Just as no man is an island, neither is any bird, insect, plant, or mammal. Many species live in close relationships with others, affecting each others ways of life. It seems logical to think that species that live closely with each other might evolve in adaptation to each other. This logic is extremely difficult to prove, since it requires direct proof of evolution in not one but two species. However, there is ample evidence to suggest that coevolution does take place.
In order to live in symbiotic or parasitic relationship, species must be adapted to each other. For example, cattle harbor bacteria in their stomachs that help them break down plant material. To live like this, the immune system of the cattle must be adapted to not kill these useful bacteria and the bacteria themselves must be adapted to live in the harsh environment of the cow's stomach. If a population of cattle moved to a new location where radically new plant material was available, they might adapt to eating this new food source. The bacteria, in turn, might then undergo adaption of their own digestive mechanisms to the new plant material. This would be an example of coadaptation. Most biologists accept coevolution on the basis of coadaptation if there is no overwhelming evidence to the contrary.
In parasitic relationships, the prey species often evolves mechanisms to defend itself against the parasite. However, the parasite may also evolve to evade these new mechanisms. This back-and-forth evolution of defense and offense, often called a coevolutionary arms race, can often result in a rapid burst of evolutionary change in both species.