Predation has been given many different definitions. For the purposes of this chapter, it is an interaction in which one free-living individual kills and derives resources from another organism. This definition includes finches that consume seeds but does not include fish that eat the siphons of clams that are unable to retract them quickly enough (assuming the clam usually survives the loss of tissue). Both broader and narrower definitions of predation are possible, and a variety can be found in ecology textbooks. Because broad definitions include herbivory and parasitism as forms of predation, the definition used here was chosen to minimize overlap with other chapters in this section. Predation probably arose early in the history of life, and since then, it has been a major source of natural selection on both parties in the interaction. Given the lethal consequences of predation, it is clear that predators will usually have some effect on the rate of increase of their prey. If prey differ in their susceptibility to predators due to heritable differences in characteristics, evolutionary change in antipredator traits will ensue. Because predators must consume prey to survive and reproduce, the selective importance of predation-related traits is obvious. Predators have undergone considerable change and diversification since the first predatory protocell evolved from what was probably a scavenging ancestor. Darwin regarded some of the clearest cases of natural selection as due to the interactions between predator and prey, and that viewpoint is also held by many current-day evolutionary biologists (e.g., Dawkins and Krebs 1979; Vermeij 1994). Predation can be regarded as the most basic interaction between populations. Herbivory and parasitism share the basic property of predation, that one organism consumes some or all of another living organism. Many cases of competition involve predation on the same set of prey species by two or more different predator species. Even when competitors consume nonliving foods, many aspects of the consumption process are similar to consumption of prey by predators. Even mutualism frequently involves one organism eating parts or products of another.
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