CSI: Changing Species Investigation

Lots of species change over time, but it’s not always due to natural selection. Students evaluate data about a population and work together to decide where natural selection is at work and where it isn’t. Two of the CSIs are examples of natural selection, one is not, and one could go either way.

In each of these examples, one population is compared to another that is a stand-in for a historic population. Until time travel is possible, this is what scientists must do – select populations that are as close to a historic population as possible.

In two of the examples, the elephants and bedbugs, the populations are influenced by human-caused factors. In both examples, the human influence is considered a part of the environment in which the populations live and not the more goal-directed interference we see in artificial selection.

The Junco case study is ambiguous—you could make a case for or against natural selection. The ambiguity makes this case study more challenging than the others. If you feel it would be too complex or confusing for your students, you may leave it out. Or you could go through it together as a wrap-up.

• “Some of the sections in this module were very writing intensive and back-to-back. I think a few well-placed species videos would break this up for them.”

1. Before you begin, review natural selection. It may be helpful to post something like this for all to see:
   In natural selection, helpful traits become more common in a population over time.
   Natural selection requires:
   1. A population where individuals have trait differences
   2. The trait differences are heritable (genetic)
   3. Some trait differences make an individual more likely to reproduce
2. Make sure students know how to calculate percentages. Review as needed.
3. Have pairs or small groups of students work together on a case. Make sure each student gets their own copy, fills in the data, and answers the questions.
   1. You may want to point out to your students that, in each case, one population is a stand-in for a historic population.
   2. You may want to work through one example as a class.
4. Jigsaw. Form groups where each student worked on a different case. Students discuss whether they think each case is an example of natural selection.
   • Students may continue refining their criteria for natural selection as they discuss the examples.
5. Whole-class discussion. Each group presents their decisions about whether each example satisfies the criteria for natural selection. They can also share whether they changed their thinking about the criteria.

• In natural selection, inherited traits that favor reproduction are passed down. Over many generations, those traits become more common while traits that do not favor reproduction become less common.

CSI cases (pdf) — Includes 4 examples. Make one example per student.

Dixon, A. F. G., & Glen, D. M. (1971). Morph determination in the bird cherry‐oat aphid, Rhopalosiphum padi L. Annals of applied biology, 68(1), 11-21.
Jones, T., Cusack, J. J., Pozo, R. A., Smit, J., Mkuburo, L., Baran, P., & Foley, C. (2018). Age structure as an indicator of poaching pressure: Insights from rapid assessments of elephant populations across space and time. Ecological indicators, 88, 115-125.
Ríos Martínez, A. F., & Costamagna, A. C. (2018). Effects of crowding and host plant quality on morph determination in the soybean aphid, Aphis glycines. Entomologia experimentalis et applicata, 166(1), 53-62.
Romero, A., & Anderson, T. D. (2016). High levels of resistance in the common bed bug, Cimex lectularius, to neonicotinoid insecticides. Journal of medical entomology, 53(3), 727-731.
Sutherland, O. R. W. (1970). An intrinsic factor influencing alate production by two strains of the pea aphid, Acyrthosiphon pisum. Journal of insect physiology, 16(7), 1349-1354.
Whitehouse, A. M. (2002). Tusklessness in the elephant population of the Addo Elephant National Park, South Africa. Journal of zoology, 257(2), 249-254.
Yeh, P. J. (2004). Rapid evolution of a sexually selected trait following population establishment in a novel habitat. Evolution, 58(1), 166-174.