Assignment 2

Random changes in the genotypes of a population are known as genetic drift. Genetic drift has the potential to change the genetic composition of populations. Carrying capacity of an environment can influence how a population changes in response to genetic drift because carrying capacity is an important factor that determines the size of a population. Large populations rarely show strong effects of genetic drift; however, genetic drift in a small population can have a great influence on allele and genotype frequencies in that population. The following exercise is designed to help you understand important aspects of genetic drift as a potential cause of microevolution. Working within PopulationGeneticsLab and referring to the detailed instructions on Assignment 2, answer the questions below.


1 .		Formulate a hypothesis below to predict the effect of a decrease in tree stand carrying capacity on allele and genotype frequencies.

2 .		Set up an experiment with default values for all of the Hardy-Weinberg conditions. Run this experiment and make note of what happens to allele and genotype frequencies. Run a series of experiments with default parameters for all conditions except tree stand size. For each experiment, reduce tree stand size by one-half (for example, run an experiment at 2000, half of the default value, then change tree stand size to 1000, and so on). What happened to allele and genotype frequencies in this experiment? How do these results compare with what you observed for allele and genotype frequencies under the default conditions? Explain these results.

3 .		What happened to the percentage of heterozygotes? Explain these results.

4 .		Set up an experiment with the number of tree stands set to the maximum value of 100 and perform a series of experiments with different tree stand carrying capacities. Study genotype and allele frequencies that result from each experiment. For each experiment, go to the Allele Distribution view and use the slider to examine allele frequency among the populations. What happened to the average genotype frequencies? What happened to the genotype frequencies in each population? What happened to the allele frequencies in each population? What happened to the average allele frequencies? Explain these results.

5 .		What happened to the average heterozygosity? What happened to the heterozygosity in each population? Explain these results.

6 .		How does population size (carrying capacity) affect the changes in genotype and allele frequencies that you observed in these experiments?

7 .		What do these experiments tell you about the effect of population size on genetic variation?

8 .		Did any of the populations that you studied become extinct? If so, which ones? Why did these populations become extinct while others avoided extinction? What can you say about the relationships between carrying capacity and the risk of extinction?