Evolution of Quantitative Traits

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Evolution of Quantitative Traits
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variation in quantitative characters many
(most?) characters affecting fitness are
polygenic -- number, identity of genes
unknown continuous traits meristic
traits threshold traits all three
types are assumed to have similar genotypic
distributions
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for a particular quantitative trait, how much of
the phenotypic variation is due to genetic
variation? VP VG VE
Phenotypic Genetic
Environmental Variance
Variance Variance heritability, h2
components of genetic variance VA additive
variance narrow-sense heritability, VD
dominance variance VI epistatic variance
h2 VM maternal variance
VG VP
VA VP
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additive alleles, heterozygote is exactly
intermediate in phenotype incomplete
dominance, heterozygote resembles one
homozygote more than the other
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qp0.5
additive variance is a function of allele
frequencies in the population environmental
variance comprises a greater proportion of
the phenotypic variance when allele
frequencies are skewed
q0.1 p0.9
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measuring h2 and additive variance
parent-offspring regression genetic covariance
of parents and offspring 0.5
slope of line b, regression coefficient
narrow-sense heritability, h2
offspring mean
each point one family
MPV parental mean
significance of h2 determined by
significance of regression
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h2 0 h2 0.5 h2 1
offspring mean
MPV MPV MPV
h2
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Clausen, Keck Hiesey 1940
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What is the response to selection? - strength of
selection - amount of additive genetic variance
X, population mean
selection differential, s X - XS
threshold for selection
Xs, mean of selected parents
Xoff, mean of offspring population
response to selection, R X - Xoff
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how much does a population change in one
generation of selection? recall with a
single gene, Dpheno DQ ---gt Dq
with a quantitative trait, R h2s
-sq2(1q) 1-sq2
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qp0.5
additive variance is a function of allele
frequencies in the population environmental
variance comprises a greater proportion of
the phenotypic variance when allele
frequencies are skewed
q0.1 p0.9
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the selection differential and the selection
gradient s, selection differential XS -
X b, selection gradient slope of best fit
line for relative fitness, w, as a function of
trait value, z b cov(w, z)/var (z) s
cov (w, z) the selection gradient enables
measurement of selection independent of trait
size (otherwise, larger traitstronger
selection) important when considering multiple
traits simultaneously
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Evolution of quantitative characters Phenotypic
variance in a trait can be partitioned into
several genetic variance components Selection
can only change traits that have additive genetic
variance (VA) Selection will change the
genetic variation in a quantitative trait in a
manner similar to one with simpler
genetics Narrow-sense heritability (which
enables the estimation of VA) can be determined
from parent-offspring regression or mass
selection studies
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Directional Selection in the Blackcap, Sylvia
atriacapilla
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novel route
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change in migratory direction is heritable, h2
0.58 0.9
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non-migratory
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populations from southern Germany are
migratory, those from the Canary Is. are
not
number of 30-minute periods of migratory
restlessness
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artificial selection increased and decreased
migratory tendency
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Stabilizing selection in the goldenrod gallfly,
Eurosta solidiginis females insert an egg
into a goldenrod bud larva induces gall
formation ---gt protection summer parasitoid
wasps winter (pupa) woodpeckers and
chickadees infer predator from type of damage
to gall 16 populations, each for four
years measure galls of survivors and dead each
spring ---gt sources of mortality ---gt
intensity, direction of selection
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opposing directional selection is equivalent to
stabilizing selection
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Stabilizing selection in the goldenrod gallfly,
Eurosta solidiginis females insert an egg
into a goldenrod bud larva induces gall
formation ---gt protection summer parasitoid
wasps winter (pupa) woodpeckers and
chickadees infer predator from type of damage
to gall 16 populations, each for four
years measure galls of survivors and dead each
spring ---gt sources of mortality ---gt
intensity, direction of selection great
variation in intensity of selection among
populations and among years
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Disruptive Selection in the large cactus finch,
Geospiza conirostris
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Geospiza conirostris on Genovese Is. four dry
season feeding modes bark-stripping to obtain
arthropods cracking seeds of Opuntia
helleri extracting seeds from ripe Opuntia
fruits to obtain the surrounding
arils tearing open rotting Opuntia pads to
obtain arthropods
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extracting seeds from ripe Opuntia fruits to
obtain the surrounding arils tearing open
rotting Opuntia pads to obtain arthropods
Grant 1986
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stripping bark to obtain insects and other
arthropods
(Grant 1986)
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Geospiza conirostris on Genovese Is. four dry
season feeding modes bark-stripping to obtain
arthropods cracking seeds of Opuntia
helleri extracting seeds from ripe Opuntia
fruits to obtain the surrounding
arils tearing open rotting Opuntia pads to
obtain arthropods birds that stripped bark had
significantly deeper beaks than those that did
not birds that cracked seeds had significantly
larger beaks than those that did not birds
that opened opuntia fruits had significantly
longer bills than those that fed on arils in
already opened fruits
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What maintains additive genetic variance in
fitness traits? non-equilibrium with respect to
selection (stabilizing, directional) mutation
-selection balance (weak selection on
any single locus) disruptive
selection/frequency-dependent selection stabiliz
ing selection/fluctuating environment antagonist
ic pleiotropy/genetic correlations
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Evolution of quantitative characters Phenotypic
variance in a trait can be partitioned into
several genetic variance components Selection
can only change traits that have additive genetic
variance (VA) Selection will change the
genetic variation in a quantitative trait in a
manner similar to one with simpler
genetics Narrow-sense heritability (which
enables the estimation of VA) can be determined
from parent-offspring regression or mass
selection studies Contrary to expectations from
animal and crop breeding, in natural
populations, traits that affect fitness
often have considerable genetic
variance Additive variance may be maintained by
mutation, stabilizing or disruptive selection,
or by antagonistic pleiotropy