Summer School – Day 7 – Darwin and Natural Selection

Started out the day by picking up where I left off on my Darwin lecture, and then moved on to adaptations, and the principles of natural selection. I frame most of this discussion around the Oldfield Mice experiment, partly because it’s a perfect example of a scientific study that demonstrates the effects of selection on populations, but also because the mice are so cute! I end up using these mice as an example all the way through evolution and speciation, so I have a bunch of slides I’ve animated showing all sorts of things happening to the mice. (Selective forces, like being caught by a hawk when the fur doesn’t match the substrate; and, later, random forces, like severe weather).

Oh! I almost forgot to mention that this is the lecture where I reveal probably the single most important biology fact of all . . . the secret connection between Charles Darwin and Abraham Lincoln. It’s common knowledge that they were born on the exact same day (February 12, 1809), but I’ve uncovered a surprising bit of information that is much less well known . . . the image at right should make it obvious what I’m talking about (all photos were found on the internet, so obviously they must be completely legit).

We completed two big lab activities today. Before lunch, we viewed a short film from HHMI about Charles Darwin and Alfred Russel Wallace, called “Making of a Theory,” along with a rotation lab that helped them explore Charles Darwin’s life, and the inspiration behind his ideas about natural selection and evolution.

I usually modify activities to some degree, to better suit how I cover the topics, but in this case, I used materials I’d found online exactly as is.

“Making of a Theory” –
“Darwin Rotation Lab” –

When we came back from lunch, I lectured about natural selection, and then we did some lab activities to demonstrate selection, and competition, in ACTION!

First, the predator lab: each group is provided with a bucket filled with a substrate of a particular color (mostly pink, mostly white, or mostly blue). We then populated these habitats with pink, white, and blue pipe cleaner “worms.” They then switched off hunting for prey in their habitat. After each round of hunting, they calculated how many of each color had survived, and the survivors were allowed to “reproduce” (so more worms of those colors were added to the subtrate).

The hypothesis? That worms that matched the substrate color would survive the best (because they were the hardest for hunters to spot), so over time, this color would be favorable, and the other colors would decline.

So, catching worms is a lot of fun, but did their results match up with this hypothesis? Turns out, not really! In the past, I’ve done this activity with students, and generally the data they collect is exactly what they expected: the pink habitats have more pink worms than other colors, white habitats have more white worms, etc. But we only made that finding for one of our substrate colors (and it wasn’t a strong effect at all):

So, how do we explain this? I gave them some time to come up with an answer . . . our new hypothesis is that most of the students weren’t hunting by sight (in which case, color would be under selection), but were hunting by touch, so color didn’t impact how easy or difficult the worms were to catch. I think that’s a pretty sound hypothesis. 🙂

Our final activity for the day is also one of the most exciting of the semester . . . *drumroll please* . . . COMPETITION LAB!!!!!!! In this lab, students use chopsticks to simulate the beak of a wading bird. All birds start out with short beaks, and have to compete against their team members to catch the most food out of a limited supply. Each generation, the most successful birds are able to reproduce. It is also possible for a mutation to take place, causing an adaptation of beak length. As you might guess, birds with longer beaks are more successful at catching food swimming in deeper water. Yes, this activity allows us to demonstrate some principles of natural selection. But it’s also just hilarious to watch!

Everything usually starts out nicely enough, with each bird just trying to catch some food:

But, sooner or later, things start to get brutal. This is COMPETITION we’re talking about, after all. (It doesn’t hurt that I goad them on throughout that “IF YOU DON’T CATCH ENOUGH FOOD YOUR BABIES WILL DIEEEEEEEEE!!!!!!!!!!”).

Honestly, I don’t remember what our results were. I think Cassandra (in the greyish green jacket) was the winner, overall. But really, they were all winners that day. Well, at least those that caught enough food to feed their little offspring hahahahaha.

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