Natural Selection and Allele Frequency Simulation (Microevolution)

Objectives: Upon completion of this lab, the student should:

  1. Understand Hardy-Weinberg equilibrium (a null model) and the assumptions of the model.
  2. Predict trends in population gene frequencies with natural selection.
  3. Improve data interpretation.
You will begin this lab by watching a video on microevolution. After the movie, you will be asked to make some predictions concerning allele frequencies in a population and natural selection in a population.

* You will have to make printouts of the tables for Allele frequencies for this lab. Allele Frequency Table and Punnett Square with no selection pressure and Punnett Square and Allele Frequency Table with selection pressure

Part 1 - The Hardy-Weinberg Equilibrium

The Hardy-Weinberg equilibrium is derived from the equation (p2 + 2pq + q2 = 1) and demonstrates how gene frequencies (2 alleles) and therefore, frequencies of alleles in a population will not change over time. It is a null model (no change) with the following assumptions: If any of the assumptions are broken allele frequencies in subsequent generations can change.

The following example will demonstrate how Hardy-Weinberg equilibrium works with the assumptions unbroken.

Suppose we have a population of 1000 diploid organisms with the alleles B and b. The number of homozygous dominant individuals (BB) is 600. The number of heterozygotes (Bb) is 350. The number of homozygous recessive individuals is 50. What are the frequencies of the alleles B and b in the population? One way to visualize this is to draw a table that tells us the frequencies of the alleles in question. Allele Frequency Table and Punnett Square with no selection pressure . Please look at the table and Punnett Square while you read the next paragraph.
In our population of diploid organisms 77.5% of the alleles are the allele "B" and 22.5% are "b". Assume our population has an equal number of females (500) and males (500), and each of them are allowed to mate and bear two offspring (total 1000 offspring). Since we know the allele frequencies, we can use Punnett Square and predict the number of offspring in the population with a specific genotype. By multiplying the frequencies of alleles, we can find the frequencies of genotypes of the offspring in our population. It now becomes evident that we have the same number of genotypes and allele frequencies in the second generation as the first.
60% BB = 600 BB, 17.5% + 17.5% = 35% Bb = 350 Bb, and 5% bb = 50 bb. Total 1000 individuals.
What happens to the frequencies of genotypes (and alleles) and number of offspring when one of the Hardy-Weinberg assumptions is broken? Suppose the selection pressure of a lethal allele is added. Let us suppose that the "b" allele is lethal when it is in the homozygous condition (bb), meaning that individuals that are homozygous recessive do not survive and therefore do not reproduce. Punnett Square and Allele Frequency Table with selection pressure Please use the Punnett Square and table while reading the next paragraph and answering the following questions.

  1. What does this do to the allele frequencies of this generation?

  2. Which allele has increased?

  3. Which allele has decreased?

  4. Is the "b" allele still in the population?

  5. What do you predict will happen to the "b" allele in successive generations?

  6. If there was no selection pressure of a lethal allele and the other assumptions of the Hardy-Weinberg model are not broken, what do you predict would happen to the allele frequencies in our population after thirty generations?

Part 2 - Effects of Selection on Gene Frequencies

Your instructor will now run the allele frequency computer simulation that will track the gene frequency of a population with the alleles "D" and "d".
  1. After viewing the simulation was your prediction correct about the allele frequency of alleles B and b when b was lethal in the homozygous condition?
  2. If your prediction did not match the computer simulation, explain where you made your error below.

 

 

 

 Part 3 - Selection Effects on a Moth Population

Your instructor will run the "Selection effects on a moth population" computer simulation. Please answer the following questions while you are viewing the demonstration.
  1. Light color in the moth Biston betularia before 1850 was a form of protective coloration. Why?
  2. What was the selection pressure that caused an increase in the black moth population?
  3. If this pressure had not occurred, what would have happened to the gene frequency of the dark moths' genotype?
  4. The common housefly became resistant to DDT an insecticide used during the 1940's and 1950's. Describe what probably happened to the frequency of the resistant gene during the years DDT was in use. On a separate piece of paper draw a graph of your hypothesis.

Part 4 - Natural Selection Experiment

Your instructor will run the experiment part of the computer program. In this simulation the following variables can be manipulated for different results:
  1. Frequency of light colored moths to the dark colored moths.
  2. Number of generations tracked.
  3. Amount of time between reports.
  4. Pollution levels.
Your instructor will simulate the effects of pollution on a moth population. After that, you will be given a scenario different from the one your instructor demonstrated. You will have to create a data table similar to the one on the computer demonstration and create a graph depicting the pollution levels and the percent of black moths.