One type of organism might develop a defense and then radiate into new adaptive zones. Then the other organism might develop a defense against the plant defense and have a burst of radiation. Populations of A and B are independent and don't interact. Both species benefit from the relationship. Many interesting coevolutionary relationships are mutualisms.
Mutualism seems particularly important in the tropics.
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. Ants A really interesting case of mutualism is that of the relationship between ants of the genus Pseudomyrmex and plants of the genus Acacia . Figs have a very unusual flowering system.