Name: ______Period: ______

Biology Lab Activity: Natural Selection and Allelic Frequency

Pre-lab Questions

1. Define:

allele—

natural selection—

2. According to Darwin, how are traits and natural selection connected?

Objectives

Simulate natural selection by using beans of two different colors.

Calculate allelic frequencies over five generations.

Demonstrate how natural selection can affect allelic frequencies over time.

Materials

Plastic container, dark colored beans or beads, light colored beans or beads, colored pencils (2 different), graph paper.

Procedure

  1. Place 50 dark beans (representing the G allele) and 50 light beans (representing the g allele) in the container.
  1. Shake the container. Without looking, remove two beans representing one rabbit’s genotype.
  1. Form two columns (not piles, you must be able to count the pairs separately) of bean pairs. One will contain the brown-gray rabbit phenotypes (genotypes GG or Gg) and the other will contain the white rabbit phenotypes (genotype gg). Remove 49 more pairs of beans from the container as described in step 2.
  1. Examine your columns. Eagles in this habitat eat 25% of the gray rabbits and 100% of the white rabbits. This represents selective pressure on your rabbit population. Randomly remove 25% of the brown-gray rabbit pairs and 100% of the white rabbits. If the number you calculate is a fraction, round to the nearest whole rabbit (nearest whole pair of beans).
  1. Count the number of all remainingbeans by color. Record the number of each allele in the data table.
  1. Calculate allelic frequencies remaining (number of each allele divided by the total remaining). This number will be a decimal.
  1. Multiply the frequency for each allele by 100, rounding to the nearest whole bean. Place this number of beans of each type back in the container, making sure the total is 100 beans. (i.e .37 = 37 beans of that color in the next trial.)
  1. Repeat steps 3-8, collecting data for five generations. If at the end of any generation you get 100% GG, change two beans to white to represent a new mutation and continue with the other generations.
  1. Graph the frequencies of each allele over five generations. Plot the frequency of the allele on the

y-axis and the number of the generation on the x-axis. Use a different colored pencil for each allele.

Data Table

Allele GAllele g

Generation / Number / Frequency / Number / Frequency
Start / 50 / .50 / 50 / .50
1
2
3
4
5

Analysis Questions

  1. Did either allele disappear? Why or why not?
  1. What does your graph show about allelic frequency and natural selection?
  1. Assume this habitat is a grassy area with sparse trees, and the main rabbit predators are eagles. What would happen to the allelic frequency if a disease caused the number of eagles to decline? Explain.
  1. If this habitat were the tundra (snow ten months a year), what would happen to the allelic frequencies? Explain.
  1. Compare your data to as many other groups as possible and explain any differences you observe.