Natural Selection and the Distribution of Traits
1. polygenic trait:
Because genes can have many alleles, polygenic traits tend to exhibit a range of phenotypes clustered around an average value. For example, skin color and human height are influenced by ~12 different genes. Natural selection can change how often an allele for a certain trait occurs within a population. Because natural selection acts in this manner, it will have the greatest effect on the genes that make the greatest contribution to an organism’s phenotype. In other words, the more polygenic the trait, the more it will be effected by natural selection.
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Human eye color, 3 genes |
Human skin color, 16 genes |
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Dimples, 1 gene |
Human blood type, 3 genes |
2a) Which trait would natural selection have the greatest impact on?
2b) Which trait would natural selection have the least impact on?
3. Normal distribution:
4. Directional selection:
5. Stabilizing selection:
6. Disruptive selection:
7. Natural selection acts on an individuals’ [genotype | phenotype] (circle one).
Normal Distribution Lab
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Shoe length (cm) |
Gender (m/f) |
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Step 1) Remove one of your shoes and measure it to the nearest centimeter. Record the shoe length and your gender on the class data sheet (see board).
Step 2) Copy the class data sheet onto the table left.
Step 3) Determine the number of shoes of each length.
Step 4) Make a graph (bar or line graph) showing the distribution of shoe length for your entire class. Show the number of students on the y-axis and the shoe length on the x-axis.
*y-axis on your graph should be numbered 0-10
*x-axis on your graph should be numbered 18-35
Step 5) Make a second graph using data for FEMALES only.
Step 6) Make a third graph using data for MALES only.
****If making a line graph, remember to graph the zeroes on your y-axis!
ANALYSIS QUESTIONS
1. Describe the shape of the class graph you made in step 5.
2. How does the distribution curve of shoe length for males differ from shoe length for females?
3. This lab is intended to relate to how natural selection affects phenotypes. If there was a great flood and the larger shoe length variation was better able to climb and live in trees, while the other phenotypes were not able to survive as well, which type of selection do you suppose your graph would resemble? How so?
4. If a deadly foot fungus was introduced to the population, and it could only survive on average size feet, what type of selection would occur? Why?