When one or both sides of a mutualism cannot survive without the other, the association is said to be obligate. The association of aphids and bacteria is obligatory for both sides: Bacteria cannot live without the shelter the aphids provide, and aphids cannot live without the nutrients provided by the bacteria.

Many associations, however, are not so tightly intertwined, and one or both participants can survive without the other. These interactions are called facultative. For the midges that pollinate cacao, the association is facultative because the midges have other sources of food, including other flowers, in the wet forests they inhabit. For cacao, however, the relationship is obligate because bees and other insects do not usually pollinate them, and so their reproduction more or less relies on visits by midges.

Many obligate mutualisms are thought to have begun as facultative relationships that became reinforced over time by natural selection. Thus, the mutation that originally made a cacao flower sprout from the tree trunk close to the ground rather than high out on a branch made the flower easier to pollinate by low-flying midges while decreasing the chances of discovery and pollination by bees. If midges were consistently more reliable pollinators than bees, the mutation would have been selected and then spread throughout the population of cacao (Chapters 21 and 22).