S318 - Honors Evolution
Labs II & III: Natural Selection on a Deck of Cards
You may wish to consult the web notes from Lecture IV as you review what we did.
In this lab we simulated a closed population using decks of cards to represent allelic identity at different loci to investigate how heritability and the response to selection are affected by environmental variance and dominance.
Game I: The "control"
You can link to the data here
Main Points
1) We estimated heritability by asking excel to calculate the slope of the best-fit line through the data points of a mid-parent vs. offspring graph. This is analagous to determining the covariance between mid-parents and offspring and dividing that quantity by the variance among mid-parents.
2) The heritability was ~1
3) The response to selection roughly equalled the selection differential
Game II: Environmental variance
You can link to the data here
Main Points
1) Introducing a random environmental effect on the phenotype lowers the heritability because it increases the total phenotypic variation in the population. As a result, the proportion of phenotypic variation that can respond to selection (additive genetic variation) decreases.
2) Because the heritability < 1, the response to selection is less than the selection differential. The environmental effects on the phenotype cloud the association between genotype and reproductive success, affecting the transmission of selective effects on the distribution across generations.
Game III: Dominance
You can link to the data here
Main Points
1) Like the previous game, introducing dominance increases the phenotypic variation in the population. As a result, the proportion of phenotypic variation that can respond to selection (additive genetic variation) decreases. This can be difficult to see. On your own, compare the variance between these two imaginary populations.
a) no dominance:
10 AA individuals with a phenotype of 100 units
20 Aa individuals with a phenotype of 50 units
10 aa individuals with a phenotype of 10 units
b) dominance (A completely dominant):
10 AA individuals with a phenotype of 100 units
20 Aa individuals with a phenotype of 50 units
10 aa individuals with a phenotype of 10 units
If you graph these two scenarios (frequency on the y-axis, trait value on the x-axis), you will see that phenotypic variance is increased with dominance.
Dominance essentially clouds, but does not remove, the association between genotype and reproductive success, affecting the transmission of selective effects on the distribution across generations. Note that phenotypes are still partly additive. The proportion of phenotypic variation that is due to these additive effects of alleles is simply reduced.
2) As before, because the heritability < 1, the response to selection is less than the selection differential.