Predation is a process by which one organism (predator) eats another organism (prey). If the prey population is abundant, the predator population also becomes abundant. If the predators are abundant and efficient, there will be a reduction in the abundance of the prey population. Prey-predator relationships can be visualised as a game in which each player tries to outwit the other. If you consider the prey population, only individuals better adapted to escape predation would survive and reproduce. So in a prey-predator population with varying degrees of efficiency (the variations are genetically determined), the prey species evolve at the avoidance of the predator and the predator species evolves towards maintaining predation and making it more efficient. Such a coevolution of the predator-prey relationship may continue indefinitely over long periods without causing extinction of both the species.
This is achieved by improving the efficiency of the predator and the resistance of the prey in a balanced way. Natural selection aims at a faster improvement of the efficiency of an inefficient predator than the resistance in an already resistant prey. But in a vulnerable victim, the resistance is evolved faster than the efficiency of an already efficient predator. Such balancing acts by natural selection do not allow any one of the two species to win the race.
There are several examples of natural selection having produced complex adaptations as a consequence of long term evolutionary process. These adaptations manifest themselves both among the predators and prey organisms. Examples among the predators are: the long and slimy tongue and the accurate aim of insectivorous lizards; the elaborate webs of the spiders; the social hunting behaviour of the lions. Cryptic toloration, fleetness, noxious substances and toxins, alarm calls and posting of sentinels can be cited as instances of adaptations evolved by prey organisms as defensive measures.