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
2
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
3
“Pictures” of hybrid zones
Hybrid zone
Population 1
Population 2
A1A1
A2A2
Geographic distance
4
Frequency of A2
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 1
Pure
population 2
Distance
5
Frequency of A2
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 1
Pure
population 2
Hybrid zone
Distance
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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”
Frequency of A2
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
Pure
population 1
Pure
population 2
Hybrid zone
Distance
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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
Sympatric zone
Species 1
Species 2
Allopatric zones
12
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
Frequency of A2
• A cline is a geographic gradient
0
30
Latitude ºN
60
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A brief review of 1-locus selection models:
relative fitnesses of genotypes and outcome of selection
• A1A1 = A1A2 > A2A2:
• A2A2 > A1A1, A1A2:
• A1A2 > A1A1, A2A2:
• A1A2 < A1A1, A2A2:
A1 fixed
A2 fixed
stable
polymorphism
unstable, A1 or A2
fixed
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Formation of a step cline in allele frequency along an
environmental gradient
Fitness
A2A2
A1A1, A1A2
Environmental Gradient
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Formation of a step cline in allele frequency along an
environmental gradient
Fitness
A2A2
Frequency A2
A1A1, A1A2
1
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
“reinforcement”
Frequency of A2
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
Pure
population 1
Pure
population 2
Hybrid zone
Distance
21
Formation of a step cline in allele frequency along an
environmental gradient with gene flow
Fitness
A2A2
Frequency A2
A1A1, A1A2
1
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 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
dispersal
Peripatric population
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/GreenishWarblers.html
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Larus gulls
•
•
In northern
Europe, where
the herring gull
and lesser blackbacked gull cooccur, they do
not interbreed.
Nevertheless,
these “species”
are connected by
a ring of
interbreeding
populations
Figure obtained
at
http://en.wikiped
ia.org/wiki/Ring
_species
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