When you look at a population of animals you may notice some variations or differences between individuals of the same species. Perhaps some are larger than others or there are differences in their coloration. These variations serve as the basis for natural selection where beneficial traits in a population that positively affect survival and reproduction will be preserved. For example, animals that can more easily camouflage with their environment evade predators and pass on their genes more often than those that do not. This camouflage coloration is a phenotype, defined as the visual expression of a trait. A genotype refers to the set of genes responsible for a phenotype. Therefore, due to natural selection certain genotypes are maintained over time, while less favorable variations disappear and this drives the evolution of populations.
We humans have studied this process and taken advantage of naturally occurring variations to purposely evolve plants and animals to farm or keep as companions. This process is known as artificial selection, the propagation and domestication of plants and animals for favorable traits. Let's look at dogs for example. Thousands of years ago, humans started selectively breeding pairs of wolves for traits like docility, protection and hunting prowess along with appearance. We continued this selective breeding over countless generations and this has culminated in scores of modern dog breeds, including the Great Dane and Chihuahua, which in turn are both very different in appearance and character to their ancestor, the wolf.
We have also implemented artificial selection in plants. Brassica oleracea was once a large, leafy plant, but it has been artificially selected in different ways to produce many common vegetables we eat today, including kale, cabbage, cauliflower and even kohlrabi. Another species in the same family, Brassica napus, is commonly known as the oilseed rape plant because of the abundant oil sequestered in its seeds, which are excellent for cooking or fuel production.
In this lab, you will artificially select traits from individual oil seed rape plants to produce two lines displaying different levels of a trait, hairiness.
- Observing Artificial Selection in Plants
- To begin the experiment, collect five two-week old grown plants from the generation one population that appear healthy. Note: Plants should appear green, not wilted, and not have damaged leaves. Experimental Hypothesis: The experimental hypothesis for this lab might be that the number of lead 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. Null Hypothesis: The null hypothesis might be 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 partners’ names.
- Record any natural differences you see between your plant samples. Note: All of the plants are the same age, so any observed variation is due to expressed phenotype, not development.
- Use a hand lens to count the number of trichomes found on the petiole of the lowest leaf, not including the round, first-grown cotyledons for each plant. Note: 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.
- Both partners will conduct counts on each plant independently and compare answers. If the counts are different, enlist a third counter or take the average of the two results.
- After this, determine the median number of trichomes for the entire class. Click Here to download Table A
- Determine the top and bottom 10% of the plants from the class trichome count. These groups will make up the two artificially selected lines: high hairiness and low hairiness, respectively.
- If more than 10% of the plants have no trichomes, select 10% of those plants randomly for the low hairiness plant line.
- Separate the two groups of selected plants and label them clearly.
- Move the plants to a well-lit area and keep them well watered until they develop flowers. Note: Flowers typically develop around 14 days.
- When the flowers are fully developed, use a pollination wand or paintbrush to rub the stigma and anther of one plant’s flowers for a few seconds.
- Using the same wand or brush, rub the inside of each flower of every other plant in the same selection line.
- Using a different wand or brush, repeat steps 11 through 12 on the other line. Note: Take care not to cross the lines. Spread the pollen only among the appropriately selected plants.
- Return the generation one plants to the growing areas. When the plants reach approximately 1 month old, they should produce mature seed pods.
- Allow the plants to dry out once the seed pods turn yellow.
- Break the seed pods open and collect the seeds.
- Obtain new pots for the generation two seeds.
- Fill the pots with moist potting soil about halfway and add a fertilizer pellet to each pot.
- Fill the pots with more soil to fill just below the lip.
- Make a shallow depression in the soil of each pot with a probe.
- Touch a clean toothpick to a gluestick and use the toothpick to transplant the seeds into the soil one by one.
- Cover the seeds with a thin layer of soil.
- Label the pots with their hairiness line and place them in the designated growing area.
- Use a plastic dropper to water the plants as needed to ensure they do not dry out.
- When the plants are two weeks old, collect five plants from each line and label them A to E.
- Then, count the trichomes on these plants as previously described (steps 4- 5). Click Here to download Table B
- Data Analysis
- 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. Label the mean.
- Then, create frequency graphs for both of the selected hairiness lines.
- Compare the frequency graphs for generation one and both hairiness lines.
- Take note of phenotypic shifts observed in the selected hairiness lines.
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