Genetic Polymorphism and Speciation

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Genetic Polymorphism and Speciation - An Adaptive
Dynamics Perspective -
?
Eva Kisdi Stefan Geritz Dept. of Mathematics,
University of Turku
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Evolutionary branching
http//users.utu.fi/evakis/addyn.htm
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Levene's Soft Selection Model

• random dispersal to two habitats
• within-habitat selection
• within-habitat competition fixed number of
  adults emerge
• random mating in the entire population

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Continuum of Alleles, Small Mutations
offspring
f1(x)
f2(x)
c1
c2
adults

1. Clonal (haploid) inheritance
2. Diploid, 1 locus with additive allelic effects on
   the phenotype
3. Diploid with assortative mating

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Clonal model
Evolutionary branching of phenotypes
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Clonal diploid models
Clonal model evolutionary branching of
phenotypes Diploid 1-locus model evolutionary
branching of alleles
(d/s)2 gt 1/c1c2
Infinite loci, /- alleles Genetic polymorphism
in each locus (Spichtig Kawecki, in
press) Two loci, continuum of alleles Initially
both loci undergo branching, but only one remains
polymorphic (Kisdi Geritz 1999)
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Genetic Polymorphism
Protected polymorphism of similar alleles under
weak selection - ?!
Fixed alleles
Hoekstra et al. 1985
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Genetic Polymorphism
Generic models of a wide class, both clonal and
diploid no polymorphism of similar alleles away
from singularities invasion implies
fixation near a singularity, protected
polymorphism or rare disadvantage of similar
alleles fine-tuning done by directional
evolution if the singularity is an ESS, then the
polymorphism is transient on the long-term
evolutionary timescale not every polymorphism is
evolutionarily permanent
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Genetic Polymorphism
Both clonal and diploid
evolutionarily stable polymorphism (no
branching) evolutionary branching
Evolutionary branching is not necessary (neither
sufficient) for evolutionarily stable
polymorphism
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Clonal model
After branching, two specialist phenotypes evolve
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Diploid model
Will assortative mating restore the simplicity of
the clonal model?
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Assortative mating
"Two-allele mechanisms" (Felsenstein,
1981) strong selection is necessary to
counterbalance recombination
Evolution to strongest heterozygote
inferiority Time window for speciation
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Population Genetics of Assortative mating
Udovic (1980) model mating locus with two
alleles (B, b) mating groups BBBb and bb p
penetrance mating within group with prob.
p, otherwise random mating r recombination
between the ecological and the mating
locus In linkage equilibrium, alleles B and b
are neutral
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Population Genetics of Assortative Mating
Ecological locus (no mutation) Mating locus B,
b Gamete frequencies
u freq(x1) 0.5 v freq(b)
in the dominant (BBBb) mating group
in the recessive (bb) mating group
overall
Zygote frequencies
except
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Population Genetics of Assortative Mating
Zygote frequencies
except
Selection by the ecological locus
relative fitnesses
Gamete frequencies by the standard Mendelian
rules within each mating group recombination
rate r 6 variables 1 (sum of the Q's is 1) 1
(symmetry, u0.5)
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Population Genetics of Assortative Mating
p 0.75 r 0.5 s 0.358 (max s with d/s3,
c10.5)
vfreq(b)
D qib - uv
Bistability of the gamete frequency dynamics
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Population Genetics of Assortative Mating
p 0.85, r 0.5 (?x given for d/s 3, c10.5)
s0.125 (?x1)
s0.194 (?x1.35)
s0.203 (?x1.4)
frequency of allele b
s0.207 (?x1.42)
s0.339 (?x2.4)
s0.239 (?x1.6)
linkage disequilibrium, D qib - uv
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Population Genetics of Assortative Mating
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Assortative mating during branching
Switch from LE to LD somewhere in the orange
band depending on the frequency of b
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Adaptive dynamics of alleles at LD
F
heterozygote deficiency before / after selection
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Assortative mating during branching
Switch from LE to LD somewhere in the orange
band depending on the frequency of b
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Adaptive dynamics of alleles at LD
LD permanent incipient speciation
LD lost
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Prospects for speciation
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Prospects for speciation
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Prospects for speciation
"Byproduct" speciation - no LD is needed
between the ecological and mating loci - sexual
selection may counterbalance disruptive
natural selection (Kirkpatrick and Nuismer
2004), and causes a bistability in the
evolutionary dynamics of assortment (Matessi
et al. 2001, Meszéna and Christiansen in
prep.) - environmental variance in the
selected trait constrains the maximum
attainable level of assortment
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An Adaptive Dynamics Perspective
... on polymorphism directional evolution takes
care of "fine tuning" polymorphism may be
transient in evolution (evolution near an ESS,
evolution to extinction) evolutionary branching
is neither necessary nor sufficient for an
evolutionarily stable polymorphism to exist ...
on speciation evolution can lead to strong
disruptive selection but can lead away from it
again (time window) competing processes
evolution of dominance, sexual dimorphism, mixed
strategies
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An Adaptive Dynamics Perspective
... on speciation - evolution to polymorphism
under disruptive selection appears to be
common - reproductive isolation depends on
genetics Alliance with population genetics
needed!
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