Other Mechanisms of Isolation Changes in Chromosome Number Changes in Chromosome Number – Polyploidization can create barriers to gene flow that are not geographic – Very important mechanism of isolation in plants 43-70% of extant plant species may have arisen by polyploidization 43-70% of extant plant species may have arisen by polyploidization – Much less common in animals but occasionally important in barrier creation – Sometimes smaller scale chromosomal changes can cause speciation as well Changes in Chromosome Number Changes in Chromosome Number – Polyploidization can create barriers to gene flow that are not geographic – Very important mechanism of isolation in plants 43-70% of extant plant species may have arisen by polyploidization 43-70% of extant plant species may have arisen by polyploidization – Much less common in animals but occasionally important in barrier creation – Sometimes smaller scale chromosomal changes can cause speciation as well

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 What causes divergence?

Mechanisms of Divergence After a barrier has been formed, genetic drift and natural selection act on mutations to cause divergence in isolated populations After a barrier has been formed, genetic drift and natural selection act on mutations to cause divergence in isolated populations Drift is most important in small populations Drift is most important in small populations – Most species are formed from few individuals – Drift may rapidly cause divergence because of bottleneck effects After a barrier has been formed, genetic drift and natural selection act on mutations to cause divergence in isolated populations After a barrier has been formed, genetic drift and natural selection act on mutations to cause divergence in isolated populations Drift is most important in small populations Drift is most important in small populations – Most species are formed from few individuals – Drift may rapidly cause divergence because of bottleneck effects

The Role of Drift This is very difficult to determine because it is tough to tease apart the magnitude of drift’s impact This is very difficult to determine because it is tough to tease apart the magnitude of drift’s impact However, it is definitely a strong force in founder effect speciation and other situations where N e is small However, it is definitely a strong force in founder effect speciation and other situations where N e is small This is very difficult to determine because it is tough to tease apart the magnitude of drift’s impact This is very difficult to determine because it is tough to tease apart the magnitude of drift’s impact However, it is definitely a strong force in founder effect speciation and other situations where N e is small However, it is definitely a strong force in founder effect speciation and other situations where N e is small

The Role of Selection Widely held view that vicariant speciation is caused by Natural Selection resulting in pre- & post zygotic incompatibility Widely held view that vicariant speciation is caused by Natural Selection resulting in pre- & post zygotic incompatibility This occurs while the taxa are allopatric This occurs while the taxa are allopatric Thus, both pre- and post zygotic isolating mechanisms are by-products of divergent selection that occurs during allopatry Thus, both pre- and post zygotic isolating mechanisms are by-products of divergent selection that occurs during allopatry Widely held view that vicariant speciation is caused by Natural Selection resulting in pre- & post zygotic incompatibility Widely held view that vicariant speciation is caused by Natural Selection resulting in pre- & post zygotic incompatibility This occurs while the taxa are allopatric This occurs while the taxa are allopatric Thus, both pre- and post zygotic isolating mechanisms are by-products of divergent selection that occurs during allopatry Thus, both pre- and post zygotic isolating mechanisms are by-products of divergent selection that occurs during allopatry

Categories of Selection Ecological Selection – side effect of adaptive divergence Ecological Selection – side effect of adaptive divergence Sexual Selection – variation in reproductive success due to variation in the ability to acquire mates Sexual Selection – variation in reproductive success due to variation in the ability to acquire mates Ecological Selection – side effect of adaptive divergence Ecological Selection – side effect of adaptive divergence Sexual Selection – variation in reproductive success due to variation in the ability to acquire mates Sexual Selection – variation in reproductive success due to variation in the ability to acquire mates

Ecological Selection Almost no evidence but probably is significant Almost no evidence but probably is significant Requires the genes underlying the adaptive differences either have pleiotropic effects resulting in reproductive isolation or be closely linked to those genes responsible for isolation Requires the genes underlying the adaptive differences either have pleiotropic effects resulting in reproductive isolation or be closely linked to those genes responsible for isolation Almost no evidence but probably is significant Almost no evidence but probably is significant Requires the genes underlying the adaptive differences either have pleiotropic effects resulting in reproductive isolation or be closely linked to those genes responsible for isolation Requires the genes underlying the adaptive differences either have pleiotropic effects resulting in reproductive isolation or be closely linked to those genes responsible for isolation

Ecological Selection Little known about genes that control pre & post zygotic isolation Little known about genes that control pre & post zygotic isolation Sperm lysin genes of marine inverts.: prezygotic isolation results from constant change in female egg proteins which is ‘chased’ by evolution of male lysin proteins Sperm lysin genes of marine inverts.: prezygotic isolation results from constant change in female egg proteins which is ‘chased’ by evolution of male lysin proteins Ground finch (Geospiza) – different beak sizes will court different species with wrong head Ground finch (Geospiza) – different beak sizes will court different species with wrong head Little known about genes that control pre & post zygotic isolation Little known about genes that control pre & post zygotic isolation Sperm lysin genes of marine inverts.: prezygotic isolation results from constant change in female egg proteins which is ‘chased’ by evolution of male lysin proteins Sperm lysin genes of marine inverts.: prezygotic isolation results from constant change in female egg proteins which is ‘chased’ by evolution of male lysin proteins Ground finch (Geospiza) – different beak sizes will court different species with wrong head Ground finch (Geospiza) – different beak sizes will court different species with wrong head

Evidence for Ecological Selection  “Parallel Speciation” Independent evolution of the “same” reproductively isolated forms in different localities suggests that ecological selection can result in speciation Independent evolution of the “same” reproductively isolated forms in different localities suggests that ecological selection can result in speciation Sticklebacks Sticklebacks Independent evolution of the “same” reproductively isolated forms in different localities suggests that ecological selection can result in speciation Independent evolution of the “same” reproductively isolated forms in different localities suggests that ecological selection can result in speciation Sticklebacks Sticklebacks

Parallel Speciation Sticklebacks

Sexual Selection Arises from competition among individuals of one sex (usually males) for access to the other sex, or from the preferences of one sex Arises from competition among individuals of one sex (usually males) for access to the other sex, or from the preferences of one sex The males secondary sex characteristics frequently play a role in both contexts The males secondary sex characteristics frequently play a role in both contexts Females choose among conspecific males on the basis of the state of one or more features in the courtship display Females choose among conspecific males on the basis of the state of one or more features in the courtship display Generally the female prefers the male with more elaborate stimuli or traits Generally the female prefers the male with more elaborate stimuli or traits Arises from competition among individuals of one sex (usually males) for access to the other sex, or from the preferences of one sex Arises from competition among individuals of one sex (usually males) for access to the other sex, or from the preferences of one sex The males secondary sex characteristics frequently play a role in both contexts The males secondary sex characteristics frequently play a role in both contexts Females choose among conspecific males on the basis of the state of one or more features in the courtship display Females choose among conspecific males on the basis of the state of one or more features in the courtship display Generally the female prefers the male with more elaborate stimuli or traits Generally the female prefers the male with more elaborate stimuli or traits

Females have a fixed maximum fecundity Females have a fixed maximum fecundity Males fecundity essentially based on number of mates only Males fecundity essentially based on number of mates only So, variance in male fitness is always > female fitness So, variance in male fitness is always > female fitness Females have a fixed maximum fecundity Females have a fixed maximum fecundity Males fecundity essentially based on number of mates only Males fecundity essentially based on number of mates only So, variance in male fitness is always > female fitness So, variance in male fitness is always > female fitness Sexual Selection – WHY?

Sexual Selection Why females have preference is controversial Why females have preference is controversial 1.Runaway selection: self reinforcing 2.Preference is adaptive: Male traits are correlated with fitness 3.Females prefer traits that will produce more competitive sons 4.Perceive louder, brighter, etc. against background noise 5.Females have sensory biases towards certain specific stimuli Why females have preference is controversial Why females have preference is controversial 1.Runaway selection: self reinforcing 2.Preference is adaptive: Male traits are correlated with fitness 3.Females prefer traits that will produce more competitive sons 4.Perceive louder, brighter, etc. against background noise 5.Females have sensory biases towards certain specific stimuli

The weight of evidence is that sexual selection is a major source of pre zygotic isolation The weight of evidence is that sexual selection is a major source of pre zygotic isolation Sexually selected characters act as a barrier to interbreeding (e.g. Physalaemus call preferred only intraspecifically) Sexually selected characters act as a barrier to interbreeding (e.g. Physalaemus call preferred only intraspecifically) High species richness of many taxa in which sexual selection appears to be diverse and intense High species richness of many taxa in which sexual selection appears to be diverse and intense The weight of evidence is that sexual selection is a major source of pre zygotic isolation The weight of evidence is that sexual selection is a major source of pre zygotic isolation Sexually selected characters act as a barrier to interbreeding (e.g. Physalaemus call preferred only intraspecifically) Sexually selected characters act as a barrier to interbreeding (e.g. Physalaemus call preferred only intraspecifically) High species richness of many taxa in which sexual selection appears to be diverse and intense High species richness of many taxa in which sexual selection appears to be diverse and intense Sexual Selection

Many species that are genetically similar and have diversified rapidly Many species that are genetically similar and have diversified rapidly These show females which look very similar yet males which look VERY different!! These show females which look very similar yet males which look VERY different!! Suggests strong female mate choice powering speciation (e.g. sea lions!) Suggests strong female mate choice powering speciation (e.g. sea lions!) Study by Barroclaugh – found the clade with sexual dimorphism contains more species Study by Barroclaugh – found the clade with sexual dimorphism contains more species Many species that are genetically similar and have diversified rapidly Many species that are genetically similar and have diversified rapidly These show females which look very similar yet males which look VERY different!! These show females which look very similar yet males which look VERY different!! Suggests strong female mate choice powering speciation (e.g. sea lions!) Suggests strong female mate choice powering speciation (e.g. sea lions!) Study by Barroclaugh – found the clade with sexual dimorphism contains more species Study by Barroclaugh – found the clade with sexual dimorphism contains more species Sexual Selection

Hawaiian Drosophila have elaborate courtship behaviors and secondary sexual characteristics Hawaiian Drosophila have elaborate courtship behaviors and secondary sexual characteristics Males meet in leks and fight to attract females Males meet in leks and fight to attract females Males have bizarre head shapes to help them win fights Males have bizarre head shapes to help them win fights Hawaiian Drosophila have elaborate courtship behaviors and secondary sexual characteristics Hawaiian Drosophila have elaborate courtship behaviors and secondary sexual characteristics Males meet in leks and fight to attract females Males meet in leks and fight to attract females Males have bizarre head shapes to help them win fights Males have bizarre head shapes to help them win fights

Sexual Selection D. heteroneura have hammerhead and butt heads during fights D. silvestris have heads just like females and grapple during fights Evolutionary scenario Ancestor of both species had normal heads Ancestor of both species had normal heads Mutation occurred to cause head-butting behavior Mutation occurred to cause head-butting behavior Mutants won more contests Mutants won more contests Mutation increased to fixation in a sub- population Mutation increased to fixation in a sub- population Additional mutations caused change in head shape for better fighting Additional mutations caused change in head shape for better fighting D. heteroneura have hammerhead and butt heads during fights D. silvestris have heads just like females and grapple during fights Evolutionary scenario Ancestor of both species had normal heads Ancestor of both species had normal heads Mutation occurred to cause head-butting behavior Mutation occurred to cause head-butting behavior Mutants won more contests Mutants won more contests Mutation increased to fixation in a sub- population Mutation increased to fixation in a sub- population Additional mutations caused change in head shape for better fighting Additional mutations caused change in head shape for better fighting

Peripatric Speciation The process by which drift effects initial changes in allele frequencies The process by which drift effects initial changes in allele frequencies Then selection acts to further diverge colony from parent population Then selection acts to further diverge colony from parent population The process by which drift effects initial changes in allele frequencies The process by which drift effects initial changes in allele frequencies Then selection acts to further diverge colony from parent population Then selection acts to further diverge colony from parent population

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

Genetic change can be very rapid in localized populations founded by few individuals cut off from gene flow Genetic change can be very rapid in localized populations founded by few individuals cut off from gene flow Differences in ecological selection can play a role because the environment of a small area is often more homogenous than the larger area Differences in ecological selection can play a role because the environment of a small area is often more homogenous than the larger area So... conflicting pressures that act on a widespread species may be less numerous and more directed at certain adaptations So... conflicting pressures that act on a widespread species may be less numerous and more directed at certain adaptations Genetic change can be very rapid in localized populations founded by few individuals cut off from gene flow Genetic change can be very rapid in localized populations founded by few individuals cut off from gene flow Differences in ecological selection can play a role because the environment of a small area is often more homogenous than the larger area Differences in ecological selection can play a role because the environment of a small area is often more homogenous than the larger area So... conflicting pressures that act on a widespread species may be less numerous and more directed at certain adaptations So... conflicting pressures that act on a widespread species may be less numerous and more directed at certain adaptations Peripatric Speciation: Remember...

Allele frequencies at certain loci will be different from the parent population due to drift and founder effects Allele frequencies at certain loci will be different from the parent population due to drift and founder effects Changes in allele freq. could create genotypes that have higher fitness that Natural Selection may select for or against Changes in allele freq. could create genotypes that have higher fitness that Natural Selection may select for or against Many species have arisen by peripatric speciation Many species have arisen by peripatric speciation eg. Peromysus maniculatus eg. Peromysus maniculatus Allele frequencies at certain loci will be different from the parent population due to drift and founder effects Allele frequencies at certain loci will be different from the parent population due to drift and founder effects Changes in allele freq. could create genotypes that have higher fitness that Natural Selection may select for or against Changes in allele freq. could create genotypes that have higher fitness that Natural Selection may select for or against Many species have arisen by peripatric speciation Many species have arisen by peripatric speciation eg. Peromysus maniculatus eg. Peromysus maniculatus Peripatric Speciation: Remember...

Peripatric Example: Peromysus maniculatus

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

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

Parapatric Speciation Divergent selection GREATER than gene flow over time Divergent selection GREATER than gene flow over time Generally a hybrid zone may be formed Generally a hybrid zone may be formed The degree of selection and gene flow depend on the size of the character clines The degree of selection and gene flow depend on the size of the character clines Divergent selection GREATER than gene flow over time Divergent selection GREATER than gene flow over time Generally a hybrid zone may be formed Generally a hybrid zone may be formed The degree of selection and gene flow depend on the size of the character clines The degree of selection and gene flow depend on the size of the character clines

Secondary Contact If populations come back into contact after steps 1 and 2, have opportunity to interbreed If populations come back into contact after steps 1 and 2, have opportunity to interbreed Hybridization is common in plants and birds Hybridization is common in plants and birds – Will hybrids be viable and fertile? – Will hybrids have characteristics of parent species or new characteristics? – Depends on outcome of speciation event If populations come back into contact after steps 1 and 2, have opportunity to interbreed If populations come back into contact after steps 1 and 2, have opportunity to interbreed Hybridization is common in plants and birds Hybridization is common in plants and birds – Will hybrids be viable and fertile? – Will hybrids have characteristics of parent species or new characteristics? – Depends on outcome of speciation event

Secondary Contact Theodosius Dobzhansky formulated the reinforcement hypothesis about the third stage of speciation Theodosius Dobzhansky formulated the reinforcement hypothesis about the third stage of speciation – If populations sufficiently diverged in allopatry, hybrids should have reduced fitness – Should be strong selection for assortive mating – Selection that reduces fitness of hybrids is Reinforcement Completes reproductive isolation Completes reproductive isolation Theodosius Dobzhansky formulated the reinforcement hypothesis about the third stage of speciation Theodosius Dobzhansky formulated the reinforcement hypothesis about the third stage of speciation – If populations sufficiently diverged in allopatry, hybrids should have reduced fitness – Should be strong selection for assortive mating – Selection that reduces fitness of hybrids is Reinforcement Completes reproductive isolation Completes reproductive isolation

Secondary Contact Reinforcement Hypothesis Reinforcement Hypothesis – Predicts that pre-mating isolation will evolve in species in secondary contact – Prezygotic isolating mechanisms prevent fertilization Mate choice, time of breeding, genetic incompatibility Mate choice, time of breeding, genetic incompatibility – Postzygotic isolation Offspring are sterile Offspring are sterile Reinforcement not necessary Reinforcement not necessary Reinforcement Hypothesis Reinforcement Hypothesis – Predicts that pre-mating isolation will evolve in species in secondary contact – Prezygotic isolating mechanisms prevent fertilization Mate choice, time of breeding, genetic incompatibility Mate choice, time of breeding, genetic incompatibility – Postzygotic isolation Offspring are sterile Offspring are sterile Reinforcement not necessary Reinforcement not necessary

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

Secondary Contact Hybridization Hybridization – Hybrids should have reduced fitness by reinforcement hypothesis – Some have normal or increased fitness – Sorghum is an important crop – Johnsongrass is closely related weed – Agriculturalists worried that if they genetically engineered herbicide resistance into sorghum it might be transferred to johnsongrass Hybridization Hybridization – Hybrids should have reduced fitness by reinforcement hypothesis – Some have normal or increased fitness – Sorghum is an important crop – Johnsongrass is closely related weed – Agriculturalists worried that if they genetically engineered herbicide resistance into sorghum it might be transferred to johnsongrass

Secondary Contact Hybridization Hybridization – Significant gene flow occurred and herbicide resistance introgressed into johnson grass – If hybrids have increased fitness, will they form their own species? – Biologists attempted to recreate the hybridization event that lead to the formation of Helianthus anomalous Found that certain crosses had higher fitness and certain had low Found that certain crosses had higher fitness and certain had low – High fitness hybrids became new species Hybridization Hybridization – Significant gene flow occurred and herbicide resistance introgressed into johnson grass – If hybrids have increased fitness, will they form their own species? – Biologists attempted to recreate the hybridization event that lead to the formation of Helianthus anomalous Found that certain crosses had higher fitness and certain had low Found that certain crosses had higher fitness and certain had low – High fitness hybrids became new species

Secondary Contact Hybridization Hybridization – Hybrid zones occur where recently diverged populations overlap – May occur by secondary contact or during parapatric (or peripatric) speciation – Hybrid zones are often present where hybrids have equal fitness to parental species – Size of hybrid zone depends on fitness of hybrids Hybridization Hybridization – Hybrid zones occur where recently diverged populations overlap – May occur by secondary contact or during parapatric (or peripatric) speciation – Hybrid zones are often present where hybrids have equal fitness to parental species – Size of hybrid zone depends on fitness of hybrids

Secondary Contact Hybridization Hybridization – Study on sagebrush in western US Basin sagebrush found at low elevations Basin sagebrush found at low elevations Mountain sagebrush found at high elevations Mountain sagebrush found at high elevations Species make contact at middle elevations and hybridize Species make contact at middle elevations and hybridize – Graham compared several fitness measures of hybrids and parentals – Found that hybrids have superior fitness in transitional habitats Hybridization Hybridization – Study on sagebrush in western US Basin sagebrush found at low elevations Basin sagebrush found at low elevations Mountain sagebrush found at high elevations Mountain sagebrush found at high elevations Species make contact at middle elevations and hybridize Species make contact at middle elevations and hybridize – Graham compared several fitness measures of hybrids and parentals – Found that hybrids have superior fitness in transitional habitats

Sympatric Speciation Among the most controversial subjects in evolutionary biology Among the most controversial subjects in evolutionary biology Complete panmictic mating and a reproductive isolating mechanism evolves within the population Complete panmictic mating and a reproductive isolating mechanism evolves within the population Among the most controversial subjects in evolutionary biology Among the most controversial subjects in evolutionary biology Complete panmictic mating and a reproductive isolating mechanism evolves within the population Complete panmictic mating and a reproductive isolating mechanism evolves within the population

Sympatric Speciation – Possible Mechanisms 1. Polyploidy 2. Disruptive selection, whereby certain homozygous genotypes have high fitness on one or the other of two resources & intermediate (heterozygotes) has low fitness 1. Polyploidy 2. Disruptive selection, whereby certain homozygous genotypes have high fitness on one or the other of two resources & intermediate (heterozygotes) has low fitness

Sympatric Speciation & Polyploidism

Hybridization in Helianthus

Sympatric Speciation – Disruptive Selection Selection may favor alleles in homozygous condition and you get assortive mating Selection may favor alleles in homozygous condition and you get assortive mating But, antagonism likely to arise from alternative selection which promotes association between alleles for adaptation and alleles for assortive mating, and recombination which destroy this association But, antagonism likely to arise from alternative selection which promotes association between alleles for adaptation and alleles for assortive mating, and recombination which destroy this association This is the greatest opponent to the process of Sympatric Speciation This is the greatest opponent to the process of Sympatric Speciation In such a model strong selection and tight linkage are required... BUT recombination will break this down! In such a model strong selection and tight linkage are required... BUT recombination will break this down! Selection may favor alleles in homozygous condition and you get assortive mating Selection may favor alleles in homozygous condition and you get assortive mating But, antagonism likely to arise from alternative selection which promotes association between alleles for adaptation and alleles for assortive mating, and recombination which destroy this association But, antagonism likely to arise from alternative selection which promotes association between alleles for adaptation and alleles for assortive mating, and recombination which destroy this association This is the greatest opponent to the process of Sympatric Speciation This is the greatest opponent to the process of Sympatric Speciation In such a model strong selection and tight linkage are required... BUT recombination will break this down! In such a model strong selection and tight linkage are required... BUT recombination will break this down!

Sympatric Speciation – Disruptive Selection Another similar model was proposed by Bush based on his work on the apple maggot flies Another similar model was proposed by Bush based on his work on the apple maggot flies Rhagoletis – economically important pest which are parasites on the fruits of trees in the hawthorn clade Rhagoletis – economically important pest which are parasites on the fruits of trees in the hawthorn clade Rhagoletis ID host by sight, tough and smell Rhagoletis ID host by sight, tough and smell Courtship and mating occur on the fruit Courtship and mating occur on the fruit Females lay eggs in fruit on the tree Females lay eggs in fruit on the tree eggs hatch in 2 days and develop after fruits fall to ground eggs hatch in 2 days and develop after fruits fall to ground insects burrow into ground, overwinter, and emurge next spring insects burrow into ground, overwinter, and emurge next spring Another similar model was proposed by Bush based on his work on the apple maggot flies Another similar model was proposed by Bush based on his work on the apple maggot flies Rhagoletis – economically important pest which are parasites on the fruits of trees in the hawthorn clade Rhagoletis – economically important pest which are parasites on the fruits of trees in the hawthorn clade Rhagoletis ID host by sight, tough and smell Rhagoletis ID host by sight, tough and smell Courtship and mating occur on the fruit Courtship and mating occur on the fruit Females lay eggs in fruit on the tree Females lay eggs in fruit on the tree eggs hatch in 2 days and develop after fruits fall to ground eggs hatch in 2 days and develop after fruits fall to ground insects burrow into ground, overwinter, and emurge next spring insects burrow into ground, overwinter, and emurge next spring

Mechanisms of Divergence – Apple and hawthorn maggot flies Are they different populations? Are they different populations? Live in sympatry on adjacent trees Live in sympatry on adjacent trees Recently diverged because apples are not native to US Recently diverged because apples are not native to US A mark-recapture allozyme study revealed they do form distinct populations A mark-recapture allozyme study revealed they do form distinct populations How have they diverged without initial isolation? How have they diverged without initial isolation? – Did they skip Step 1? – Apple and hawthorn maggot flies Are they different populations? Are they different populations? Live in sympatry on adjacent trees Live in sympatry on adjacent trees Recently diverged because apples are not native to US Recently diverged because apples are not native to US A mark-recapture allozyme study revealed they do form distinct populations A mark-recapture allozyme study revealed they do form distinct populations How have they diverged without initial isolation? How have they diverged without initial isolation? – Did they skip Step 1?

Mechanisms of Divergence – Apple and hawthorn maggot flies There are other mechanisms for speciation besides allopatric There are other mechanisms for speciation besides allopatric Through assortive mating there is only 6% gene flow among populations Through assortive mating there is only 6% gene flow among populations Separated in time by pupating at different times of year Separated in time by pupating at different times of year They are able to maintain distinct populations even with gene flow because of strong natural selection They are able to maintain distinct populations even with gene flow because of strong natural selection – Apple and hawthorn maggot flies There are other mechanisms for speciation besides allopatric There are other mechanisms for speciation besides allopatric Through assortive mating there is only 6% gene flow among populations Through assortive mating there is only 6% gene flow among populations Separated in time by pupating at different times of year Separated in time by pupating at different times of year They are able to maintain distinct populations even with gene flow because of strong natural selection They are able to maintain distinct populations even with gene flow because of strong natural selection

Genetics of Differentiation and Isolation What changes in the genome are necessary for speciation? What changes in the genome are necessary for speciation? Most F 1 hybrids are sterile Most F 1 hybrids are sterile – Postzygotic isolation is pronounced Which genes are responsible for sterility? Which genes are responsible for sterility? If one sex is sterile, it is usually the heterogametic sex If one sex is sterile, it is usually the heterogametic sex – Human males have Y – Bird females have W What changes in the genome are necessary for speciation? What changes in the genome are necessary for speciation? Most F 1 hybrids are sterile Most F 1 hybrids are sterile – Postzygotic isolation is pronounced Which genes are responsible for sterility? Which genes are responsible for sterility? If one sex is sterile, it is usually the heterogametic sex If one sex is sterile, it is usually the heterogametic sex – Human males have Y – Bird females have W

Genetics of Differentiation and Isolation Haldane’s Rule Haldane’s Rule – Pattern of sterility is in heterogametic sex, regardless of which is male or female Haldane’s Rule Haldane’s Rule – Pattern of sterility is in heterogametic sex, regardless of which is male or female

Genetics of Differentiation and Isolation Why does Haldane’s Rule work? Why does Haldane’s Rule work? – Consider an autosomal locus A and an X-linked locus B – Individuals from one species are fixed for A 1 and B 1 – Sister species fixed for A 2 and B 2 – A 2 and B 1 interact to cause inviability – If females from first species mate with second: A 1 A 2 B 1 males and A 1 A 2 B 1 B 2 females – Males 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 locus B – Individuals from one species are fixed for A 1 and B 1 – Sister species fixed for A 2 and B 2 – A 2 and B 1 interact to cause inviability – If females from first species mate with second: A 1 A 2 B 1 males and A 1 A 2 B 1 B 2 females – Males are inviable, females are viable

Genetics of Differentiation and Isolation Quantitative Trait Loci (QTL) Mapping Quantitative Trait Loci (QTL) Mapping – Most traits involved in reproductive isolation are quantitative – QTL mapping attempts to locate genes with small effects on quantitative traits – Find genetic markers that are unique to each parental species and the value of that trait in hybrids Quantitative Trait Loci (QTL) Mapping Quantitative Trait Loci (QTL) Mapping – Most traits involved in reproductive isolation are quantitative – QTL mapping attempts to locate genes with small effects on quantitative traits – Find genetic markers that are unique to each parental species and the value of that trait in hybrids

Genetics of Differentiation and Isolation Quantitative Trait Loci (QTL) Mapping Quantitative Trait Loci (QTL) Mapping – If statistically significant associations are found between traits and markers that have been mapped, implies that at QTL near the marker contributes to that trait – Try to identify “speciation genes” in a diverse array of organisms to understand the genetics behind speciation Quantitative Trait Loci (QTL) Mapping Quantitative Trait Loci (QTL) Mapping – If statistically significant associations are found between traits and markers that have been mapped, implies that at QTL near the marker contributes to that trait – Try to identify “speciation genes” in a diverse array of organisms to understand the genetics behind speciation