1. Bio 508: Evolution Robert Page Slides Courtesy of Dr. Voss
Response to Selection
Bio 508: Evolution
Robert Page
Slides Courtesy of Dr. Voss

2. Heritability
h2 = VA/Vp
Proportion of total phenotypic variance attributable to
additive genetic variance
Narrow-sense heritability.
Va = variance due to additive genetic effects
Vp = variance in the phenotype
Breeding value = the sum of the average effect of alleles… the value of an individual as judged by the mean value of its progeny
Additive variation is the main reason young resemble their parents
Describes the extent to which offspring resemble their
parents

3. Detecting the genetic component of phenotypic variation
Drosophila wing length
Resemblance between parents and offspring indicates degree to which a trait is inherited
heritability (narrow sense):
h2 = VA VP
Offspring average (O)
* Narrow sense heritability = regression slope (AKA regression coefficient)
bOP
parental average (P)
h2 =
bOP
Fraction of variation in parents that
is explained by variation in their genes (VA).

4. h2 = 0.9 Bill depth in the Ground Finch 1976 1978
Point is that estimated separate years and regression coefficient remained the same ~ 0.90
Just another example of parent-offspring regression…
Midparent = mean(parents)

5. Almost all characteristics in almost all species are
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.
Just an example slide…
Can make the point that estimates higher in lab b/c environmental variation is often specifically controlled for

6. 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.
Normally distributed trait = z
Measured in units of SD
Only shaded region selected to breed
Shaded triangle z = mean of whole population
Open triangle z = mean of selected parents
S = selection differential

7. Truncation Selection Breed only these 16% 68% 14% 14%
Normally distributed trait = z
Measured in units of SD
Only shaded region selected to breed
Shaded triangle z = mean of whole population
Open triangle z = mean of selected parents
S = selection differential
Results in directionality with a magnitude that is proportional to h^2
Standard deviation
units
S = selection differential

8. Response to selection when bOP = 1
selected
nonselected
16% > 1
Response to selection ( R ) = the difference in the mean between generations
When h^2 = 1… R = S… b/c offspring mean will equal midparent value
Note standard deviation ( = 2 cm

9. Response to selection for a less variable population
Point is that under the same intensity of selection in which only individuals > 1 SD above the mean breed… S is smaller and so is R
Here h^2 still = 1… so R still = S
Note standard deviation ( = 1 cm

10. Response to selection when when bOP < 1
Here h^2 is relatively low
Thus, R < S… mean of the next generation is only slightly greater than the mean of the first

11. Response to selection under a more intense selection program
2% > 2
Here only breeding the top 2%
S and R are larger than in the other examples

12. Summary of Graphs
Response of a quantitative trait to selection depends on:
the relationship between fitness and phenotype
the phenotypic variance
the degree to which the trait is heritable
(1) from above determines in nature which individuals breed and which don’t (I.e., selection differential)
(2) Phenotypic variance determines the scale (I.e. units of SD)
(3) R is proportional to h^2 and S
R = h2 S

13. Selection on polygenic characters
How do selection response
and heritability change over
time?
R = h2S
generation h2 (low line) 1–
10–
26–
53–
Point is that under rigorous selection… variation may be depleted at a rate that is too fast for mutation to rescue
Example is arbitrary… point is about h^2 for the negatively selected line….
• Long-term selection may
eventually exhaust standing
additive genetic variation
• Continued response depends on mutational input

14. Response to Selection for Increased Bristle #
316% increase in phenotype !
Responses to artificial selection for increased number of abdominal bristles in six laboratory populations of Drosophila melanogaster
Mean bristle numbertt in females is shown for two sets of populations separately
Selection terminated at the points indicated by circles
Note that the response slowed in later generations of selection

15. Just a review slide… here blue is not selected and pink is selected
Tbar = overall population mean
T* = mean of selected individuals

16. Relationship among heritability, R, and S
The midoffspring and midparent values are indicated both as dots on the scatterplot and as diamonds on the y and x-axes
The red symbols represent the 10 families with the largest midparent values.
Pbar = the average midparent value for the entire population
P* = the average midparent value of the families with the largest values
Obar = the average offspring value for the entire population
O* = the average midoffspring value for the families with the largest midparent values