We can adapt the Hardy-Weinberg to investigate what happens to allele frequencies in a
population that is evolving. To do this, it is necessary to introduce a new term, selection
coefficient. It is defined as "the relative advantage or disadvantage of a genotype with
respect to survival and reproductive success." You can also think of it as the relative
selection advantage of a specific allele. For example, if there are two alleles present in a
population for a particular trait and one allele is 10% more likely to survive than the
other allele, then the selection coefficient for that allele is +0.1.
14. Use the spreadsheet posted in the lab folder on Brightspace to determine how the
selection coefficient (s) influences the phenotype of future generations. Substitute
increasingly large values for s (between 0 and 1). List the values you used for s and
describe what happened to the frequencies of p and q over the subsequent five generations.
In areas with primarily dark-colored substrate, dark-coat mice have a selective
advantage over light-coat mice. Therefore, mice with one or more copies of the
dominant Mclr D allele have a selective advantage over mice with two copies of the
Mclr d allele. Even small selective advantages can have a profound effect on allele
frequencies in a population if given enough time.
15. In a population of 500 light-coat individuals (dd), an individual is born with a mutation
(D) in one of their two copies of the Mclr with the effect of producing a dark-coat
phenotype. What is the initial frequency of the D in this population?
16. In the same population of 500 individuals, how many generations following the first
appearance of a dark-coat mutation it would take for 95% of the mice to express the
dominant dark-coat phenotype, given a 5% advantage (s = 0.05)? Rock pocket mice have
approximately one litter of pups a year, so the number of generations will be equal to the
number of years. Note - you will not be able to use the graph on the Main Page tab since it
only goes up to 100 generations. So, you will need to look at column D (Dominant
phenotype) of the worksheet called Main Worksheet. Scroll down until the value is greater
than 0.95.
17. How many generations following the appearance of a dark-coat mutation would it take
for 95% of the mice to express the dominant dark-coat phenotype if dark-coat mice enjoy a
10% advantage (s = 0.1) over their light-coat cousins?
18. What would the selection coefficient need to be for 95% of the mice to have the
dominant phenotype in just 75 years?
19. Notice that even when selective advantage for D is strong, the less favored allele
doesn't simply disappear from the population (frequency = 0). Why?
