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Response to Selection
Bio 508: Evolution
Robert Page
Slides Courtesy of Dr. Voss
Heritability
h2 = VA/Vp
Proportion of total phenotypic variance attributable to
additive genetic variance
Describes the extent to which offspring resemble their
parents
Detecting the genetic component
of phenotypic variation
Drosophila wing length
Offspring average (O)
Resemblance between parents and
offspring indicates degree to which
a trait is inherited
heritability (narrow sense):
h2
VA
=
VP
h2 = bOP
bOP
parental average (P)
Fraction of variation in parents that
is explained by variation in their genes (VA).
Bill depth
in the
Ground Finch
1978
1976
h2 = 0.9
Heritability is often
measured in the lab;
estimates are higher
than would be
expected in nature.
Almost all characteristics
in almost all species are
genetically variable to
some extent.
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