MODELS OF SPECIATION Sympatric Speciation

MODELS OF SPECIATIONSympatric Speciation: Speciation without restriction to gene flow. Development of reproductive isolation without geographicbarriers. Requires assortative mating and a stable polymorphism.MODEL OF SYMPATRIC SPECIATIONANCESTRALPOPULATIONReproductively isolated,geographically sympatricpopulationsTHE APPLE MAGGOT: A CASE OF SYMPATRIC SPECIATION?The Apple MaggotDistribution of “races” of R. pomnella1

SYMPATRIC SPECIATION IN RHAGOLETISEcological speciation:adaptation to different hostplants drives reproductiveisolation – temporalisolation driven by hostplant phenologyBrought to you by the Feder Lab at ND!!!SYMPATRIC SPECIATION IN CICHLID FISHES FROMNIGARAGUAN LAKESFrom: Kirkpatrick. 2000. Nature 408:298-299, Wilson et al., 2000. Proc. R. Soc. Lond. B 267:2133-2141HOW CAN REPRODUCTIVE ISOLATIONDEVELOP IN THE ABSENSE OF BARRIERS TOGENE FLOW? Sympatric speciation in animals is acontroversial mechanism. Host-race speciation requires either temporalor ecological isolation. Both of these mechanisms provide barriers togene flow. (For example the apple andHawthorne races of Rhagoletis have aneffective migration rate of approx. 6%)2

HOW MANY GENES ARE REQUIREDFOR SPECIATION TO OCCUR?FITNESSTHE PROBLEM WITH A ONE-LOCUS MODEL OF SPECIATIONAAAaaa In order for a single locus model of speciation to work,populations would have to evolve towards lowerfitness.THE BATESON-MULLER-DOBZHANSKY MODEL FORTHE GENETIC BASIS OF POST-MATINGREPRODUCTIVE BARRIERSAABBAAbbaaBBAaBb This two-locus model relies on epistatic interactions amonggenes.3

“INSTANTANEOUS SPECIATION” Chromosomal duplications: As manyas 47% of flowering plant species arepolyploid. Multiplication of the number ofchromosomes can occur in at least twoways: Autopolyploidy – duplication of thechromosomes of a single species(nondisjunction during meiosis). Allopolyploidy – Duplication of acombination of chromosomes fromdifferent species.INFECTIOUS SPECIATION The bacterium Wolbachiacan cause cytoplasmicincompatibility. When infected male hosts(W ) mate with uninfectedfemales (W-) no viableoffspring are produced. Different strains ofWolbachia areincompatible.CONTRASTING THE“CLASSICAL” MODEL OFGENETIC SPECIATION WITHTHE ‘INFECTIOUS’ MODELThe Classical ModelThe Infectious Model4

EVIDENCE OF THE CLASSIC VIEW OF SPECIATION:ACCUMULATION OF SMALL DIFFERENCES OVER TIMEREINFORCEMENT Sexual selection on males and natural selection onfemales may favor indiscriminant mating betweenpopulations. Without complete pre- and post-zygotic isolatingmechanisms, repeated hybridization can fuse genepools. Reinforcement (i. e., selection for positive assortativemating; disruptive selection) can occur if the fitnesscost from lost mating opportunities is balanced by afitness gain in offspring viability.REINFORCEMENT Reinforcement leads to pre-mating isolation. Hybridization reduces the strength of reinforcement byhomogenizing gene pools. Reinforcement must occur sufficiently fast to counterhybridization and complete the speciation process.5

PREZYGOTIC REPRODUCTIVE ISOLATION EVOLVESMORE RAPIDLY IN SPECIES THAT ARE SPATIALLYOVERLAPPING:EVIDENCE FOR REINFORCEMENTFROM: Coyne & Orr 1997Adaptive radiation is the evolution of ecological andphenotypic diversity within a rapidly multiplying lineage.It involves the differentiation of a single ancestor into anarray of species that inhabit a variety of environmentsand that differ in traits used to exploit thoseenvironments. It is regarded as the hallmark of adaptive evolutionand may well be the most common syndrome in theorigin and proliferation of taxa.Dolph Schluter, 2000The Ecology of Adaptive Radiation some time after a rather distinctive new adaptivetype has developed it often becomes highlydiversified. the same sort of diversification follows,and in this case begins almost immediately, when agroup spreads to a new and, for it, ecologically openterritory.- Simpson, 19536

WHAT PRECIPITATES AN ADAPTIVE RADIATION?ECOLOGICAL OPPORTUNITY: the invasion of unutilizedecological niches leads to rapid diversification, e.g.,colonizing a remote archipelago, surviving a massextinction.KEY INNOVATION: the acquisition of a novel adaptivetrait (behavioral, morphological, or physiological) allowsorganisms to exploit previously unavailable ecologicalniches.EVIDENCE FOR THE ECOLOGICAL OPPORTUNITY HYPOTHESISFig. 7-7 IN: Schluter 2000. The Ecology of Adaptive Radiation. Oxford Univ. Press. OxfordSeeing this graduation and diversity of structurein one small, intimately related group of birds,one might really fancy that, from an originalpaucity of birds in this archipelago, one specieshas been taken and modified for different ends.Darwin, 18427

Character Displacement: Members of one lineage constrainphenotypic evolution in members ofother lineagesThere are two sides to this coin:1) It may promote divergencebetween closely related specieswhen there are unexploitedecological niches available.2) It may constrain divergence whenthere are no unexploited nichesHAWAIIANHONEYCREEPERS 800 SPECIES OF HAWAIIAN DROSOPHILID FLIES8

HAWAIIAN SILVERSWORDALLIANCE 28 SPECIES DERIVED FROM ASINGLE COMMON ANCESTOR.THESE PLANTS OCCUPY DIVERSE FORMS INCLUDE, VINES,HABITATS RANGING FROMTREES, ERECT SHRUBS, ROSETTES,EXPOSED LAVA TO WETAND HERBACEOUS MATSFOREST.SILVERSWORDSOne signature of an adaptive radiationis a burst of diversification over arelatively short period of timeCICHLID FISHES IN AFRICANRIFT VALLEY LAKES: LAKE TANGANYIKA: 140 SPP. LAKE MALAWI: 500 SPP. LAKE VICTORIA: 250 SPP.9

Lake Malawi CichlidsPhotos by Fredrik Hagblom10

Sexual SelectionTrophicEcologicalADAPTIVE RADIATION AND CONVERGENTEVOLUTIONCONVERGENT EVOLUTION IN THE CICHLID RADIATION11

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LimneticBenthicLimneticBenthicLimneticBenthic13

AUSTRALIAN MARSUPIALSDEMONSTRATE AN ADAPTIVERADIATION IN THE ABSENSE OFCOMPETITIVE INTERACTIONSWITH PLACENTAL MAMMALSTHE AMNIOTIC EGG: Perhaps one of thegreatest keyinnovations of alltimeANOTHER KEY INNOVATION:WINGS14

ADAPTIVE RADIATION OF MAMMALSDiversification of themajor Mammalianlineages occurred ina relatively shortperiod of time.15

ADAPTIVE RADIATION OF ANIMALSRapid diversification of animals corresponds to majorenvironmental changes Warming and retreat of glaciersOxygenation of oceanIncreased availability of phosphorousNiche expansionCambrian CommunityEvidence of Predation16

ADAPTIVE RADIATIONS AND RAPID EVOLUTIONAdaptive radiations are often characterized by: Ecological opportunity Acquisition of novel adaptive traits Competitive interactions among closely related taxa Convergent/Parallel evolution Rapid phenotypic diversification17

SYMPATRIC SPECIATION IN CICHLID FISHES FROM NIGARAGUAN LAKES HOW CAN REPRODUCTIVE ISOLATION DEVELOP IN THE ABSENSE OF BARRIERS TO GENE FLOW? Sympatric speciation in animals is a controversial mechanism. Host-race spe