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.
Coadaptation
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.
Coevolutionary Arms Races
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.
