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Detecting the genetic component
of phenotypic variation
Genetically variable characters can be altered by selection.
The response to selection is proportional to the amount of
genetic variation in the character.
Truncation Selection
68%
Breed only
these 16%
14%
Standard deviation
units
14%
S = selection differential
Response to selection when bOP = 1
selected
nonselected
16% > 1
Note standard deviation ( = 2 cm
Response to selection for
a less variable population
Note standard deviation ( = 1 cm
Response to selection when when bOP < 1
Response to selection under a
more intense selection program
2% > 2
Summary of Graphs
Response of a quantitative trait to selection depends on:
1. the relationship between fitness and phenotype
2. the phenotypic variance
3. the degree to which the trait is heritable
R = h2 S
Selection on polygenic characters
How do selection response
and heritability change over
time?
R = h2S
generation
1–9
10–25
26–52
53–76
h2 (low line)
0.50
0.23
0.10
0.15
• Long-term selection may
eventually exhaust standing
additive genetic variation
• Continued response depends
on mutational input
Response to Selection for Increased Bristle #
316% increase in phenotype !
Relationship among heritability, R, and S
N = 30 mice
(0 or 1)
Absolute fitness
Mean fitness
(10/30 = 0.33)
[(20 x 0) + (10 x 1)] / 30 = 0.33
The slope of the best fit line
is the selection gradient.
Ground finches and seeds
during the drought of 1977
Heritability of
beak depth
in Geospiza
fortis.
Finches before and after the drought
Selection on multiple traits and correlated characters
Drought of 76-77
Darwin’s finches
3-Dimensional Selection Gradient
narrow
wide
shallow
deep
Two-Dimensional Graphs
Selection can
only move in
direction shown
by this arrow.
most fit 2-character
phenotype, but
can’t be achieved
because characters
are correlated