Rough-skinned newts carry toxin levels far beyond what would be needed to deter most predators — enough to kill dozens of humans. Their primary predator, the common garter snake, has evolved resistance to this toxin. What best explains why newts carry such extreme toxicity?
ANewts evolved maximum toxicity to deter the widest possible range of predators, and snakes evolved resistance independently
BReciprocal selection in an evolutionary arms race — resistant snakes selected for higher toxicity, which selected for greater resistance, in an escalating cycle
CNewts evolved extreme toxicity due to abiotic environmental pressures unrelated to predation
DGarter snakes drove toxicity upward by preferentially consuming the least toxic newts, but snake resistance is a separate, non-coevolutionary adaptation
This is the textbook example of antagonistic coevolution. Toxic newts select for resistant snakes; resistant snakes survive to eat even more toxic newts, selecting for higher toxicity. Each species' extreme trait is only explicable as a response to the other — neither makes sense in isolation. Option D is close but wrong: the escalation requires that snake resistance itself also coevolves in response to newt toxicity, not that resistance arose independently.
Question 2 Multiple Choice
Darwin predicted the existence of a specific moth species based only on observing a Malagasy orchid with a 30-centimeter nectar spur. This prediction is grounded in which principle of coevolution?
ADiffuse coevolution — orchids adapt to a guild of pollinators, so any matching tongue length becomes likely
BConvergent evolution — orchids and moths independently evolve matching structures due to shared environmental pressures
CPairwise mutualistic coevolution — tight morphological matching between interacting species produces predictable trait correspondence
DThe Red Queen hypothesis — continuous escalation in spur length is driven by parasite pressure on the orchid
Darwin's prediction followed from the logic of pairwise mutualistic coevolution: if the orchid evolved a 30-cm spur, only a pollinator with a correspondingly long proboscis could access the nectar and pollinate it. Reciprocal selection between this specific orchid and its specific pollinator produces tight morphological matching — each species' traits mirror the other's. This is fundamentally different from diffuse coevolution (which produces generalized traits) and from convergence (which involves adaptation to a shared environment, not to each other).
Question 3 True / False
The Red Queen hypothesis predicts that coevolving species must continue evolving simply to maintain their current fitness relative to their coevolutionary partner.
TTrue
FFalse
Answer: True
In antagonistic coevolution (e.g., host-parasite, predator-prey arms races), any gain by one species reduces the relative fitness of the other. This means both species must keep evolving just to stay even — like running to stay in place, as the Red Queen does in Lewis Carroll. Standing still evolutionarily means falling behind as the partner evolves. This hypothesis is especially well-supported in host-parasite systems, where rapid parasite evolution maintains pressure on host immune defenses.
Question 4 True / False
Coevolution typically involves tight pairwise relationships between exactly two specific species.
TTrue
FFalse
Answer: False
Diffuse coevolution describes reciprocal selection between a species and an entire community of interacting partners rather than a single counterpart. A plant may evolve chemical defenses in response to a guild of herbivorous insects, not any single species. Recognizing diffuse vs. pairwise coevolution matters: pairwise coevolution predicts tight trait matching and vulnerability when one partner is lost, while diffuse coevolution produces generalized strategies and more robustness to the loss of individual interactors.
Question 5 Short Answer
What distinguishes coevolution from ordinary adaptation to the abiotic environment, and why does this distinction change predictions about evolutionary outcomes?
Think about your answer, then reveal below.
Model answer: In ordinary adaptation, a species evolves in response to fixed or slowly changing environmental factors (climate, terrain, nutrient availability). In coevolution, each species is itself a major selective pressure on the other — so as species A evolves, it changes the selective environment faced by species B, which then evolves, changing the environment for A again. This reciprocal dynamic can produce escalating arms races (where neither species reaches a stable optimum) or tight morphological matching (in mutualisms). It means evolutionary change in one species can be driven entirely by evolutionary change in another, independent of any abiotic shift — and losing one partner can strand the other with traits that no longer make sense.
The key insight is that the 'environment' driving evolution includes other evolving species. This creates dynamic, co-dependent evolutionary trajectories that have no parallel when adapting to static physical environments. Predictions differ: abiotic adaptation tends toward a stable local optimum; coevolution can sustain indefinite evolutionary change or lock species into dependencies that make extinction of one partner catastrophic for the other.