In order to have coevolution, there must be selection by both the plants and the insect herbivores. The relationships between populations of two interacting species, A and B, can be described by making a chart of all possible types of interactions. Both species benefit from the relationship. Commensalism and mutualism are often grouped together and called symbiosis . Many interesting coevolutionary relationships are mutualisms.
Mutualism seems particularly important in the tropics.
There are no obligate ant-plant mutualisms north of 14 degrees. There are no nectarivorous or frugivorous bats north of 32-33 degrees. There are no orchid bees north of 24 degrees. Numbers of plant species with extrafloral nectaries decline rapidly north of 24 degrees. Slight perturbations in conditions or population numbers and one species or the other could become extinct.