Abstract
Reciprocal adaptation is the hallmark of arms race coevolution, but the symmetry of evolutionary change in each species is often untested even in the best-studied battles between natural enemies. We tested whether prey and predator exhibit a symmetrical pattern of local co-adaptation in the classic example of a geographic mosaic of coevolution between toxic newts (Taricha granulosa) and resistant garter snakes (Thamnophis sirtalis). Contrary to conventional wisdom, landscape variation in the newt toxin TTX is best predicted by neutral population divergence and not predator resistance, whereas snake resistance is clearly explained by prey toxin levels. Prey populations structure variation in levels of TTX, which in turn structures selection on predators—implying that neutral processes including gene flow, rather than reciprocal adaptation, are the primary source of variation across the coevolutionary mosaic.