Elizabeth and Nathan will continue to spend summers in the Sierra Nevada mountains studying the trade-offs that shape these insects’ physiology – and increasingly, that will mean documenting how their populations change as the climate warms because of human activities. During dry, warm years, the team has already observed the local extinction of beetle populations inhabiting their warmer study sites. The beetles at these sites mostly have the efficient PGI 4-4 genotype (the “jeep” version that usually does best in stressful situations), but that isn’t enough to protect them from the stress caused by consistently higher temperatures.

What will happen to the willow leaf beetles of the Sierras when warm years are the rule rather than the exception? While many species are expected to migrate to cooler environments in response to global warming, for this beetle, the only place to go is up – up the mountain that is, to cooler, higher altitudes. But this species already lives quite high and moving even higher means moving into an environment with extremely low percentage of oxygen in the atmosphere. Elizabeth and Nathan’s most recent work suggests that the beetles may already be close to the limit of their oxygen tolerance in their current situation.

The long-term prospects for the beetles (and the other species in their ecosystem) are still unclear, though increasingly illuminated by Nathan and Elizabeth’s work. “We are seeing the effects of human induced climate change in a pristine habitat,” says Elizabeth. “These are wilderness areas. But you go out there and in dry years, and there are no wildflowers. The beetles are a bit of a canary in a coal mine. You might ask, “Why should I care about the willow leaf beetle?’ Well, the beetle is an indicator of how much climate change is impacting even pristine environments; it’s a marker of things to come.”

Discussion and extension questions:

1. In your own words, explain what an evolutionary trade-off is. Describe the evolutionary trade-off associated with heat shock proteins (HSPs).
2. Research and describe one example of an evolutionary trade-off not mentioned in this article.
3. Describe four different lines of evidence that Elizabeth and Nathan collected in order to show that the PGI gene is undergoing natural selection. Which of these came from experiments in the lab and which came from observational field studies?
4. As the climate continues to warm, what would you expect to happen to the frequency of the two PGI alleles (1 and 4) in beetle populations living at the warmer, lower elevation site in the Sierra Nevada? What would you expect to happen to the frequency of the two PGI alleles (1 and 4) in beetle populations living at the cooler, higher elevation site in the Sierra Nevada? What evolutionary processes (mutation, migration, natural selection, and/or genetic drift) are likely to cause these changes? Explain your answer.
5. Advanced: Do some research on the genetic basis of and trade-offs associated with sickle cell anemia. Imagine that you are a scientist studying malaria resistance associated with sickle cell trait. To show that natural selection for malaria resistance is acting on this trait, what sort of evidence would you need to collect? Provide at least one potential line of evidence for each of the following levels: genotype, phenotype, performance, fitness, and gene frequency change.