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Species and Speciation, Pt. 2

Chapter 15

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Secondary contact, hybrid zones and reinforcement - 1

What 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 speciation

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Secondary contact, hybrid zones and reinforcement - 2

2. Two populations hybridize (this is particularly common in plants but also frequent in animals)

a) Hybrids are inviable or infertile – postzygotic isolation – “good” biological species

b) Hybrids have reduced fitness – “semispecies”

c) Hybrids are fit in the contact zone

d) Hybrids and “parentals” are equally fit everywhere, or one type is most fit everywhere – homogenization

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“Pictures” of hybrid zones

Population 1 Population 2

A1A1 A2A2

Hybrid zone

Geographic distance

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A 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 indistinguishable

Distance

Fre

quen

cy o

f A

2

Pure population 2

Pure population 1

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A 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”

Distance

Fre

quen

cy o

f A

2

Pure population 2

Pure population 1

Hybrid zone

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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 unfit

Distance

Fre

quen

cy o

f A

2

Pure population 2

Pure population 1

Hybrid zone

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Reinforcement

• 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)

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Evidence 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)

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Prezygotic 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)

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Character 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 occurring

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Sympatry and allopatry

Species 1 Species 2

Sympatric zone

Allopatric zones

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Character displacement in pheromones of Drosophila serrata (Higgie et al. 2000)

• In Australia, D. serrata and D. birchii occur both sympatrically and allopatrically

• The pheromones (used for species recognition ?) produced by D. serrata are different between zones of allopatry and sympatry

• In laboratory populations started with a mixture of flies from allopatric populations of the two species, D. serrata evolved pheromone profiles similar to wild D. serrata from sympatric populations, in nine generations

• This experiment is remarkable in that it shows evolution of character displacement in the laboratory

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How important is reinforcement? - 1

• Although the examples just cited provide support that reinforcement does occur at least sometimes, genetic models suggest that reinforcement might not very common

• Natural selection cannot strengthen postzygotic isolation by “direct” selection – that would require an increase in the frequency of alleles that reduce fertility or survival of hybrids (i.e., natural selection for low fitness alleles)

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How important is reinforcement? - 2• Reasons why reinforcement of prezygotic isolation may be unlikely

– No reason for isolating alleles to spread – selection for isolating alleles occurs only in hybrid zone - hard to see why such alleles should spread from contact zone to come to characterize whole species, which is typically the case

– Gene flow into hybrid zone opposes selection – alleles for prezygotic isolation that are being selected for in the hybrid zone will be “swamped” by movement of other alleles into the hybrid zone

– Problem of relatively fit backcrosses ( = weak selection) – if backcross individuals are relatively fit and most matings in the hybrid zone occur between backcross individuals or backcross individuals and “parentals”, the hybrid zone may be broad and selection for reinforcement may be weak

– Hard to complete the process – any degree of prezygotic isolation reduces the effectiveness of further selection because it reduces the frequency of hybrids

– Recombination in hybrids breaks down linkage disequilibrium – if fitness locus and isolating locus are different, then evolution of reproductive isolation requires linkage disequilibrium between the loci

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A “short course” on clines

• A cline is a geographic gradient

Latitude ºN

Fre

quen

cy o

f A

2

0 6030

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A brief review of 1-locus selection models:relative fitnesses of genotypes and outcome of selection

• A1A1 = A1A2 > A2A2: A1 fixed

• A2A2 > A1A1, A1A2: A2 fixed

• A1A2 > A1A1, A2A2: stable

polymorphism

• A1A2 < A1A1, A2A2: unstable, A1 or A2 fixed

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Formation of a step cline in allele frequency along an environmental gradient

Environmental Gradient

Fitn

ess

A2A2

A1A1, A1A2

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Formation of a step cline in allele frequency along an environmental gradient

Environmental Gradient

Fitn

ess

Freq

uenc

y A

2

A2A2

0

1

A1A1, A1A2

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Parapatric Speciation – 1

• The step cline in the previous slide looks like the step cline that can be formed in a 2º contact zone (see next slide)

• But in this case, we are talking about a cline that forms along a gradual environmental gradient (without 2º contact of allopatric populations)

• We might call this a 1º “contact zone”• In practice, because individuals disperse, the

change in allele frequencies will not be instantaneous – so the cline might look like this:

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A 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 unfit

Distance

Fre

quen

cy o

f A

2

Pure population 2

Pure population 1

Hybrid zone

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Formation of a step cline in allele frequency along an environmental gradient with gene flow

Environmental Gradient

Fitn

ess

Freq

uenc

y A

2

A2A2

0

1

A1A1, A1A2

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Parapatric Speciation – 2

• The formation of a step cline in a population distributed continuously across an environmental gradient is Phase 1 in parapatric speciation – populations on either side of the cline diverge while in contact

• Phase 2 may occur if heterozygotes formed in the region of 1º contact are unfit and the cline is stable (which should be the case if alternative homozygotes are most fit on either side of the cline)– Populations on either side of the cline continue to diverge because gene

flow through the cline is opposed by low fitness of “hybrids” and divergent selection on either side of the cline – this eventually leads to incidental reproductive isolation and speciation

– Or, reinforcement occurs in the “hybrid” zone, leading to reproductive isolation and speciation

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Does parapatric speciation really happen?

• Hard to know – there are plenty of hybrid zones and many appear to be “tension zones” in which hybrids have reduced fitness and two forms on either side of hybrid zone are adapted to different environments (Barton and Hewitt 1989)

• But we don’t know whether these are 1º rather than 2º contact zones

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Peripatric speciation - 1

• A special type of allopatric speciation in which small allopatric populations are created at the periphery of the main range of a much larger “parent” population

Parent population

Peripatric populationdispersal

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Peripatric speciation - 2

• First proposed by Ernst Mayr and later adopted by Stephen Jay Gould

• Both liked it because they believed that speciation might be rapid in small populations (founder hypothesis)

• Gould also liked it because if most speciation occurred quickly in small, geographically restricted populations then the fossil record might reveal abrupt (geologically speaking) change without transitional forms (“punctuated equilibrium”)

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Should evolution be faster in small populations ?

• Rate of change of allele frequency by selection alone is independent of population size

• Large populations produce more mutations per unit time, so if adaptation and speciation depend on occurrence of new favorable mutations, large populations are better (Darwin) – also favorable mutations are less likely to be effectively neutral in larger populations

• Rate of replacement of neutral alleles by drift is independent of population size

• However, change in allele frequency from generation to generation by drift is greater in small populations – and small founder populations may be genetically different from “parent” population (founder effect speciation)

• But founder effect is likely to be minimal unless founder population is very small and population stays small for a number of generations

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“Ring” species: a special case of allopatric speciation

• A series of populations (subspecies) distributed around a geographic “barrier”, such that hybridization occurs between adjacent populations – except where the ring “closes” and populations are reproductively isolated

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The greenish warbler (Phylloscopus trochiloides)

• The origin of greenish warblers is believed to be on the southern edge of the Tibetan Plateau. From there, populations spread east and north and west and north. Adjacent populations interbreed around the ring, except where the two subspecies meet in Siberia, where they are reproductively isolated. You can find out much more about this system and hear the songs of the various subspecies at http://www.zoology.ubc.ca/~irwin/GreenishWarblers.html

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Larus gulls

• In northern Europe, where the herring gull and lesser black-backed gull co-occur, they do not interbreed. Nevertheless, these “species” are connected by a ring of interbreeding populations

• Figure obtained at http://en.wikipedia.org/wiki/Ring_species