Questions: Predator-Prey Coevolution and Evolutionary Arms Races

Question 1

Garter snakes in a high-newt-toxicity region show high tetrodotoxin resistance; snakes in a nearby low-toxicity region show low resistance. This geographic variation best illustrates:

• A. Genetic drift causing random differences in sodium channel genes across isolated populations
• B. Directional selection acting independently within each population without any influence from newts
• C. Reciprocal coevolution: local toxin levels drive local resistance levels, and local resistance shapes selection on local toxin levels
• D. Character displacement between sympatric snake species competing for the same prey

Answer: (C)

The geographic mosaic of toxicity and resistance is a signature of coevolution, not independent evolution. Where newts are highly toxic, snakes face intense selection for resistance; where newts are less toxic, the cost of maintaining resistance outweighs its benefit and resistance is lower. This reciprocal mapping of each species' trait onto the other's trait across space is exactly what coevolutionary arms race theory predicts. Genetic drift (A) would produce random patterns, not the correlated toxin-resistance mapping observed. Independent selection (B) could produce local adaptation but not the tight correspondence between prey toxicity and predator resistance.

Question 2

Why don't predator-prey arms races escalate indefinitely, with prey eventually becoming perfectly defended against all predators?

• A. Arms races reach a stable equilibrium when both species become optimally adapted, after which evolution stops
• B. Predators and prey stop interacting reproductively once defenses become extreme, preventing further coevolution
• C. Fitness costs of defense and counter-adaptation create opposing selection pressures that constrain escalation
• D. Natural selection only favors arms races in small isolated populations; large populations evolve toward neutrality

Answer: (C)

Arms race escalation is limited by fitness trade-offs on both sides. Toxin production is metabolically expensive for prey; toxin resistance can impair normal nerve function in predators. At some threshold, the cost of further escalation exceeds the selective benefit — meaning that more extreme defenses or counter-adaptations would actually reduce fitness rather than increase it. This cost-benefit balance sets an evolutionary limit. Additionally, ecological shifts can break arms races entirely: predators may switch prey, or environmental change may alter the interaction. 'Optimal adaptation against all threats' is never achieved because the optimization is local, dynamic, and costly.

Question 3

The 'life-dinner principle' predicts that prey generally face stronger selection pressure in a predator-prey arms race than predators do.

• T. True
• F. False

Answer: True

The life-dinner principle (due to Dawkins and Krebs) captures a fundamental asymmetry: prey are running for their lives while predators are only running for their next meal. A prey individual that fails to escape dies and leaves no offspring — total reproductive failure. A predator individual that fails to catch one prey simply goes hungry and tries again — a minor fitness cost. This asymmetry means selection on prey to improve defenses is consistently stronger than selection on predators to improve attack. As a result, prey typically lead arms races and predators follow, though both sides continuously evolve.

Question 4

In an evolutionary arms race, when prey evolve a new defense, the predator population evolves a counter-adaptation within the same generation in response.

• T. True
• F. False

Answer: False

Evolutionary arms races unfold over many generations, not within a single generation. Evolution requires heritable variation, differential survival and reproduction, and the accumulation of changes across generations. When prey evolve better defenses (e.g., higher toxicity), this shifts the selection gradient on predators — but predators with better counter-adaptations must arise by mutation or recombination, survive better, reproduce more, and increase in frequency over many generations. The arms race is thus a slow ratchet across evolutionary time, not a rapid within-generation response. This is part of why the Red Queen hypothesis emphasizes continuous evolutionary change: each side must keep evolving to maintain fitness against a constantly-changing partner.

Question 5

Explain the Red Queen hypothesis and what it predicts about the evolutionary trajectories of both predator and prey over time.

Model answer: The Red Queen hypothesis (named from *Through the Looking-Glass*: 'it takes all the running you can do, to keep in the same place') holds that coevolving species must continuously evolve just to maintain their current relative fitness. In a predator-prey arms race, when prey improve their defenses, poorly-adapted predators are eliminated and better-adapted predators increase in frequency — now placing stronger selection on prey to improve further. Neither species can 'win' permanently: any adaptation is eventually countered, requiring further adaptation. The predicted trajectory is continuous escalation of both defense and counter-offense, with neither lineage achieving a stable optimum. The practical result is the extraordinary diversity of antipredator defenses and predator attack strategies in nature — both are products of this reciprocal evolutionary ratchet.