Over winter break students were told to read Chapter 11 in Holt Biology textbook or Chapters 15 & 16 in the Biology: Life on Earth text. I started class with a slideshow on some of the early scientists like Linneaus, Comte de Buffon, William Smith (geologist), Lamark (gave us the word biology) and Darwin and then moved on to the evidence for evolution: natural selection, comparative anatomy, embryo similarities, and molecular evidence (similarities in DNA). We also discussed genetic flow, the founders effect, bottleneck events and other selection effects.
We did two labs, both found in the Holt Biology online teacher resources. For Lab 11.6 Microevolution and Antibiotic-Resistant Bacteria, students used light colored (white & yellow) cubes to represent bacteria and dark colored (brown and black) cubes to represent antibiotic resistant bacteria. They started out with 20 light colored cubes in a bowl and then rolled two dice which determined the next event for their bacteria population. A roll of 4, 5, 6 means nutrients are plentiful and the population doubles, but a roll of 3 means one bacterium mutates and becomes resistant to antibiotic A (students replaced one light colored cube with a dark one). Other events included low nutrients (two thirds of the bacteria die), cleaning products are used killing 90% of the bacteria, Antibiotic A is introduced killing all but the antibiotic-resistant bacteria. The lab handout has a table of dice rolls matched with events. Students kept track of their population until it was wiped out (which can happen in just a few rolls of the dice), or they continued until they reached 20 rolls of the dice. If their population died out quickly they just started over. We had one population that evolved to consist entirely of antibiotic-resistant bacteria that just kept growing since there was no way to wipe it out – cleaning products kills 90% and low nutrients cut down the population but since they are resistant to antibiotics, there were no events that killed them all.
The second lab, 11.3 Modeling Alleles, investigated the bottleneck effect on cheetahs. Before class I prepared two bottles (for 2 groups), each containing 100 beads total – 10 beads of 10 different colors. Each color represents one trait (allele), such as fast legs, good night vision, weak immune system, etc. Students randomly assigned one color to each trait in the handout. The 100 beads in the bottle represents the gene pool of the original population of cheetahs. A disaster occurs and only 20-30 cheetahs survive – students simulate this by slowing pouring out beads into a bowl until they have roughly 20-30 beads. They record the color of beads and traits that survived (were poured into the bowl). Some students actually still had all 10 colors in this first generation after the bottleneck effect, while others only had 6 colors, or 60% of the genetic diversity of the original population (100 beads). For the second generation of cheetahs, students blindly grabbed a bead from the bowl, recorded the color and then placed it back in the bowl. They did this 10 times and usually lost a few more traits in the process. Before doing the third generation they removed all the color beads that hadn’t been passed on to the second generation and then repeated the drawing of beads from the bowls. The lab handout asked a number of questions about their remaining cheetahs and problems that might arise with the decreased genetic diversity.
We actually finished with 30 minutes to spare, so I handed out the timeline cards from the middle school class and asked students to try to put them in order. The cards had major events, such as first fish, first land animals, first flowering plants, largest mass extinction, etc.