Download Population Genetics – Modeling Natural Selection Purpose – To

Population Genetics – Modeling Natural Selection
Purpose – To demonstrate the roles of selection and chance in gene change in populations. In the peas models of “gene pools”
considered in this lab we will derive (sample) “future generations” from initial populations with particular frequencies of pea colours
(each colour representing a different allele).
Question – How do genetic drift and natural selection influence the allele frequency within a population?
Procedure
Note – Each beaker containing peas of 2 colours represents a collection of any number of gametes produced by a population. Each
pea represents a gamete carrying a particular allele (colour). Removing two peas from the beaker, therefore, represents fertilization
and “birth” of an individual. Note that because any two peas can be removed, “mating” in our models is at random. This is unlikely to
occur in most real populations. A flip of a coin will decide whether it lives or dies. Each beaker represents one population; each pea
represents one allele. Yellow peas are dominant. Remember that each individual is diploid, therefore each is represented by 2 peas.
Part A – Genetic Drift Model
For our genetic drift model use the initial population with a 1:1 ratio of pea alleles.
1. Randomly select 2 individuals (4 peas)
2. Determine the allele frequency of A of these peas. (i.e. 1:1, 3:1, 1:3, or fixed)
3. Record the allele frequency of A in your data table for trial #1.
4. Replace the peas into the 1:1 beaker.
5. Select a new pair of individuals from the beaker that represents the new frequency of A.
6. Repeat steps 1-5 until the population becomes fixed (i.e. all A or all a) OR until you have recorded 10 generations.
7. Repeat this model 5 times (5 trials).
Part B – Natural Selection Models
Natural Selection can be added to the above model by simply assigning a survival value to particular genotypes. In the models below a
selection pressure of 50% in maintained against certain genotypes by flipping a coin to determine their fate each time they appear. For
the purpose of the exercise we will assume that heads = die, and tails = live. In other words, if AA individuals are being selected
against, each time one is drawn a coin is flipped. If the call is heads, the genotype is thrown back into the beaker; if tails it becomes
part of this population.
I - Selection Against the Dominant Allele
1. Follow the steps from part A with the following exception: If AA or Aa appear, flip the coin. If aa appears, add it to the new
population, which represents the first generation.
2. Remember to flip a coin for each individual.
3. If they survive they become part of the next founding population.
4. You need to get 2 surviving individuals before recording the results for each generation.
II - Selection Against the Recessive Allele
The procedure is the same as in the model above except that only genotype aa is selected against (i.e. AA and Aa individuals live).
Repeat 5 times.
III - Selection Against Both Homozygotes (Selection for Heterozygotes)
The procedure is the same as in the model above except that genotypes AA and aa both have a 50% chance of death. Repeat 5
times.
IV - Selection Against Heterozygotes (Selection for homozygotes)
The procedure is the same as the model above except that only Aa has a 50% chance of death. Repeat 5 times.
Analysis
1. Construct graphs for each model on graph paper.
2. Explain how allele frequencies are affected by each of the 5 mechanisms, particularly rates and direction of change.
3. Discuss the effect of a greatly increased population size on each of the mechanism, particularly on mechanisms of genetic drift
and natural selection II.
4. Discuss the effect of a greatly increased selection pressure on mechanisms I and IV.