coevolution

Evolution of those structures and behaviours within a species that can best be understood in relation to another species. For example, some insects and flowering plants have evolved together: insects have produced mouthparts suitable for collecting pollen or drinking nectar, and plants have developed chemicals and flowers that will attract insects to them. Parasites often evolve and speciate with their hosts.

Coevolution occurs because both groups of organisms, over millions of years, benefit from a continuing association (symbiosis) and will evolve structures and behaviours that maintain this association.