When associations produce fitness benefits and/or costs to the organisms involved, their interactions will generate natural selection on one another, potentially producing evolutionary changes in one or both species. Because parasites reduce host fitness, they impose selection on the host to defend itself. In turn, parasites experience selection to overcome host defenses, which generates selection for more effective host defenses and so on. The result is often a coevolutionary arms race. Arms races also occur between predators or herbivores and their prey.
Animals and plants have evolved a wide range of biochemical, behavioral, and life history adaptations that reduce the impacts of enemies. That such defensive features are in fact evolutionary responses to enemy pressure is supported by their geographic distribution. For example, waterfleas (Daphnia) from lakes with predatory fish show genetically determined predator avoidance behavior that is not present in conspecifics from fishless lakes. Many seaweeds from the tropics, where herbivore pressure is generally intense, are better defended against grazing than their relatives from more temperate areas. Conversely, selection on parasites to circumvent host defenses is illustrated by parasitic flukes that infest snails in New Zealand lakes. The flukes are better able to infect snails from their own than from other lakes, suggesting that they have adapted to the traits of the local hosts with which they have experience. Such local adaptation between hosts and parasites is common.
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