Future City Lab: Understanding Tradeoffs

Tell students they are city planners, architects, or real estate tycoons working with the local government to build houses. The government will only provide funding if the project does not have a negative impact on the environment. Ask students what problems they anticipate. Expect them to come up with price as one of the potential constraints. The constraints this activity models is price, embodied energy, and insulation. You will need to define embodied energy here (see above). Explain that insulation is an important consideration because after the house is built, the vast majority of the energy is spent on heating/cooling.

Introduce students to the rules: they must pick one material each for the foundation, the roof, and the walls. There are cards describing the materials and the scores.

Students are tasked (in pairs or groups of three) with building a house. Each card has a score out of 10 for each of the three parameters, with 10 being the best possible score. Their task is to build a house that has a total score of above 15 for each parameter (averaging above 5 per house component). In other words: for this scenario, if the embodied energy, cost-effectiveness, and insulation scores are not all above 15, the government won’t provide funding and students must to go back to the drawing board.

Note: this is designed to be impossible. At most, students can only optimize two parameters at the cost of the other.

After students have been given enough time to struggle with this problem, have them share out what their takeaways were from the activity: why was the activity so hard (impossible)? What does this show? Reveal to the students that you were illustrating the concept of a tradeoff. Ask them: knowing this, what do you think the definition of a tradeoff is? (A tradeoff is a compromise: gaining a specific benefit comes at a cost.)

Ask (turn and talk before share out): Here we illustrated what kinds of factors constrain what sorts of houses can be built. What constraints exist for a living thing? (Potential answers may include: time, space, food, energy.)

Then pose this question (students may not be able to answer yet): How do tradeoffs help us understand natural selection?

Potential misconception to watch out for later in the lesson: students will tend to think on the organismal level rather than population level. So while an individual organism may spend energy on one thing over another, this is NOT the choice of the individual organism. Rather, the groups of organisms that have struck the most beneficial physiological compromise have a selective advantage.

Present the students with two sets of bird species (handout). For each bird, it shows their habitat, diet, and mating behavior. Using the framing device of an “energy budget,” ask students to first determine the patterns they notice, then identify any tradeoffs.

Some patterns: organisms in cold environments spend less energy on courtship, organisms with high-energy food have more mating partners.

The energy tradeoffs inherent have to do with energy input and output. Organisms in cold regions with scarce food must spend energy on staying warm and foraging/hunting. This comes at the cost of seeking out more or better mates. Reciprocally, organisms that live in warm environments with high-energy food are favored to spend some of that energy on finding an ideal mate. Note that the white-fronted amazon doesn’t fit the pattern perfectly. That’s okay—this is an overly simplified model.

Have groups share out and then have a discussion. What is an evolutionary tradeoff? How is it similar and different than the tradeoffs when building a house?

An evolutionary tradeoff is one where the natural world selects for a trait that confers an advantage in one way, but comes at a cost. Arguably, every adaptation is a tradeoff.

Here is a moment to clarify that in the house activity, the decision-maker was the contractor, a human. In the case of energy tradeoffs in evolution, the decision maker is natural selection. Whichever balance is most favorable confers a reproductive advantage.

Present students or groups of students with these problems, varying in difficulty, to help them in practicing thinking in terms of evolutionary tradeoffs. The answers are provided below each problem.

1.) Blind Cave Fish

The Mexican cave fish is thought to have diverged from its surface-dwelling cousin only about one million years ago, and yet it has no eyes at all! What can account for the loss of eyes (so-called regressive evolution) in a dark environment? Make sure to explain using the language of natural selection, not Lamarckian evolution!

Eyes are extremely energy costly, accounting for an estimated 15% of energy expenditure for a fish of this size. Eyes are also prone to infection. If the eyes are not providing the advantage of sight, that energy is going to waste. Fish that devoted their energy to other biological functions (like perhaps reproduction) had a selective advantage.

2.) Golden Eagle

Golden eagles are large predatory birds. They primarily eat fish, reptiles, squirrels, and other small rodents. Golden eagles use the “sustained grip method” to kill prey: grabbing the prey and squeezing with talons. They almost never eat large prey like mountain goats, but when they do, they use a different method: they pick up the goats and try to make them lose their balance, so they fall off the cliff. The eagle then flies to the base of the cliff for its feast. Why is this method specific to the goats and not seen with snakes, which also live in mountainous habitats?

This is a great example of “Optimal Foraging Theory,” which proposes that evolution selects for the method that results in the highest net gain in energy: the calories from food minus the calories of energy spent obtaining that food. Dropping a goat yields a large amount of food for relatively little effort. Conversely, flying up and down a mountain is too much energy to spend for a snake that could be easily killed with talons alone.

3.) Alzheimer’s in the Caribbean

In the slums of Brazil and elsewhere, low-quality water results in many children dying from complications due to diarrhea. These children deteriorate due to dehydration and/or malnutrition. In the same population, a rare gene mutation called APOE4 is much more common, when compared to anywhere else in the world. This mutation increases the likelihood of Alzheimer’s disease later in life. What is a possible evolutionary explanation?

APOE4 has been shown to prevent developmental problems in children due to malnutrition brought on by excessive diarrhea. Why is this tradeoff favorable? Alzheimer’s affects people usually after they reproduce. Therefore, in a population where children die from this specific type of malnutrition, natural selection favors the survival of the children to reproductive age at the cost of dementia later in life.

4.) Beech Drop

The beech drop is both a plant and a parasite. It sucks the nutrients out of the roots of the beech tree to sustain itself. We can safely assume that since beech drops are plants, they are descended from plants with green leaves. Beech drops have scaly leaves with no chlorophyll. Surely photosynthesis would allow for a secondary source of energy – why might natural selection have favored the loss of leaves?

Parasites must specialize in order to evade the defenses of their host. This costs energy. In the case of the beech drop, they grow specialized structures called haustoria that can penetrate the cell walls of plants. Photosynthesis is not a passive activity – growing functional leaves with the required specialized structures and making chlorophyll take considerable nutrients and energy. Plants compete for sunlight. Specializing in parasitism clearly has a selective advantage over a “jack of all trades” strategy.

5.) Aging

A wealth of scientific evidence points to the hypothesis that aging (also called senescence) is programmed into our genes, much like a piece of new technology like an iPhone has “planned obsolescence.” How is this possible, given what you know about natural selection?

This is again an energy tradeoff. The energy it takes to maintain and repair an organism’s body comes at the cost of energy spent on reproduction. There is no selective advantage to living twice as long unless one also reproduces more than twice as much.

6.) Male Wolf Spiders

The male wolf spider courts its mate by furiously drumming their legs against dry leaves. Recent studies have shown this activity comes at the direct cost of immune function of the male spider. Why might this help explain why drumming is attractive to the female spider?

This tradeoff is precisely why drumming is called an “honest trait.” Only a truly healthy male can afford to spend energy on this courtship activity. Therefore, a drumming male that isn’t sick is showing considerable fitness.

We see that all systems – from inanimate ones like houses to natural organisms – operate in a system of tradeoffs in the pursuit of maximum efficiency. What differences do we see in human choice over building materials and the natural process of natural selection? Can one be said to be more “rational” than another, or are their drawbacks to both?