breeding, genetic Mate choice, time of breeding, genetic

incompatibilityincompatibility– Postzygotic isolationPostzygotic isolation• Offspring are sterile Offspring are sterile • Reinforcement not necessaryReinforcement not necessary


• Study by Coyne and Orr showed that prezygotic Study by Coyne and Orr showed that prezygotic isolation occurs more often in sympatric than isolation occurs more often in sympatric than allopatric speciesallopatric species

• Study by Coyne and Orr showed that prezygotic Study by Coyne and Orr showed that prezygotic isolation occurs more often in sympatric than isolation occurs more often in sympatric than allopatric speciesallopatric species


• HybridizationHybridization– Hybrids should have reduced fitness by Hybrids should have reduced fitness by

reinforcement hypothesisreinforcement hypothesis– Some have normal or increased fitnessSome have normal or increased fitness– Sorghum is an important cropSorghum is an important crop– Johnsongrass is closely related weedJohnsongrass is closely related weed– Agriculturalists worried that if they Agriculturalists worried that if they

genetically engineered herbicide genetically engineered herbicide resistance into sorghum it might be resistance into sorghum it might be transferred to johnsongrasstransferred to johnsongrass

• HybridizationHybridization– Hybrids should have reduced fitness by Hybrids should have reduced fitness by

reinforcement hypothesisreinforcement hypothesis– Some have normal or increased fitnessSome have normal or increased fitness– Sorghum is an important cropSorghum is an important crop– Johnsongrass is closely related weedJohnsongrass is closely related weed– Agriculturalists worried that if they Agriculturalists worried that if they

genetically engineered herbicide genetically engineered herbicide resistance into sorghum it might be resistance into sorghum it might be transferred to johnsongrasstransferred to johnsongrass


• HybridizationHybridization– Significant gene flow occurred and herbicide Significant gene flow occurred and herbicide

resistance introgressed into johnson grass resistance introgressed into johnson grass – If hybrids have increased fitness, will they If hybrids have increased fitness, will they

form their own species?form their own species?– Biologists attempted to recreate the Biologists attempted to recreate the

hybridization event that lead to the formation hybridization event that lead to the formation of of Helianthus anomalousHelianthus anomalous• Found that certain crosses had higher fitness Found that certain crosses had higher fitness

and certain had lowand certain had low– High fitness hybrids became new speciesHigh fitness hybrids became new species

• HybridizationHybridization– Significant gene flow occurred and herbicide Significant gene flow occurred and herbicide

resistance introgressed into johnson grass resistance introgressed into johnson grass – If hybrids have increased fitness, will they If hybrids have increased fitness, will they

form their own species?form their own species?– Biologists attempted to recreate the Biologists attempted to recreate the

hybridization event that lead to the formation hybridization event that lead to the formation of of Helianthus anomalousHelianthus anomalous• Found that certain crosses had higher fitness Found that certain crosses had higher fitness

and certain had lowand certain had low– High fitness hybrids became new speciesHigh fitness hybrids became new species


• HybridizationHybridization– Hybrid zones occur where recently Hybrid zones occur where recently

diverged populations overlapdiverged populations overlap– May occur by secondary contact or during May occur by secondary contact or during

parapatric (or peripatric) speciationparapatric (or peripatric) speciation– Hybrid zones are often present where Hybrid zones are often present where

hybrids have equal fitness to parental hybrids have equal fitness to parental speciesspecies

– Size of hybrid zone depends on fitness of Size of hybrid zone depends on fitness of hybridshybrids

• HybridizationHybridization– Hybrid zones occur where recently Hybrid zones occur where recently

diverged populations overlapdiverged populations overlap– May occur by secondary contact or during May occur by secondary contact or during

parapatric (or peripatric) speciationparapatric (or peripatric) speciation– Hybrid zones are often present where Hybrid zones are often present where

hybrids have equal fitness to parental hybrids have equal fitness to parental speciesspecies

– Size of hybrid zone depends on fitness of Size of hybrid zone depends on fitness of hybridshybrids


• HybridizationHybridization– Study on sagebrush in western USStudy on sagebrush in western US• Basin sagebrush found at low elevationsBasin sagebrush found at low elevations

• Mountain sagebrush found at high elevationsMountain sagebrush found at high elevations

• Species make contact at middle elevations and Species make contact at middle elevations and hybridizehybridize

– Graham compared several fitness measures of Graham compared several fitness measures of hybrids and parentalshybrids and parentals

– Found that hybrids have superior fitness in Found that hybrids have superior fitness in transitional habitatstransitional habitats

• HybridizationHybridization– Study on sagebrush in western USStudy on sagebrush in western US• Basin sagebrush found at low elevationsBasin sagebrush found at low elevations

• Mountain sagebrush found at high elevationsMountain sagebrush found at high elevations

• Species make contact at middle elevations and Species make contact at middle elevations and hybridizehybridize

– Graham compared several fitness measures of Graham compared several fitness measures of hybrids and parentalshybrids and parentals

– Found that hybrids have superior fitness in Found that hybrids have superior fitness in transitional habitatstransitional habitats

SympatricSpeciationSympatricSpeciation• Among the most Among the most

controversial controversial subjects in subjects in evolutionary evolutionary biologybiology

• Complete panmictic Complete panmictic mating and a mating and a reproductive reproductive isolating isolating mechanism evolves mechanism evolves within the within the populationpopulation

• Among the most Among the most controversial controversial subjects in subjects in evolutionary evolutionary biologybiology

• Complete panmictic Complete panmictic mating and a mating and a reproductive reproductive isolating isolating mechanism evolves mechanism evolves within the within the populationpopulation

Sympatric Speciation – Possible Mechanisms

Sympatric Speciation – Possible Mechanisms

1.1. PolyploidyPolyploidy

2.2. Disruptive selection, whereby certain Disruptive selection, whereby certain homozygous genotypes have high homozygous genotypes have high fitness on one or the other of two fitness on one or the other of two resources & intermediate resources & intermediate (heterozygotes) has low fitness(heterozygotes) has low fitness

1.1. PolyploidyPolyploidy

2.2. Disruptive selection, whereby certain Disruptive selection, whereby certain homozygous genotypes have high homozygous genotypes have high fitness on one or the other of two fitness on one or the other of two resources & intermediate resources & intermediate (heterozygotes) has low fitness(heterozygotes) has low fitness

Sympatric Speciation & Polyploidism

Sympatric Speciation & Polyploidism

Hybridization in HelianthusHybridization in Helianthus

Sympatric Speciation – Disruptive Selection


• Selection may favor alleles in homozygous condition Selection may favor alleles in homozygous condition and you get assortive matingand you get assortive mating

• But, antagonism likely to arise from alternative But, antagonism likely to arise from alternative selection which promotes association between selection which promotes association between alleles for adaptation and alleles for assortive alleles for adaptation and alleles for assortive mating, and recombination which destroy this mating, and recombination which destroy this associationassociation

• This is the greatest opponent to the process of This is the greatest opponent to the process of Sympatric Speciation Sympatric Speciation

• In such a model strong selection and tight linkage In such a model strong selection and tight linkage are required... BUT recombination will break this are required... BUT recombination will break this down!down!

• Selection may favor alleles in homozygous condition Selection may favor alleles in homozygous condition and you get assortive matingand you get assortive mating

• But, antagonism likely to arise from alternative But, antagonism likely to arise from alternative selection which promotes association between selection which promotes association between alleles for adaptation and alleles for assortive alleles for adaptation and alleles for assortive mating, and recombination which destroy this mating, and recombination which destroy this associationassociation

• This is the greatest opponent to the process of This is the greatest opponent to the process of Sympatric Speciation Sympatric Speciation

• In such a model strong selection and tight linkage In such a model strong selection and tight linkage are required... BUT recombination will break this are required... BUT recombination will break this down!down!



• Another similar model was proposed by Bush based Another similar model was proposed by Bush based on his work on the apple maggot flieson his work on the apple maggot flies

• Rhagoletis – economically important pest which are Rhagoletis – economically important pest which are parasites on the fruits of trees in the hawthorn cladeparasites on the fruits of trees in the hawthorn clade

• Rhagoletis ID host by sight, tough and smellRhagoletis ID host by sight, tough and smell

• Courtship and mating occur on the fruitCourtship and mating occur on the fruit

• Females lay eggs in fruit on the treeFemales lay eggs in fruit on the tree

• eggs hatch in 2 days and develop after fruits fall to eggs hatch in 2 days and develop after fruits fall to groundground

• insects burrow into ground, overwinter, and emurge insects burrow into ground, overwinter, and emurge next springnext spring

• Another similar model was proposed by Bush based Another similar model was proposed by Bush based on his work on the apple maggot flieson his work on the apple maggot flies

• Rhagoletis – economically important pest which are Rhagoletis – economically important pest which are parasites on the fruits of trees in the hawthorn cladeparasites on the fruits of trees in the hawthorn clade

• Rhagoletis ID host by sight, tough and smellRhagoletis ID host by sight, tough and smell

• Courtship and mating occur on the fruitCourtship and mating occur on the fruit

• Females lay eggs in fruit on the treeFemales lay eggs in fruit on the tree

• eggs hatch in 2 days and develop after fruits fall to eggs hatch in 2 days and develop after fruits fall to groundground

• insects burrow into ground, overwinter, and emurge insects burrow into ground, overwinter, and emurge next springnext spring


– Apple and hawthorn maggot fliesApple and hawthorn maggot flies• Are they different populations?Are they different populations?

• Live in sympatry on adjacent treesLive in sympatry on adjacent trees

• Recently diverged because apples are not Recently diverged because apples are not native to USnative to US

• A mark-recapture allozyme study revealed A mark-recapture allozyme study revealed they do form distinct populationsthey do form distinct populations

• How have they diverged without initial How have they diverged without initial isolation?isolation?– Did they skip Step 1?Did they skip Step 1?

– Apple and hawthorn maggot fliesApple and hawthorn maggot flies• Are they different populations?Are they different populations?

• Live in sympatry on adjacent treesLive in sympatry on adjacent trees

• Recently diverged because apples are not Recently diverged because apples are not native to USnative to US

• A mark-recapture allozyme study revealed A mark-recapture allozyme study revealed they do form distinct populationsthey do form distinct populations

• How have they diverged without initial How have they diverged without initial isolation?isolation?– Did they skip Step 1?Did they skip Step 1?


– Apple and hawthorn maggot fliesApple and hawthorn maggot flies• There are other mechanisms for speciation There are other mechanisms for speciation

besides allopatricbesides allopatric

• Through assortive mating there is only 6% Through assortive mating there is only 6% gene flow among populationsgene flow among populations

• Separated in time by pupating at different Separated in time by pupating at different times of yeartimes of year

• They are able to maintain distinct They are able to maintain distinct populations even with gene flow because populations even with gene flow because of strong natural selectionof strong natural selection

– Apple and hawthorn maggot fliesApple and hawthorn maggot flies• There are other mechanisms for speciation There are other mechanisms for speciation

besides allopatricbesides allopatric

• Through assortive mating there is only 6% Through assortive mating there is only 6% gene flow among populationsgene flow among populations

• Separated in time by pupating at different Separated in time by pupating at different times of yeartimes of year

• They are able to maintain distinct They are able to maintain distinct populations even with gene flow because populations even with gene flow because of strong natural selectionof strong natural selection

Genetics of Differentiation and Isolation


• What changes in the genome are necessary What changes in the genome are necessary for speciation?for speciation?

• Most FMost F11 hybrids are sterile hybrids are sterile– Postzygotic isolation is pronouncedPostzygotic isolation is pronounced

• Which genes are responsible for sterility?Which genes are responsible for sterility?

• If one sex is sterile, it is usually the If one sex is sterile, it is usually the heterogametic sexheterogametic sex– Human males have YHuman males have Y– Bird females have WBird females have W

• What changes in the genome are necessary What changes in the genome are necessary for speciation?for speciation?

• Most FMost F11 hybrids are sterile hybrids are sterile– Postzygotic isolation is pronouncedPostzygotic isolation is pronounced

• Which genes are responsible for sterility?Which genes are responsible for sterility?

• If one sex is sterile, it is usually the If one sex is sterile, it is usually the heterogametic sexheterogametic sex– Human males have YHuman males have Y– Bird females have WBird females have W



• Haldane’s Rule Haldane’s Rule – Pattern of sterility is in heterogametic sex, Pattern of sterility is in heterogametic sex,

regardless of which is male or femaleregardless of which is male or female

• Haldane’s Rule Haldane’s Rule – Pattern of sterility is in heterogametic sex, Pattern of sterility is in heterogametic sex,

regardless of which is male or femaleregardless of which is male or female



• Why does Haldane’s Rule work?Why does Haldane’s Rule work?– Consider an autosomal locus A and an X-linked Consider an autosomal locus A and an X-linked

locus Blocus B

– Individuals from one species are fixed for Individuals from one species are fixed for AA11 and and

BB11

– Sister species fixed for Sister species fixed for AA22 and and BB22

– AA22 and and BB11 interact to cause inviability interact to cause inviability

– If females from first species mate with second: If females from first species mate with second: AA11AA22BB11 males and males and AA11AA22BB11BB22 femalesfemales

– Males are inviable, females are viableMales are inviable, females are viable

• Why does Haldane’s Rule work?Why does Haldane’s Rule work?– Consider an autosomal locus A and an X-linked Consider an autosomal locus A and an X-linked

locus Blocus B

– Individuals from one species are fixed for Individuals from one species are fixed for AA11 and and

BB11

– Sister species fixed for Sister species fixed for AA22 and and BB22

– AA22 and and BB11 interact to cause inviability interact to cause inviability

– If females from first species mate with second: If females from first species mate with second: AA11AA22BB11 males and males and AA11AA22BB11BB22 femalesfemales

– Males are inviable, females are viableMales are inviable, females are viable



• Quantitative Trait Loci (QTL) Mapping Quantitative Trait Loci (QTL) Mapping – Most traits involved in reproductive Most traits involved in reproductive

isolation are quantitativeisolation are quantitative– QTL mapping attempts to locate genes QTL mapping attempts to locate genes

with small effects on quantitative traitswith small effects on quantitative traits– Find genetic markers that are unique to Find genetic markers that are unique to

each parental species and the value of that each parental species and the value of that trait in hybrids trait in hybrids

• Quantitative Trait Loci (QTL) Mapping Quantitative Trait Loci (QTL) Mapping – Most traits involved in reproductive Most traits involved in reproductive

isolation are quantitativeisolation are quantitative– QTL mapping attempts to locate genes QTL mapping attempts to locate genes

with small effects on quantitative traitswith small effects on quantitative traits– Find genetic markers that are unique to Find genetic markers that are unique to

each parental species and the value of that each parental species and the value of that trait in hybrids trait in hybrids



• Quantitative Trait Loci (QTL) Mapping Quantitative Trait Loci (QTL) Mapping – If statistically significant associations are If statistically significant associations are

found between traits and markers that found between traits and markers that have been mapped, implies that at QTL have been mapped, implies that at QTL near the marker contributes to that traitnear the marker contributes to that trait

– Try to identify “speciation genes” in a Try to identify “speciation genes” in a diverse array of organisms to understand diverse array of organisms to understand the genetics behind speciationthe genetics behind speciation

• Quantitative Trait Loci (QTL) Mapping Quantitative Trait Loci (QTL) Mapping – If statistically significant associations are If statistically significant associations are

found between traits and markers that found between traits and markers that have been mapped, implies that at QTL have been mapped, implies that at QTL near the marker contributes to that traitnear the marker contributes to that trait

– Try to identify “speciation genes” in a Try to identify “speciation genes” in a diverse array of organisms to understand diverse array of organisms to understand the genetics behind speciationthe genetics behind speciation