Artificial Selection

To begin the experiment, collect five two-week old grown plants from the generation one population that appear healthy. That is, plants that are green, not wilted, and do not have damaged leaves. The experimental hypothesis for this lab is that the number of leaf trichomes varies naturally, but the average number will increase over generations if only plants with the highest number of trichomes are interbred or decrease if those with the least are interbred.

The null hypothesis is that the average number of trichomes will not change over subsequent generations after artificial selection. Label each plant with a letter from A to E and with both partner's names. Record any natural differences you see between your plant samples.

All of the plants should be the same age, so this variation is due to expressed phenotype, not development. Using a hand lens, count the number to trichomes found on the petiole of the lowest leaf, not including the round, first-grown cotyledons for each plant. It can be helpful to shine a light onto the plant and to hold it against a dark background, such as a lab bench, during the trichome analysis.

To ensure accuracy, both partners should conduct the counts independently, and compare answers. If the counts are different, enlist a third counter, or take the average of the two results. Record the agreed upon counts in the table and report the data to your instructor for compilation.

When all of the data has been compiled, determine the median number of trichomes for the entire class. To create two different lines of artificially-selected plants from generation one, determine the top and bottom 10%of the plants from the class trichome count. These plants will be called the high hairiness and the low hairiness lines, respectively.

For the low hairiness line, it is possible that more than 10%of the plants counted will have no trichomes. If this is the case in your experiment, select 10%of plants randomly for the low trichome group for the low hairiness plant line. Separate the two groups of selected plants and label them clearly, then move all of the plants into a well-lit area and keep the plants well watered until the develop flowers, which is typically and around 14 days old.

When the plants produce fully developed flowers, each student should take a pollination wand or paintbrush and rub it gently against the anther and stigma of one plant's flowers for a few seconds. Then, rub the same brush inside each flower of every other plant in the same selection line. Using a new wand, repeat the pollination for each plant until all of the plants have been pollinated.

Being careful not to cross the lines and to spread the pollen only among the appropriately selected plants. Then place them back into the growing area. When the generation one plants are approximately one month old, they should have produced mature seed pods.

Allow the plants to dry out once the seed pods begin to turn yellow. Next, remove the seed pods from each plant. Break open the seed pods and collect new empty pods to plant the seeds.

Fill the pods with moist potting soil half-way. Add a fertilizer pellet to each pot and then add more soil to fill just below the lip. Make a shallow depression in the soil using a probe, the touch a clean toothpick to a gluestick and use this to transplant the seeds into the soil one by one.

Finally, cover the seeds with a thin layer of soil. Label the pots based on their hairiness line, and then place them in a designated growing area such as under a grow lamp or on a windowsill. Water them using a plastic dropper and repeat this often to ensure they don't dry out.

When the generation two plants are two weeks old, collect five plants from each line and label them A to E.Then count the trichomes in the same manner as for the first generation plants and record the trichome count results in the table, along with the class average for each line. To analyze the trichome leaf variations between the plant lines and generations, first create a frequency graph based on the generation one class data with the trichome count on the x axis and the number of plants on the y axis, and label the mean. Then create frequency graphs for both the selected lines of generation two and compare them to each other as well as to the graph for generation one.

Looking at the frequency graphs for each generation, you will notice a phenotypic shift toward a higher trichome count in the high hairiness group, while the low hairiness line would have shifted to a lower trichome count. You will also have noticed differences in the means between the generations and between the hairiness lines that mirror the shifts in distribution. What do you think the distributions would look like if the selection continued over more generations?