response to selection
DESCRIPTION
Response to Selection. Bio 508: Evolution Robert Page Slides Courtesy of Dr. Voss. Heritability. h 2 = V A /V p. Proportion of total phenotypic variance attributable to additive genetic variance . Describes the extent to which offspring resemble their parents. - PowerPoint PPT PresentationTRANSCRIPT
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Response to Selection
Bio 508: EvolutionRobert Page
Slides Courtesy of Dr. Voss
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h2 = VA/Vp
Describes the extent to which offspring resemble theirparents
Proportion of total phenotypic variance attributable toadditive genetic variance
Heritability
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Resemblance between parents and offspring indicates degree to which a trait is inherited
heritability (narrow sense):
h2 = VA
VP
h2 = bOP
Drosophila wing length
Offs
prin
g av
erag
e (O
)
bOP parental average (P)
Detecting the genetic componentof phenotypic variation
Fraction of variation in parents that is explained by variation in their genes (VA).
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19761978 h2 = 0.9
Bill depthin the
Ground Finch
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Almost all characteristicsin almost all species aregenetically variable tosome extent.
Heritability is often measured in the lab;estimates are higherthan would be expected in nature.
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Detecting the genetic componentof 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.
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Truncation Selection
S = selection differential
Breed only these 16%
Standard deviation units
68%
14% 14%
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nonselected
selected
Response to selection when bOP = 1
Note standard deviation ( = 2 cm
16% > 1
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Note standard deviation ( = 1 cm
Response to selection for a less variable population
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Response to selection when when bOP < 1
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2% > 2
Response to selection under a more intense selection program
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Summary of Graphs
Response of a quantitative trait to selection depends on:
1. the relationship between fitness and phenotype2. the phenotypic variance3. the degree to which the trait is heritable
R = h2 S
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Selection on polygenic characters
R = h2S
generation h2 (low line)1–9 0.5010–25 0.2326–52 0.1053–76 0.15
How do selection response and heritability change over time?
• Long-term selection may eventually exhaust standing additive genetic variation
• Continued response depends on mutational input
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Response to Selection for Increased Bristle #
316% increase in phenotype !
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Relationship among heritability, R, and S