The clearest examples of coevolution come from ecologically coupled pairs of species.

In practice each species will experience, and exert, selective pressures on many other species. The evolution of a species will be an aggregate response to all its mutualists and competitors, and any evolutionary change may not be easy to explain in terms of any one competitor. The process is called diffuse coevolution. It undoubtedly operates in nature; indeed, it may be the main force shaping the evolution of communities of species. But it is difficult to study, and its importance is consequently controversial.

The image opposite is a nest of Formica ants in Naomi's laboratory in Harvard.