Sélection artificielle

Trichomes are hair-like structures on plant leaves that vary naturally among individuals. In artificial selection experiments, trichomes serve as a measurable phenotypic trait. By selecting plants with high or low trichome counts and breeding them separately, researchers can observe how traits change across generations, demonstrating the principles of artificial selection and phenotypic shifts.

Use a hand lens to examine the petiole of the lowest leaf, excluding cotyledons. Shine light onto the plant and hold it against a dark background to enhance visibility. Both partners should count independently on each plant; if counts differ, either enlist a third counter or average the two results to ensure accuracy and reduce observer bias.

The experimental hypothesis predicts that average trichome numbers will increase or decrease across generations depending on which plants are selected for breeding. The null hypothesis states that average trichome numbers will remain unchanged across generations after artificial selection, providing a baseline to test whether selection actually produces measurable evolutionary change.

After determining the median trichome count for the entire class, identify the top 10% of plants with the highest trichome counts for the high hairiness line and the bottom 10% with the lowest counts for the low hairiness line. If more than 10% have no trichomes, randomly select 10% of those plants for the low hairiness group. Keep these lines separate to prevent cross-pollination.

When flowers fully develop, use a pollination wand or paintbrush to transfer pollen by rubbing the stigma and anther of one plant's flowers, then applying the same wand to other flowers within the same line. Use a different wand for the other line to prevent cross-contamination. This controlled pollination ensures that only plants within each selection line breed together.

Allow seed pods to dry until yellow, then break them open to collect seeds. Fill new pots halfway with moist potting soil and add a fertilizer pellet. Fill pots to just below the lip, make a shallow depression, and use a gluestick-coated toothpick to transplant seeds individually. Cover with a thin soil layer and water as needed to prevent drying.

Create frequency graphs with trichome counts on the x-axis and plant numbers on the y-axis for generation one and both hairiness lines. Comparing these graphs reveals how the distribution of trichome counts shifts in response to artificial selection. A successful experiment shows the high hairiness line shifting toward higher counts and the low hairiness line toward lower counts, demonstrating selection visualization plants and data analysis.