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.
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.