Assignment 4

It is possible for natural selection to affect allele and genotype frequencies in several ways depending on the mode of natural selection occurring within the population. Different modes of natural selection can select for or against a particular trait, thus affecting the balance of phenotypes in the population. For example, in a mode of natural selection called directional selection, selection favors individuals with one of the extreme phenotypes—a phenotype at either end of the range of phenotypes. For example, all black or all white moths are at the extremes while brown moths represent an intermediate phenotype. In stabilizing selection, extreme phenotypes are selected against and intermediate phenotypes have higher rates of reproduction or survival. Conversely, diversifying (disruptive) selection favors individuals with a range of extreme phenotypes over individuals with an intermediate phenotype.

In the following experiment you will investigate how fitness—the probability that a particular phenotype will survive and produce offspring (which is a measure of survival and reproduction of different genotypes)—affects changes in allele frequency in the population. Working within PopulationGeneticsLab and referring to the detailed instructions on Assignment 4, answer the questions below.


1 .		Begin an experiment with default conditions for Hardy-Weinberg equilibrium for all parameters except the number of tree stands and genotype frequencies. Set the number of stands to 100. Change genotype frequency to set up several different experiments under conditions of directional selection for dark color, directional selection for light color, balancing selection, and diversifying selection. For each experiment, look at genotype and allele frequencies. Try experiments with the different conditions of selection and initial allele frequencies near zero and one. What happens to allele frequency in the case of directional selection? Does directional selection for dark color produce the same result as directional selection for light color?

2 .		What happens to allele frequency in the case of balancing selection?

3 .		What happens to allele frequency in the case of diversifying selection?

4 .		Are small differences in fitness effective in changing allele frequencies? Conduct additional experiments with varying proportions of tree types to help answer this question.

5 .		Under what mode of selection might genetic variation be maintained? Explain your answer.

6 .		Under what mode of selection might there be a "founder effect," where the final allele frequency depends on the initial allele frequency? Explain your answer.