Species and Speciation, Pt. 2Secondary contact, hybrid zones and reinforcement - 1Secondary contact, hybrid zones and reinforcement - 2“Pictures” of hybrid zonesA hybrid zone in which hybrids and “parentals” are equally fit globally – the width of the hybrid zone and the steepness of the cline in allele frequency will depend on the amount of time since 2º contact and on the dispersal distance of individuals. This is unstable – eventually the two populations become indistinguishableA hybrid zone in which hybrids are most fit in the hybrid zone, but each “parental” type is most fit in its own geographic region – the width of the hybrid zone depends on the geographic region where hybrids are superior and on individual dispersal distances. This is stable – produces a “step cline”A hybrid zone in which hybrids are unfit and each “parental” type is most fit in its own geographic region – the width of the hybrid zone depends on individual dispersal distances. This is stable – produces a “step cline”, provides conditions for “reinforcement” Concordant step clines are produced for other loci that are differentiated between populations and linked to fitness loci - other loci may introgress, provided hybrids are not too unfitReinforcementEvidence for reinforcement - 1 (Coyne and Orr 1997)Prezygotic isolation in allopatric versus sympatric species pairs of Drosophila (Coyne and Orr 1997) (Fig. 15.13)Character displacement is evidence for reinforcementSympatry and allopatryCharacter displacement in pheromones of Drosophila serrata (Higgie et al. 2000)How important is reinforcement? - 1How important is reinforcement? - 2A “short course” on clinesA brief review of 1-locus selection models: relative fitnesses of genotypes and outcome of selectionFormation of a step cline in allele frequency along an environmental gradientSlide 19Parapatric Speciation – 1A 2º contact zone in which hybrids are unfit and each “parental” type is most fit in its own geographic region – the width of the hybrid zone depends on individual dispersal distances. This is stable – produces a “step cline”, provides conditions for “reinforcement” Concordant step clines are produced for other loci that are differentiated between populations and linked to fitness loci - other loci may introgress, provided hybrids are not too unfitFormation of a step cline in allele frequency along an environmental gradient with gene flowParapatric Speciation – 2Does parapatric speciation really happen?Peripatric speciation - 1Peripatric speciation - 2Should evolution be faster in small populations ?“Ring” species: a special case of allopatric speciationThe greenish warbler (Phylloscopus trochiloides)Larus gulls1Species and Speciation, Pt. 2Chapter 152Secondary contact, hybrid zones and reinforcement - 1What happens if allopatric populations come back into contact (= 2º contact)?1. Two populations no longer recognize each other as conspecifics and do not mate with each other – prezygotic isolation – “good” biological species, “pure” allopatric speciation3Secondary contact, hybrid zones and reinforcement - 22. Two populations hybridize (this is particularly common in plants but also frequent in animals)a) Hybrids are inviable or infertile – postzygotic isolation – “good” biological speciesb) Hybrids have reduced fitness – “semispecies”c) Hybrids are fit in the contact zoned) Hybrids and “parentals” are equally fit everywhere, or one type is most fit everywhere – homogenization4“Pictures” of hybrid zonesPopulation 1 Population 2A1A1A2A2Hybrid zoneGeographic distance5A hybrid zone in which hybrids and “parentals” are equally fit globally – the width of the hybrid zone and the steepness of the cline in allele frequency will depend on the amount of time since 2º contact and on the dispersal distance of individuals.This is unstable – eventually the two populations become indistinguishableDistanceFrequency of A2Pure population 2Pure population 16A hybrid zone in which hybrids are most fit in the hybrid zone, but each “parental” type is most fit in its own geographic region – the width of the hybrid zone depends on the geographic region where hybrids are superior and on individual dispersal distances.This is stable – produces a “step cline”DistanceFrequency of A2Pure population 2Pure population 1Hybrid zone7A hybrid zone in which hybrids are unfit and each “parental” type is most fit in its own geographic region – the width of the hybrid zone depends on individual dispersal distances.This is stable – produces a “step cline”, provides conditions for “reinforcement”Concordant step clines are produced for other loci that are differentiated between populations and linked to fitness loci - other loci may introgress, provided hybrids are not too unfitDistanceFrequency of A2Pure population 2Pure population 1Hybrid zone8Reinforcement•When hybrids have reduced fitness, we may expect natural selection to favor the evolution of reproductive isolation – because hybridization reduces fitness relative to mating with one’s own kind.•This process of selection for reproductive isolation to complete the process of speciation is known as reinforcement (Dobzhansky 1937)9Evidence for reinforcement - 1(Coyne and Orr 1997)•Pairs of sister species in Drosophila•Classify each pair as allopatric or sympatric•Measure genetic distance (assumed to be correlated with time since common ancestor)•Measure degree of prezygotic isolation for each pair•Prediction:–Sympatric species pairs will be more likely than allopatric species pairs to be prezygotically isolated when genetic distance is relatively small – because reinforcement can only happen in sympatry (when “species” hybridize)10Prezygotic isolation in allopatric versus sympatric species pairs of Drosophila (Coyne and Orr 1997) (Fig. 15.13)•Prezygotic isolation estimated from mate choice tests. Value of 0 means different populations freely interbreed; value of 1 mean no interbreeding (=100% prezygotic isolation)11Character displacement is evidence for reinforcement•In general, we expect that hybridization will be less likely the more dissimilar two populations are•Therefore, when two species occur both allopatrically and sympatrically, we expect them to be more different in sympatry than in allopatry if reinforcement is occurring12Sympatry and