Explain Step 11. Study the bar graphs of each generation. Discuss the following questions (possible student responses are included).
Which, if any, colors of paper dots survived better than others in the second- and third-generation beginning populations of paper dots?
Answers will vary depending on the color of the fabric that the students used. The beginning populations for the second and third generations should include more dots that are of colors similar to the fabric and fewer dots that are of colors that stand out against the fabric. The change between the first and third generations should be more dramatic than the change between the first and second generations.
What might be the reason that predators did not select these colors as much as they did other colors?
Some colors were better camouflaged than others—they blended into the environment.
What effect did capturing a particular color dot have on the numbers of that color in the following generations?
When an individual is removed from a population and dies, in this case through predation, that individual no longer reproduces. The students should realize that heavy predation leads to a decrease in the size of the population and in the size of the gene pool.
Step 12. Allow the students enough time to resort the colored dots into the appropriate bags. Be sure the students recount the dots in each bag and replace missing dots. Have a three-hole punch and construction paper on hand to replace lost dots.
Elaborate This portion of the activity provides you with an opportunity to assess the learners' understanding of evolution and the mechanisms by which it occurs. Before the students begin to work on these tasks, display a piece of Fabric A and a piece of Fabric B and ask the learners to post their third generation bar graphs beside the fabric that they used. The learners now will benefit by comparing their results with those from other teams that used the same fabric as well as with those from teams that used a different fabric. These comparisons will give them more data with which to construct explanations for the results that they see.
How well do the class data support your team's conclusions in Step 11?
Students need to be able to analyze the relationship between their response in Step 11 and the cumulative data. The specific response should address the relationship between the team data and the class data.
Imagine a real-life predator-prey relationship and write a paragraph that describes how one or more characteristics of the predator population or the prey population might change as a result of natural selection.
The students should explain that variation exists in populations. Individuals with certain characteristics are better adapted than other individuals to their environment, and consequently survive to produce offspring; less well-adapted individuals do not. The offspring, in turn, possess characteristics similar to those of their parents, and that makes them better adapted to the environment as well. These two concepts are the basis of natural selection, and they explain how populations evolve.
Little variation in a population of organisms would mean that fewer differences would be expressed in the offspring. Fewer differences would mean that individuals would have similar advantages and disadvantages in the prevailing environmental conditions. This similarity, in turn, would mean that their survival and reproductive rates would be similar, so few heritable differences then would be passed on to the next generation.
Evaluate Have the students write one paragraph that summarizes their understanding of biological evolution. Refer to the learning outcomes and the National Science Education Standards. Expect that students will describe that in a population of organisms, variation exists among characteristics that parents pass on to their offspring. Individuals with certain characteristics might have a slight advantage over other individuals and thus live longer and reproduce more. If this advantage remains, the difference would be more noticeable over time. These changes could eventually lead to new species. The process of natural selection, then, provides an explanation for the relatedness of organisms and for biological change across time.