speciation. the history of life we have been considering how the four evolutionary forces affect...

SpeciationSpeciation

The History of LifeThe History of Life

• We have been considering how the four We have been considering how the four evolutionary forces affect populations evolutionary forces affect populations – Mutation, natural selection, genetic drift, Mutation, natural selection, genetic drift,

migrationmigration

• Now we will examine how these forces Now we will examine how these forces lead to the formation of species and lead to the formation of species and higher taxahigher taxa “How and why lineages “How and why lineages diversify through time”diversify through time”

• We have been considering how the four We have been considering how the four evolutionary forces affect populations evolutionary forces affect populations – Mutation, natural selection, genetic drift, Mutation, natural selection, genetic drift,

migrationmigration

• Now we will examine how these forces Now we will examine how these forces lead to the formation of species and lead to the formation of species and higher taxahigher taxa “How and why lineages “How and why lineages diversify through time”diversify through time”

The History of LifeThe History of Life

• Darwin’s “The Origin of Species”Darwin’s “The Origin of Species”

• It is very important to remember that It is very important to remember that the same forces that cause the same forces that cause microevolution also cause microevolution also cause macroevolutionmacroevolution

• The mechanisms are identical, only the The mechanisms are identical, only the scale is greaterscale is greater

• Darwin’s “The Origin of Species”Darwin’s “The Origin of Species”

• It is very important to remember that It is very important to remember that the same forces that cause the same forces that cause microevolution also cause microevolution also cause macroevolutionmacroevolution

• The mechanisms are identical, only the The mechanisms are identical, only the scale is greaterscale is greater

Most Basic & Debated Question

Most Basic & Debated Question

• ““Is speciation an adaptive process?”Is speciation an adaptive process?”

• No doubt different sp. occupy different No doubt different sp. occupy different niches and have different coadapted niches and have different coadapted genetic traitsgenetic traits

• But, this does not necessarily mean But, this does not necessarily mean that speciation is inherently adaptive that speciation is inherently adaptive whereby Natural Selection builds whereby Natural Selection builds isolating mechanisms because they isolating mechanisms because they achieve these resultsachieve these results

• ““Is speciation an adaptive process?”Is speciation an adaptive process?”

• No doubt different sp. occupy different No doubt different sp. occupy different niches and have different coadapted niches and have different coadapted genetic traitsgenetic traits

• But, this does not necessarily mean But, this does not necessarily mean that speciation is inherently adaptive that speciation is inherently adaptive whereby Natural Selection builds whereby Natural Selection builds isolating mechanisms because they isolating mechanisms because they achieve these resultsachieve these results

SpeciationSpeciation

• Reproductive isolation evolves as a by-Reproductive isolation evolves as a by-product of genetic changes that occur product of genetic changes that occur for other reasons, so speciation is for other reasons, so speciation is incidental, non-adaptive consequence incidental, non-adaptive consequence of divergence between populations.of divergence between populations.

• Reproductive isolation evolves as a by-Reproductive isolation evolves as a by-product of genetic changes that occur product of genetic changes that occur for other reasons, so speciation is for other reasons, so speciation is incidental, non-adaptive consequence incidental, non-adaptive consequence of divergence between populations.of divergence between populations.

How Does Speciation Occur?How Does Speciation Occur?

• Speciation may be viewed as a three-Speciation may be viewed as a three-step processstep process

1.1. Population isolationPopulation isolation

2.2. Divergence of phenotypes and genotypesDivergence of phenotypes and genotypes

3.3. Reproductive isolationReproductive isolation

• We will examine each step to We will examine each step to understand how speciation understand how speciation progressesprogresses

• Speciation may be viewed as a three-Speciation may be viewed as a three-step processstep process

1.1. Population isolationPopulation isolation

2.2. Divergence of phenotypes and genotypesDivergence of phenotypes and genotypes

3.3. Reproductive isolationReproductive isolation

• We will examine each step to We will examine each step to understand how speciation understand how speciation progressesprogresses

Modes of speciation (=how species are isolated)

Modes of speciation (=how species are isolated)

• Two basic groups, along with some special cases Two basic groups, along with some special cases that don’t fit neatly into the paradigm:that don’t fit neatly into the paradigm:

• Those that rely on geographic isolation Those that rely on geographic isolation ((AllopatricAllopatric))

• Those that rely on selection (Those that rely on selection (SympatricSympatric and and ParapatricParapatric) )

• Also:Also:Speciation through hybridizationSpeciation through hybridizationSpeciation through chromosomal Speciation through chromosomal

changes (or polyploidy events)changes (or polyploidy events)

• Two basic groups, along with some special cases Two basic groups, along with some special cases that don’t fit neatly into the paradigm:that don’t fit neatly into the paradigm:

• Those that rely on geographic isolation Those that rely on geographic isolation ((AllopatricAllopatric))

• Those that rely on selection (Those that rely on selection (SympatricSympatric and and ParapatricParapatric) )

• Also:Also:Speciation through hybridizationSpeciation through hybridizationSpeciation through chromosomal Speciation through chromosomal

changes (or polyploidy events)changes (or polyploidy events)

Allopatric SpeciationAllopatric Speciation

• Most Common Mode of SpeciationMost Common Mode of Speciation

• Thousands of examples knownThousands of examples known

• Evolution of reproductive barriers Evolution of reproductive barriers between populations that are separated between populations that are separated by a geographic barrier from by a geographic barrier from exchanging genes at more than a exchanging genes at more than a negligible ratenegligible rate

• Most Common Mode of SpeciationMost Common Mode of Speciation

• Thousands of examples knownThousands of examples known

• Evolution of reproductive barriers Evolution of reproductive barriers between populations that are separated between populations that are separated by a geographic barrier from by a geographic barrier from exchanging genes at more than a exchanging genes at more than a negligible ratenegligible rate


• Physical IsolationPhysical Isolation– Migration tends to homogenize genotypesMigration tends to homogenize genotypes– When gene flow is stopped, speciation When gene flow is stopped, speciation

may occurmay occur– Migration-selection balance of water Migration-selection balance of water

snakes in Lake Eriesnakes in Lake Erie• Populations did not diverge into banded Populations did not diverge into banded

and plain forms because migration kept and plain forms because migration kept gene flow activegene flow active• If snakes were prevented from immigrating If snakes were prevented from immigrating

to the island, the two forms might diverge to the island, the two forms might diverge into separate species by selection, into separate species by selection, mutation, and driftmutation, and drift

• Physical IsolationPhysical Isolation– Migration tends to homogenize genotypesMigration tends to homogenize genotypes– When gene flow is stopped, speciation When gene flow is stopped, speciation

may occurmay occur– Migration-selection balance of water Migration-selection balance of water

snakes in Lake Eriesnakes in Lake Erie• Populations did not diverge into banded Populations did not diverge into banded

and plain forms because migration kept and plain forms because migration kept gene flow activegene flow active• If snakes were prevented from immigrating If snakes were prevented from immigrating

to the island, the two forms might diverge to the island, the two forms might diverge into separate species by selection, into separate species by selection, mutation, and driftmutation, and drift


• Physical IsolationPhysical Isolation

• Speciation may occur by Speciation may occur by VicarianceVicariance – A new barrier to gene flow arisesA new barrier to gene flow arises

Mountain range or riverMountain range or river•Or Or Dispersal (also called Peripatric)Dispersal (also called Peripatric)

– Individuals migrate to new area and a Individuals migrate to new area and a separated from their source separated from their source population, population, oror individuals on the edge individuals on the edge of parent species range become of parent species range become isolatedisolated

• Physical IsolationPhysical Isolation

• Speciation may occur by Speciation may occur by VicarianceVicariance – A new barrier to gene flow arisesA new barrier to gene flow arises

Mountain range or riverMountain range or river•Or Or Dispersal (also called Peripatric)Dispersal (also called Peripatric)

– Individuals migrate to new area and a Individuals migrate to new area and a separated from their source separated from their source population, population, oror individuals on the edge individuals on the edge of parent species range become of parent species range become isolatedisolated

Allopatry Via DispersalAllopatry Via Dispersal

• Geographic isolation through dispersalGeographic isolation through dispersal– 850 species of Hawaiian 850 species of Hawaiian DrosophilaDrosophila– Exploit a vast number of niches and have Exploit a vast number of niches and have

highly varied body formshighly varied body forms– Founder Hypothesis of speciationFounder Hypothesis of speciation• Hawaiian Islands have varied ages and they Hawaiian Islands have varied ages and they

arose in a west to east sequencearose in a west to east sequence

• Founder flies inhabited western island and Founder flies inhabited western island and emigrated eastward as other islands aroseemigrated eastward as other islands arose

• Geographic isolation through dispersalGeographic isolation through dispersal– 850 species of Hawaiian 850 species of Hawaiian DrosophilaDrosophila– Exploit a vast number of niches and have Exploit a vast number of niches and have

highly varied body formshighly varied body forms– Founder Hypothesis of speciationFounder Hypothesis of speciation• Hawaiian Islands have varied ages and they Hawaiian Islands have varied ages and they

arose in a west to east sequencearose in a west to east sequence

• Founder flies inhabited western island and Founder flies inhabited western island and emigrated eastward as other islands aroseemigrated eastward as other islands arose

• Geographic isolation through dispersalGeographic isolation through dispersal– Test founder hypothesis with geology of Test founder hypothesis with geology of

Hawaiian Islands and phylogeny of fliesHawaiian Islands and phylogeny of flies– Predictions of founder hypothesisPredictions of founder hypothesis• Closely related species will occupy Closely related species will occupy

adjacent islandsadjacent islands• Branching sequence should partially Branching sequence should partially

coincide with coincide with island emergence sequenceisland emergence sequence– Used mtDNA to estimate phylogenyUsed mtDNA to estimate phylogeny– Founder hypothesis was supportedFounder hypothesis was supported

• Geographic isolation through dispersalGeographic isolation through dispersal– Test founder hypothesis with geology of Test founder hypothesis with geology of

Hawaiian Islands and phylogeny of fliesHawaiian Islands and phylogeny of flies– Predictions of founder hypothesisPredictions of founder hypothesis• Closely related species will occupy Closely related species will occupy

adjacent islandsadjacent islands• Branching sequence should partially Branching sequence should partially

coincide with coincide with island emergence sequenceisland emergence sequence– Used mtDNA to estimate phylogenyUsed mtDNA to estimate phylogeny– Founder hypothesis was supportedFounder hypothesis was supported

Allopatry Via DispersalAllopatry Via Dispersal

Direction of Colonization

Allopatry Via VicarianceAllopatry Via Vicariance

• Geographic isolation through Geographic isolation through vicariancevicariance– Ranges of marine organisms were split Ranges of marine organisms were split

when the Central American Isthmus closed when the Central American Isthmus closed 3 Mya3 Mya

– Did this vicariant event cause the Did this vicariant event cause the speciation patterns seen today in speciation patterns seen today in snapping shrimp?snapping shrimp?• 7 pairs of morphospecies separated by 7 pairs of morphospecies separated by

IsthmusIsthmus

• Geographic isolation through Geographic isolation through vicariancevicariance– Ranges of marine organisms were split Ranges of marine organisms were split

when the Central American Isthmus closed when the Central American Isthmus closed 3 Mya3 Mya

– Did this vicariant event cause the Did this vicariant event cause the speciation patterns seen today in speciation patterns seen today in snapping shrimp?snapping shrimp?• 7 pairs of morphospecies separated by 7 pairs of morphospecies separated by

IsthmusIsthmus

• Geographic isolation through Geographic isolation through vicariancevicariance– A mtDNA phylogeny demonstrates that A mtDNA phylogeny demonstrates that

formation of Central America did appear to formation of Central America did appear to split species in twosplit species in two

– Also revealed Cryptic SpeciesAlso revealed Cryptic Species• Species that cannot be distinguished by Species that cannot be distinguished by

morphologymorphology– Species were not all the same ageSpecies were not all the same age• Isthmus arose disjunctly over a long time Isthmus arose disjunctly over a long time

periodperiod

• Geographic isolation through Geographic isolation through vicariancevicariance– A mtDNA phylogeny demonstrates that A mtDNA phylogeny demonstrates that

formation of Central America did appear to formation of Central America did appear to split species in twosplit species in two

– Also revealed Cryptic SpeciesAlso revealed Cryptic Species• Species that cannot be distinguished by Species that cannot be distinguished by

morphologymorphology– Species were not all the same ageSpecies were not all the same age• Isthmus arose disjunctly over a long time Isthmus arose disjunctly over a long time

periodperiod

Allopatry Via VicarianceAllopatry Via Vicariance

Snapping ShrimpSnapping Shrimp

P = PacificC = Carribian

Sister species ‘cut’ by Panamanian land bridge formation

Pacific and Carribian sister species cannot produce viable offspring

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

speciation. the history of life we have been considering how the four evolutionary forces affect...

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