Download Geospiza conirostris

Pi = Ai + Di +
= VA + VD + VE + some other stuff (covariances)
What is parental phenotype?
Pi = Ai + Di + EiP
Offspring
VP
Ei
What is offspring phenotype?
Oi = 1/2 Ai + EiO
Parents
CovO,P = 1/2 VA + 1/2 Cov (A,D) + 1/2 Cov (A,EP ) + Cov (A,EO ) + Cov (D,EO ) + Cov (EP,EO )
CovO,P = 1/2 VA + “G by E terms” + covariance in environment
Sibling
Siblings have the same parents
Sibling
They have resemblance through
both parents---AND it is possible for
both to get the same alleles. In that
case their phenotypes will be influenced
by Dominance in the same way.
Covsiblings = 1/2 VA + 1/4 VD
Offspring
How does a population
respond to selection?
On average,
Offspring = h2 Parents
Mid-Parent
If we only allow some parents to breed (e.g. above the mean)
Then the offspring will be larger. By how much?
offspring = h2 Parents
R = h2 s
Mean of Parents
Threshold for Survival
s
Mean of Surviving Parents
Mean of Offspring
R
R = h2 s
Often: h2 = R / s
s -- Selection Differential
With a single gene the change in phenotype is the change in
allele frequency:
 sq 2 (1  q)
q 
1  sq 2
With a quantitative trait:
R = h2 s
How big are selection differentials?
R = h2 s
Selection differentials
How much heritability is there?
Why is that important?
How do traits differ?
R = h2 s
Or resemblance among relatives
Graph of successive generations of
phenotype.
Change in ‘oil content’ = R = h2 s
R1 ~ R30 ~ R70
Closer look shows decline in rate
of change
the selection differential and the selection gradient:
s, selection differential = XS - X
, selection gradient = slope of best fit line for
relative fitness, w, as a function of trait value, z
 = [cov(w, z)]/var (z)
s = cov (w, z)
the selection gradient enables measurement of selection independent
of trait size (otherwise, larger trait=stronger selection)
important when considering multiple traits simultaneously
Different Types of Selection
Directional Selection in the Blackcap, Sylvia atriacapilla
novel route
change in migratory direction is heritable, h2: 0.58 – 0.9
non-migratory
populations from southern
Germany are migratory,
those from the Canary Is.
are not
artificial selection increased and decreased migratory tendency
Stabilizing selection in the goldenrod gallfly, Eurosta solidiginis
females insert an egg into a goldenrod bud
larva induces gall formation ---> 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
 sources of mortality
 intensity, direction of selection
parasitoids attack small galls; birds attack large galls
opposing directional selection is equivalent to stabilizing selection
Stabilizing selection in the goldenrod gallfly, Eurosta solidiginis
females insert an egg into a goldenrod bud
larva induces gall formation ---> 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
---> sources of mortality
---> intensity, direction of selection
*great variation in intensity of selection among populations
and among years
Disruptive Selection in the large cactus finch, Geospiza conirostris
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
extracting seeds from ripe
Opuntia fruits to obtain the
surrounding arils
tearing open rotting Opuntia pads
to obtain arthropods
Grant 1986
stripping bark to obtain
insects and other
arthropods
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
feeding efficiency
seed-size hardiness
resource gradient
Evolution of correlated characters
selection acts on individuals, not traits
few traits are completely independent—
e.g., forelimbs and hindlimbs
similar developmental pathways, similar genes
e.g., size of red shoulder patch on a Red-Winged Blackbird
pigment precursor may be involved in multiple
biochemical pathways
pleiotropy (one gene, many traits)
polygeny (many genes, one trait)
---> many loci, many traits
genetic
correlations
linkage disequilibrium can produce genetic correlations
locus A only affects trait z1, locus B only affects trait z2
D=0
no
correlation
D = +0.15
D = -0.15
positive
correlation
negative
correlation
pleiotropy can produce genetic correlations
locus A (with additive alleles) affects both trait z1 and z2
phenotypic correlations may also arise from environmental effects
rG and rE
both positive
positive rG
negative rE
no rG
negative rE
initial selection study --- measure several features
problems of interpretation: how important is what you’ve measured?
observe change in trait
-- selection on measured trait
-- selection on a correlated trait that wasn’t measured
failure of trait to change
-- no selection
-- no additive variance
-- opposing selection
-- genetic correlation
easy to measure phenotypic variance and covariance
but only genetic variance and covariance relevant to evolution
Evolution of correlated characters
selection on any trait can be partitioned into a direct
component (changes due to phenotypic/genotypic
variation in the trait) and an indirect component due
to genetic covariation with other traits
the magnitude and direction of direct selection may differ
from overall selection because of indirect effects
consequently:
a trait may change solely because of selection on some
other trait -- correlated response to selection
a trait may fail to change (despite measurable selection)
because of opposing selection on some other, correlated
trait --- constraints on trait evolution
Model for quantitative trait evolution
single trait:
several traits:
R = h2 s
amount of phenotypic change (R),
depends on amount of VA (h2) and
strength of selection (s)
z = GP-1s
= Gb
si = 3 Pijbij =
z is the trait vector (z1 z2 z3 …zn)
s is still selection differential (z – zs)
G, P are the genotypic and phenotypic
variance-covariance matrices
b is the selection gradient
Pi1b1 + Pi2b2 + Pi3b3 + …… + Pinbn
direct
indirect
b1 is the partial regression coefficient
Directional natural selection on Geospiza fortis in 1976-77 and 1984-86.
standardized selection coefficients
differential
gradient
s

SE
1976-77 (n=632)
weight
wing length
tarsus length
bill length
bill depth
bill width
+0.74
+0.72
+0.43
+0.54
+0.63
+0.53
+0.477
+0.436
+0.005
-0.144
+0.528
-0.450
0.146
0.126
0.110
0.174
0.214
0.197
1984-86 (n=549)
weight
wing length
tarsus length
bill length
bill depth
bill width
-0.11
-0.08
-0.09
-0.03
-0.16
-0.17
-0.040
-0.015
-0.047
+0.245
-0.135
-0.152
0.101
0.084
0.076
0.095
0.136
0.125
Grant & Grant 1995
Evolution 49:241
Evolutionary genetics of
feeding behavior in the
garter snake, Thamnophis
elegans
two populations:
coastal -- eat slugs
inland -- no slugs occur; eats fish and aquatic amphibians
(Arnold 1981)
feeding response to slugs is influenced by genes
coastal – eat slugs
inland – avoid slugs
Genetic correlations between responses to different prey odors in two
populations of Thamnophis elegans
Hyla
Batrachoseps
Taricha
fish
slug
leech
Hyla
---
1.10
-0.24
0.18
0.88
1.01
Batrachoseps
0.81
---
0.07
1.00
1.34
0.98
Taricha
-0.45
0.57
---
0.09
-0.55
-0.88
fish
0.89
1.27
0.02
---
0.59
0.84
slug
-0.03
0.56
-0.79
0.19
---
0.89
leech
0.07
0.77
-0.01
-0.38
0.89
---
coastal = above diagonal; inland = below diagonal
avoid
accept
Response to slugs (food)
avoid
Response to leeches (risk)
accept
H
accept
avoid
Response to leeches (risk)
L
avoid
accept
Response to slugs (food)
L
Response to leeches (risk)
accept
Selection against eating leeches is stronger than
selection for eating slugs (slugs are rare)
avoid
H
avoid
accept
Response to slugs (food)
L
Response to leeches (risk)
accept
Selection for eating slugs is stronger than
selection against eating leeches (slugs are common)
avoid
H
avoid
accept
Response to slugs (food)
Traits may not evolve independently because of genetic
correlations due to pleiotropy or linkage disequilibirum
A trait may change as a consequence of direct selection, or
as a correlated response to selection on a different trait
A trait undergoing selection may fail to change because of a
constraint operating through a genetically correlated
character
Partial regression is a statistical method that enables us to
separate direct selection on a trait () from total
selection (s)
The selection gradient () and the selection differential (s) may
differ in magnitude and sign