Summer School – Day 8 – Evidence of Evolution

Today we covered sort of a hodge-podge of things, but there was a common thread – ways in which we can see evidence of evolution, both on long time scales, as well as short ones.

First things first, though – Exam #2. Before the exam, we played a game of Pictionary, using the following prompts:

AlleleAnaphase IICharacter vs TraitDirectional Selection
Disruptive SelectionFunction of tRNAsGene FlowGenetic Drift
Haploid CellHomologous ChromosomesIncomplete DominanceIntegumentary System
Metastatic TumorNervous SystemPhases of MitosisProphase I
Recessive PedigreeReplicationTranscriptionTranslation

Again, no photos from that day, but I’ll recreate some of the drawings . . . answers at the bottom of the post. ūüôā

After the exam, I walked them through the evidence of evolution: how the fossil record demonstrates graduate change over time, in the form of transitional fossils; how geographical patterns are explained by common ancestry, such as the radiation of marsupials in Australia; how we can track the number of mutations over time in a sequence of DNA, and use this to estimate how long ago two species diverged from a common ancestor. And my favorite: comparative anatomy. At this point in the lecture, I always tell the story of the “ah-HA” moment in my own life, when this understanding clicked for me. When I was young, I spent a lot of time at the L.A. Zoo. Eventually, in high school, I’d become a student volunteer there, but even before that, I spent a lot of weekends at the zoo, sometimes attending some really cool educational programs. There’s one class I remember really clearly . . . the woman giving the program showed us a diagram of a whale’s skeleton, including the bones in the fin. Then, she showed us a diagram of the bones in a human hand . . . same configuration, both inherited from a common ancestor way back in our evolutionary history.

After lunch, we continued with evolution, but on a slightly different tack: infectious diseases. At first, it might not seem related, but when we talk about the influenza virus, and how it evolves to overcome our immune system’s adaptations, BOOM! Evolution that we can watch happening in real time. Plus, students tend to find the topic of disease really interesting. Bubonic plague, malaria, Lyme disease . . . fascinating stuff.

Today’s lab activity explored a couple of types of disease transmission, in the context of a role-playing exercise. I had the class “attend” a convention: the International Association for the Breeding of Dragons (since they are experienced dragon breeders after last week’s inheritance lab). Unfortunately for them, however, one of the conventioneers showed up carrying a nasty case of contagious Dragon Pox, and the following morning, there was a problem with food poisoning in the hotel’s breakfast buffet.

Figuring out how to do this lab took some ingenuity on my part. I found a few examples online of disease transmission labs, with the general idea of seeing how quickly a disease can travel through a population, depending on the method of transmission. Transmission would happen by exchanging “body fluids,” represented by cups of water. One “infected” individual (who wouldn’t know they were infected) would have a cup with water and sodium hydroxide, and after a sort of “musical chairs” in which students would randomly contaminate one another’s water cups, all of the water would be tested with phenolpthalein solution . . . the water of anyone who’d been infected by the disease would turn pink. Only problem: I didn’t have any phenolpthalein on hand.

So, I improvised. Instead of using sodium hydroxide, I filled the “infected” cup with 3 parts water to 1 part vinegar. It wasn’t enough vinegar so that the smell would be obvious, but it was enough that, even after mixing with two or three other “uninfected” samples of water, there would be enough acidity to see a difference using standard pH test strips. I had the students “mingle,” and everyone exchanged body fluids two times. Then, each of them had to visit the “doctor,” to be tested for Dragon Pox. Once we knew who had been infected, they tried to figure out the identify of Patient Zero – or, the original source of the infection. In our case, four of the ten students were infected, and we were only able to narrow it down to two people, but of course, I knew all along that Greg was the bearer of the disease.

As if that wasn’t bad enough, the “next morning” at the convention, I had them eat a big breakfast from the hotel’s buffet. I had a list of food items, labeled with photographs . . . in order to take a helping, they took an eye-dropper full of water out of that item’s cup. Again, I’d laced two of the cups (the ones containing “strawberries”) with vinegar. Turns out, a lot of people like strawberries, and almost all of them ended up with a case of food poisoning. This time, they compared notes as to who ate which items, and they were able to determine that the strawberries were tainted.

It went well, and the vinegar/pH paper system worked out perfectly. I’ll definitely do that again in the future, instead of trying to source less common chemicals. I think I’ll also add another section, to simulate airborne transmission, although I haven’t yet thought through how to do it.

We ended the day by watching a film – it’s one of my favorites for this class: “Why Sex?” This one follows another couple and their path to parenthood, while exploring the ways in which humans, and other animals, benefit from the process of sexual reproduction.

As usual on test days, we didn’t cover a much material as usual today, but no worries! There will be more fun tomorrow.

Selected Materials

110 Disease Transmission Lab
110 Disease Transmission Buffet

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