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Week5Speciation

- 3speciesconceptso 1)Morphospeicesconcept

Species=morphologicallysimilarorganisms Advantageswidelyapplicable Disadvantagesarbitraryandinconsistentdefinitions e.g.asexemplifiedbydifferenthappy-facespidersinHawaii

o 2)Biologicalspeciesconcept Species=groupsofinbreedingorganisms(reproductive

isolationseparatesspecies)

Advantageclearevolutionarymeaning Disadvantagescannotbeappliedtoextinctspecies,

asexualspecies,plantswithhybridization

o 3)Phylogeneticspeciesconcept Species=smallestmonophyleticgroupoforganisms(evolutionaryindependence) Advantageclearevolutionarymeaningandapplicableto

anytypeoforganisms

Disadvantagephylogeneticinfoisneededtomakestatementsaboutspeciation

- Speciescanemergenceviao 1)anagenesischangeofentiregroupwithoutbranchingo 2)cladogenesischangeofspeciesviabranching

- Speciationisaprocessdegreeofreproductiveisolationvariesalongacontinuum

- Speciationrequiresthedisruptionofgeneflowandtheevolutionofreproductiveisolatingmechanisms(RIMs)- ClassicViewofSpeciation

o 1)Barriertogeneflowdevelopso 2)Gradualaccumulationofgeneticdifferencesthroughmutation,

drift,andnaturalselection

o 3)GeneticdivergenceleadstoRIMsasabyproducto possibly4)secondarycontactleadingtohybridizationor

reinforcement

- 4modesofspeciation

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- 1)Allopatryo Speciationduetogeographicisolation(notjustphysicaldistance

spatialrestrictionofgeneflowcounts)

o EvolutionofRIMsbetweenthesepopulationsthatareisolatedo E.g.SnappingshrimpvicariancearoundbetweenduetoemergenceofIsthmusofPanamathatseparatedshrimp

- 2)Peripatryo Speciationinaperpherlaisolatedudetodisperalorcolonization

events

o a.k.a.FounderEffecto Driftandnaturalselectionmaycauserapiddivergencefromthe

parentalpopulation

o E.g.DrosophilaspeciationinHawaiiolderislandshavemoreancientlineages

- Me

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chanismsofdivergenceinallopatry

o 1)Geneticdriftrandomsamplingafterseveralgenerationstakeoverofonetypeofallele

o 2)Naturalselection&sexualselection

o Driftandselectiontogetherpeakshift

peakshiftcanleadtocoexistenceoftwodifferentspeciesthroughsecondarysympatry

o 3)Reinforcementthroughless-fithybrids

- Reproductiveisolationcanbeeithero Prezygoticstemmingfromdifferencesintimingofbreedingormatechoiceo Postzygoticduetodysfunctionaldevelopmentorsterilityin

hybrids

- 3)Parapatryo ecologicalspeciationthroughreproductiveisolationfrom

divergentnaturalselectionbetweenniches

o RIMsbyproductofecologicaldivergenceandselectionagainsthybridfavorspositiveassortativemating,facilitatingspeciation

o Possibilywidespreadinmarineorganisms(e.g.sculpinsalongPacificcoastduetodifferentsubtidal/intertidalhabitats)

- 4)Sympatry

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o Polyploidye.g.inplantswithmutationgivingdiploidgametesfromdiploidparents

o Host-switching(e.g.applemaggotflyswitchingfromhawthorntoappletrees)andhabitatswitching(e.g.differentpalmsin

calcareousvs.volcanicsoilsleadingtodifferentfloweringseasons

ORe.g.cichildsinAfricanRiftlakes)o Hybridization(e.g.hybridsunflowerspecieshomoploidhybrid)

Maintainingspeciesboundariesthroughreinforcementhybridshavelowfitnesssothereisselectionforpositive

assortativematingleadingtoprezygoticisolation

- Hybridzoneso Hybridizationcrossesbetweengeneticallydifferentiatedformso Introgressionmovementofgenesbetweenspecies(orwell-

differentiatedpopulations)mediatedbybackcrossing(crossingof

hybridwithparenttoyieldoffspringclosertoparentalgenotype)

o Oftenhybridzoneswhencloselyrelatedspeciesexperiencesecondarycontacte.g.atboundarieswithcrows

- Possibleoutcomesofsecondarycontacto 1)Nohybridization(nohybridzone)o 2)Hybridizationbutnointrogression(narrowhybridzone)o 3)Limitedintrogression(widerhybridzone)o 4)Reunificationofgenepool

- Ringspeciesgeographicvariationandreproductiveisolationsinsalamanderswhenspeciesgoallaroundcoastandmakesecondary

contact

-

Prematingisolationpassiveresponsetoeecologicaldivergence

o E.g.beaksizesinfinchesmeandifferentcalls- Summary

o Reproductiveisolationismostwidelyuseddefinitionbutcantbeappliedtoplants,microbes,fossilspecies

o Driftandselectioncanexplainspeciationincombowithphysicalisolatedbydispersalorvicariance

o Identifyingmodeofspeciationforaparticularspeciescanbehard

Readings

Ecology and the origin of species

- Schluter

- Ecological speciation happens when divergent selection leads directly orindirect to evolution of reproductive isolation

- Ecological speciation occurs in allopatry or in sympatry- Reproductive isolation - by-product mechanism leading to premating

and/or postmating isolation

- Classic scenario - where reproductive isolation build sin allopatry asbyproduct of a different environment. Then reinforcement of this

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isolation is driven by reduced hybrid fitness which completes thespeciation process during the sympatric phase

Chromosomal rearrangements and speciation- Rieseberg

- common belief is that speciation occurs when a population becomesfixed for one or more chromosomal rearrangements that reduce fitnesswhen they are heterozygous

- Rieseberg argues that this these mutations can only become fixedthrough drift in small inbred populations

- instead, she argues that rearrangements reduce gene flow bysuppressing recombination and extending the effects of linked isolationgenes

- chromosomal rearrangements on fitness of heterozygous hybrids tend tocause sterility in plants more than animals

- chromosomal rearrangement suppression might work synergisticallywith isolation genes to diminish gene flow over larger chromosomalregions and contribute to sympatric or parapatric speciation.

Adaptive Evolution and Explosive Speciation: The Cichlid Fish Model- Kocher

- Cichlid fish are a good model to understand the genetic basis ofvertebrate speciation

- They show a capacity for rapid radiation and sympatric speciation- ** debates about whether cichlids from different populations with pre-

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zygotic behavioral isolating mechanisms that can be mated in a labsetting are two separate species

- rapid radiation of cichlids makes it hard to reconstruct a tree based onone gene - therefore averaging many individual gene trees allows

inference of true relationships among species

- radiation in 3 steps- 1) habitat divergence based on different rock vs. sand habitats- 2) different feeding apparatus development by natural selection- 3) color differences indicating the action of sexual selection- 3 major selective forces: ecological selection, sexual selection and

genetic conflicts- ecological selection - different jaw morphologies for different modes of

feeding, habitat preferences, behavioral niche partitioning- sexual selection - male traits evolving in response to female mate

preferences leading to a runaway process resulting in rapid divergence

in male display traits among discrete populations- genetic conflicts - gene competition within genome, or intragenomic

conflict over sex ratio to influence sex of offspring- genomic technologies can help identify speciation genes- cichlid species are usually separated by reduced hybrid fitness or pre-

mating isolating mechanisms- many studies focus on post-mating barriers or acute hybrid inviability- further studies could lead to a more general understanding of

speciation and this can be achieved with the use of genomics inevolutionary biology

TheRiseofFishesbyMichaelBenton

- Chordataincludesallanimalswithanotochord- ThesubphylumVertebrata,whichcontains>99%ofallchordatespecies,

hasvertebraeinadditiontonotochord

- Firstjawlessfishesperhapsdiedoutduetofeedingmechanisms,whichlimitedevolutionaryprospectssuckingandraspingatflesh

- WealthoffossilsintheOldRedSandstoneinScotland- Dearthoftransitionalfossilsshowingthestagesofjawdevelopment- Theoryisthatjawsevolvedbymodifyingslenderbonesinthethroat

regionthatsupportedthegillarchesofjawlessfish

- MostcommonfishestodayareintheclassOsteichthyesbonyfishes

FourFeetontheGroundbyMichaelBenton

- Whymoveontoland?- Perhapsfornewresourceslikefoodandoxygen- Newstructuralsupportmechanismstomaintainshape,keeporgansin

place,orenablemovementneededtodevelopmentforlobe-finstoevolveintoamphibians

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- Newmusclestocopewithlandstressthatisdirecteddown- Locomotionstemmingfrompropulsioninadifferentwayalso

development

- Divorcefromwaterreptiles- Amphibiansdevelopedtheamnioticeggwhichprovidedprotectionandiswhatfreedthemfromtheirdependenceonwaterforreproduction- Amnioticeggsemi-permeableshellforasafeenvironmentandalso

differentfluidfilledsacsthatprotecttheembryo(Amnion),enclosethe

foodsupply(yolk),gatherthewaste(allantois),andsurroundeverything

(chorion)

- Reptilesdidnotneedtolayeggsinwaterbutthisrequiredinternalfertilization

- Itappearscleidoiceggonlyemergedonce