Experiment 1: Genetic Variation
Genetic variation is simply the genetic difference within or between populations, in the gene pool and/or gene frequency.
Consider the following two populations of butterflies (Figure 2):
Assumptions: Both populations contain the same four colors of butterflies, thus the gene pool is the same. However, the distribution of colors within that population is different, thus their gene frequencies are different.
Procedure:
1. Pour 50 blue beads and 50 red beads into a 250 ml beaker. Without looking, randomly take 50 beads from the 250 mL beaker and place them in a 100 mL beaker (this is beaker #1).
2. Pour 50 green beads and 50 yellow beads into a second 250 mL beaker. Without looking, randomly take 20 beads from the 250 mL beaker and place them in the other 100 mL beaker (this is beaker #2).
Note: When done, return beads to their respective beakers (1 or 2) for use in the next experiment.
Post-Lab Questions
1. What is the gene pool of beaker #1?
2. What is the gene pool of beaker #2?
3. What is the gene frequency of beaker #1?
4. What is the gene frequency of beaker #2?
5. What can you say about the genetic variation between these populations?
Experiment 2: Natural Selection
Natural selection is a selection pressure that effects phenotypes in one of three ways:
1. It will create an adaptive advantage.
2. It will create an adaptive disadvantage.
3. It will remain entirely neutral.
A classic example to illustrate natural selection comes from England. Prior to the Industrial Revolution, the native moths were normally a light color, though darker versions of the same species existed. The lighter color blended with the light bark of the local trees, while the darker moths experienced a higher predation rate - they were easier for birds to spot and fewer survived to reproduce. As England entered the Industrial Revolution they began burning fossil fuels with little regard to the pollutants they were emitting. The trunks of the trees became coated with soot and their color darkened. The lighter moths became more conspicuous and the darker moths were better camouflaged. The proportion of white to dark moths changed.
Procedure:
1. Print the two sheets of paper marked Blue Habitat (Figure 5) and Red Habitat (Figure 6), from your manual (found at the end of this procedure).
2. Place 50 red and 50 blue beads into a 100 mL beaker.
3. Mix them well and pour them onto the sheet marked Red Habitat.
4. Keep the beads that fall onto habitat that matches their color.
5. For each bead that you keep (and return to the beaker), add another bead of the same color to the beaker (discard the rest).
6. Repeat this three times.
7. Record the remaining colors.
Do you observe a selective advantage for the red or blue beads? Why?
8. Repeat the process using the Blue Habitat with the remaining beads.
What beads remain now?
1. How did the distribution of phenotypes change over time?
2. Is there a selective advantage or disadvantage for the red and/or blue phenotypes?
3. What phenotypic results would you predict if starting with the following population sizes?
A. 1000:
B. 100:
C. 10:
4. Assume that you live in a country with 85 million people that consistently experiences an annual growth rate of 4.2%. If this population continues to grow at the same rate for the next 5 years, how many people will live in the country (round to the nearest whole number).
Experiment 3: Sickle Cell Anemia Inheritance Patterns
Sickle cell anemia is a genetic disease (one base pair mutation that changes a protein). It is more common in those of African ancestry. "S" will represent the normal dominant allele and "s" the recessive sickle allele. They are co-dominant alleles - SS is normal, Ss is not fatal, ss is debilitating, painful and often fatal.
Procedure:
1. Place 25 red (S) and 25 blue (s) beads into the 100 mL beaker and mix well.
2. Randomly (without looking) remove two beads. Repeat 10 times (without returning the beads to the beaker), each time recording if it was a SS, Ss or ss.
3. Remove each ss from the population - they died.
4. The remaining beads survived and reproduced.
5. Count how many red and blue beads remained (separately) and place twice that number back in the beaker.
6. Repeat the process seven times.
Post-Lab Questions
1. What is the remaining ratio of alleles?
2. Have any been selected against?
3. Given enough generations, would you expect one of these alleles to completely disappear from the population? Why or why not?
4. Would this be different if you started with a larger population? Smaller?
5. After hundreds or even thousands of generations both alleles are still common in those of African ancestry. How would you explain this?
6. The worldwide distribution of sickle gene matches very closely to the worldwide distribution of malaria (Figure 7).
What is the significance of this?