Species and Speciation, Pt. 2

1
Species and Speciation, Pt. 2

• Chapter 15

2
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

3
Secondary contact, hybrid zones and reinforcement
- 2

• Two populations hybridize (this is particularly
  common in plants but also frequent in animals)
• Hybrids are inviable or infertile postzygotic
  isolation good biological species
• Hybrids have reduced fitness semispecies
• Hybrids are fit in the contact zone
• Hybrids and parentals are equally fit
  everywhere, or one type is most fit everywhere
  homogenization

4
Pictures of hybrid zones
Hybrid zone
Population 1
Population 2
A1A1
A2A2
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
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
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
reinforcementConcordant 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
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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 ones 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)

10
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
Sympatric zone
Species 1
Species 2
Allopatric zones
13
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

Frequency of A2
0
60
30
Latitude ºN
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A brief review of 1-locus selection
modelsrelative fitnesses of genotypes and
outcome of selection

• A1A1 A1A2 gt A2A2 A1 fixed
• A2A2 gt A1A1, A1A2 A2 fixed
• A1A2 gt A1A1, A2A2 stable polymorphism
• A1A2 lt A1A1, A2A2 unstable, A1 or A2 fixed

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Formation of a step cline in allele frequency
along an environmental gradient
A2A2
Fitness
A1A1, A1A2
Environmental Gradient
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Formation of a step cline in allele frequency
along an environmental gradient
A2A2
Fitness
A1A1, A1A2
1
Frequency A2
0
Environmental Gradient
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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 reinforcementConcordant 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
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Formation of a step cline in allele frequency
along an environmental gradient with gene flow
A2A2
Fitness
A1A1, A1A2
1
Frequency A2
0
Environmental Gradient
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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 dont 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

Peripatric population
dispersal
Parent population
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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/Gre
  enishWarblers.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/Ri
  ng_species