The peppered moth of industrial England—and the Galapagos finches on Daphne Major during the 1976 drought—are common examples used for teaching natural selection. What unites them is their method: observational work.
John Endler's work, however, gives us insight into natural selection through experimental methods. Like the observational work just mentioned, his experiments showed the rapidity of natural selection when pressures change abruptly. However, Endler also pioneered evolutionary field experiments (Losos 2017).
Endler was interested in a particular type of fish known for developing colourful spots—the guppies. He spent years visiting the island of Trinidad to map the locations and types of guppies he could find in the many pools of the island's rivers. This is where I began my lesson with my students. We looked at pictures of the pools in Trinidad and images of guppies.
Then, I told them about the pattern that Endler observed, which I wrote down:
I then drew the stock and flow model below. The model represents the number of colourful spots accumulating in a population of guppies. If the inflow is faster than the outflow, the genetics for more spots will accumulate in the population. If the outflow is higher than the inflow, then the genetics for more spots will be discarded from the population.
What was Endler's hypothesis for his observations?
In other words, what was driving more spots in one population and less in another? This is what I asked my students to think about. Specifically, I wanted them to add their own hypothesis to the stock and flow model. This was before I told them Endler's hypothesis, which I added to the model like this:
The hypothesis is then, that sexual selection was driving an increase in colourful spots, whereas predation was driving a decrease. Whichever pressure was stronger, would determine the observed number of spots in a population.
What did Endler do about his hypothesis? By serendipity (and years of hard work), Endler came across a lone pool on Trinidad with no guppies at all. It was populated by some predators of guppies—killifish—but they were weak predators of the fish.
What if he transplanted guppies from a pond with strong predators to this lonely pool of weak predators? That's what he intended to find out. As I told my students this, I drew a diagram of the experiment:
He then left the pond to return two years later to collect the data. Again, before moving on, I asked the students to predict the outcome—using the stock and flow model for their reasoning.
In the meantime, Endler set up a parallel experiment in greenhouses back in the USA. I began drawing the graph of the results but stopped at the point where the experiment began.
In the greenhouse Endler allowed guppy populations to grow without predation. Then, he exposed some populations to weak predation, some to strong predation, and some were left as a control without predation. I asked my students to predict the lines of the results. When they were done and we had discussed their ideas, I gave them the results (See graphs on the internet for clearer drawn lines).
After two years in Trinidad, the guppy population had also evolved significantly more blue and iridescent spots now they were exposed to weaker predation pressure. So, it wasn't a clear-cut case of spots versus no spots. The colours seemed to matter. So, I then showed the students the full results by colour of the spot. I got this bar chart from Save My Exams IB revision.
The bar chart requires attention, and was good practice for my students to attend to details in data charts. Did the results match the hypothesis in the model we made? Not quite, my students thought. For example, blue and iridescent spots seem to be affected by both sexual selection and predation. Red spots, however, may be affected by sexual selection, but not predation pressures. Yellow spots don't seem too attractive to mates or predators. If you want to learn more about teaching using stock and flow diagrams and variation theory check out my books.
My books: Difference Maker | Biology Made Real, or my other posts.
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References
Losos, Jonathan B. 2017. Improbable Destinies: Fate, Chance, and the Future of Evolution. New York: Riverhead Books.
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