Predators are a strong selective force in nature. Because natural selection favors organisms that are best at surviving and reproducing in a given environment, the genes of prey organisms that can most efficiently escape predation will be favored in subsequent generations.
There is a saying that when a fox is chasing a rabbit that the fox is running for his dinner, while the rabbit is running for his life. This means that predators must always be evolving to be better at catching prey, and prey must always be evolving to be better at outrunning or outsmarting predators.
The evolutionary arms race, as it is known, between predators and prey exemplifies the role that natural selection has in the theory of evolution. Once a prey evolves a mechanism to effectively avoid predators, it is likely that predators will evolve an improved ability to counteract the antipredator response. If predators are too efficient, they risk wiping out their prey, but if they are not efficient enough, the will starve to death.
As a result, prey have developed a number of ways to escape predators. Increased agility and speed is seen in animals such as rabbits and zebras. Other animals have developed strategies by which they better cope with predation pressure, which are listed below.
Camouflaged prey either blend into the background, or have the same form as a non-prey item like a stick or a bird dropping. The more closely a prey item resembles its background, the more difficult it is for the predator to see and capture. Camouflage is, therefore, an extremely effective defense mechanism that has evolved in response to predatory pressure.
Aposematic, or warning, coloration has also developed in response to predation. Brightly colored animals tend to be distasteful or poisonous. Predators effectively learn to associate the bright color pattern on these organisms with being inedible and will avoid them in the future. Poison dart frogs are an excellent example of aposematic coloration.
Mimicry has evolved partially as a response of predators to aposematically colored animals. Viceroy butterflies appear identical to monarch butterflies, which are aposematic. Monarchs, however, are poisonous and birds will not eat them. So, by looking like monarchs, viceroys enjoy the benefits of being brightly colored without actually being poisonous.
Predation pressure can also influence the behavior of prey. Many species of animals have evolved calls and startle responses to ward off predators. Other animals have become nocturnal or undergo daily migrations to avoid encountering predators. Still others have evolved stings or nasty chemical defenses.
Animals are not the only organisms involved in the predator-prey evolutionary arms race. Plants have also evolved defenses against predators. Sticky leaves, sharp spines, and toxic sap are all common defenses in the plant world.
Whether plant or animal, predators are a selective force that strongly influence the evolutionary response of prey.