Darwin's Model of Natural Selection:
Time Frame and Sequence
This activity will take two class periods if students do some of the work at home.
Ten days prior to the activity you will need to plant several pots of Wisconsin Fast Plants ™ so that they have true leaves by the time of the activity.
Prior to this activity, students will have systematically reviewed the components of the Darwinian Model. This activity is intended to provide opportunities for students to explore the ideas of variation and heritability in greater detail.
Begin with a fairly simple demonstration to develop the idea that although a passing glance may make it seem that all members of a species are exactly alike, small variations exist. In fact, for most traits there is a wide range of variation, rather than simply two character states as is often seen in genetics problems. Pass out a handful of sunflower seeds to each group of 2-3 students and ask them to count the stripes. It will quickly become apparent to students that this is not a straightforward task and they will probably begin to ask questions such as:
"Do we count both sides or just one?"
It will be important to negotiate the rules for counting as a class. Once these are established, let the students work for about 10 minutes counting and putting the seeds into piles based on the number of stripes they have. As students are finishing this task, walk around with a test tube rack holding large tubes marked from zero to ten (or with the highest number that students have found). Have students place their piles of seeds into the corresponding test tube. Once all groups have added their seeds, initiate a discussion about what they see. The seeds in the test tubes will form a normal distribution that should be fairly obvious. Ask students if they are surprised to see so much variation in number of stripes. Give students a chance to name other characteristics of living things that vary over a wide range.
Once this discussion has provided students with an opportunity to consider variability, have each group of students examine a pot of Fast Plants™ and complete the handout. They will need to look at more than just one set of plants so it will be necessary to have them trade plants with other groups. Again the goal of this part of the activity is to give students a chance to look at variation carefully and to have them examine the trichomes on fast plant leaves in particular because they are the focus of the artificial selection experiment that will be used as an example in the next part of the activity. While they are working, walk around and make sure they are able to see the trichomes on the plants. At the end of the period have students share their observations with the whole class.
As a homework assignment, pass out the "Statistical Tools" handout. This assignment is intended to be a review for students on the mathematical concepts of mean, median, mode, and range. It contains a brief description of these ideas and a graph with questions that students can use to practice finding mean, median and mode.
Begin class by going over the homework assignment on statistical tools. Use this opportunity to clear up any questions students have about graphs and how to read them, because the work for the rest of the day depends on this skill.
Pass out the Fast Plant Graphs handout. Before setting students off to work in groups on this assignment, remind them what the trichomes are on fast plants. Also, take a moment to explain what they are seeing in the graphs using an overhead projector. On each page there are five graphs. There are two different processes represented: selection and reproduction. We have attempted to make the distinction clear on the handout, but it warrants some explanation for students. On the graph pages, each of the larger graphs (#1, 3, 5) represents all of the plants in a particular generation. The smaller graphs (#2 & 4) show those plants that were selected as the parental plants for the second and third generations. It is important to distinguish between two processes represented by the graphs: selection of parental plants and reproduction by those plants. Once students have been shown what the graphs represent, allow them about 20 minutes to work in groups on the handout. It is likely that students will have many questions once they get started, mostly related to what the graphs represent.
After students are finished with questions #1-6 on the handout, call the group back together and discuss student answers. There are two more questions for students to answer after this discussion. These can be done with any remaining class time or as homework.
Teaching Strategies and Student Ideas
Sunflower seed counting
This exercise is intended to provide students with an opportunity to examine a trait carefully in order to see the wide range over which it varies. Many students have probably never had this experience and therefore often think of all the members of a particular species as being almost exactly alike. Students should not be asked to count the seeds for more than about 10 minutes, however, because the task can quickly become tedious and then the lesson has the potential to be lost.
The rules for counting are not really important, except that everyone should be operating with the same criteria. We have found that even when students don't perform this task with a great deal of care, the point about variation is still clearly made and the normal distribution can be observed in the test tubes.
Fast plant observation
Students will have to look very carefully to see the trichomes and may have to look at multiple plants before they find any. Although they are visible to the naked eye, it may be helpful to provide hand lenses so that students can see them more clearly. Also, holding the plants up against a dark background may help. They should look on the petiole of the first true leaves as directed on the handout. Depending on their prior experience with plants, it may be necessary to define some of these terms.
A second part of the activity is to have students look at other varying traits. You might point out to them that all the seeds were planted at the same time so variations in height, etc. are a not a function of some plants being older than others. Encourage them to think about traits other than structural ones that may vary. It will be interesting also when the students think about traits that do not vary. They will likely have a hard time coming up with things that are absolutely the same across all the plants. Frequently students will say things like, "they are all green, they all have roots, etc." and while these statements are true, a close look will reveal that they are not all exactly the same shade of green or some roots go deeper than others. This is a good opportunity to talk about the tiny variations that are so important to natural selection and how it was common in Darwin's time (and even today) to think of members of a particular species as being the same except for trivial imperfections. In fact, moving away from this type of essentialism is often cited as one of Darwin's major achievements.
Fast plant graphs
The fast plant graphs are intended to address some common ideas that students have about variation. One of these is the notion that offspring will look exactly like their parents (question 1). The hairy lineage graphs are particularly useful here. The range of variation in the offspring far exceeds that seen in the parents. Students should see that although on average the offspring of the hairiest plants have more hairs, there are still individual plants with very low numbers of hairs as well as plants that are hairier than any of the parental plants.
Another idea that is addressed by this activity is that variation is diminished as selection takes place. In answering questions 2 and 3 students should see that generally, variation is maintained even as the mean value in the population changes.
Questions 4 and 5 are intended to get students to think about a change in the population over time. A very tenacious misconception about species change is that individual change leads to species change. The concept of a shifting mean in the population is a key idea in understanding how species change over time. Use these questions to emphasize the idea that selection results in slow changes to the way the population as a whole may look. That is, the mean may only shift slightly from one generation to the next. Over many generations these small shifts in the proportion of individuals with particular trait variations can result in dramatic changes to the population as a whole.
One other common idea that students bring to the study of natural selection is a bias toward traits with only two variations. In typical genetics instruction, students are exposed to many examples of human and other traits that can take one of two forms. Because of that introduction to variation they are likely to think that most traits vary in this way. In fact it is the other way around. Far more traits exhibit variation over a wide range than not. In other words, most traits are either continuous (such as leaf color) or have a large number of different variations (such as trichome number). One result of this idea is that students think of selection as an all or none event. For example, they might think that all of the plants with hairs either lived or died as a result of some selective pressure instead of thinking about the degree of hairiness as an important factor. Of course, this is artificial selection, but the point can still be made. This may be the first experience students have had with traits that have multiple variations, so don't be surprised if the implications of the idea take some time to develop. For the rest of the unit it will be important to keep reminding students of these graphs and the idea that most traits exhibit more than two variations.
NEXT PAGES: Student handouts