Evolutionary Ecology of Mutualism

Much of the current research on mutualism centers on the ecology and population dynamics of mutualism, but, as indicated above, the evolution of these interactions has attracted considerable attention as well. Issues central to the study of the evolutionary ecology of mutualism include: their evolutionary origin and stability, the evolution of specificity and obligacy, and natural selection and the evolution ofmutualistic traits. While evolutionary biologists study these and many other topics concerning mutualism, evolutionary ecologists have focused much oftheir attention in recent years on one question in particular: what are the biological mechanisms that may prevent overexploitation of one mutualistic species by another, thereby averting the degradation of mutualism into parasitism or predation.?

Mutualism inherently involves conflicts of interest between interacting species when there is interspecific linkage of benefits and costs. Overexploitation and conflicts of interest can lead to the ecological and evolutionary destabilization of mutualism. In benefiting by extracting a cost from its partner, a species might increase its own benefit and its partner's cost to the point at which the partner no longer has a net benefit. For example, yucca moths may pollinate and deposit eggs in yucca flowers, but if moths deposit so many eggs that most or all seeds are consumed by larvae, then the costs of seed consumption may converge with the benefits of seed production. In either case, if a species increases its benefit and as a result, its partner's cost increases beyond its limits of tolerance, it could cause extinction of the partner. If interactions are obligate, this could mean extinction of both species. Destabilization of mutualism through conflicts of interest may arise independent of the evolution of cheater genotypes within either of the populations of partner species.

Theoretically, the evolutionary stability of mutualism requires that a conspecific 'cheater' genotype, within either partner species, not be able to spread to fixation and eliminate the 'mutualistic' genotype. This problem mirrors similar barriers to the evolution of intraspecific cooperation. Cheaters are individuals that increase their fitness, relative to their conspecifics, by reducing their cost:benefit ratio and thereby the benefit:cost ratio of their partner. They can do so by reducing benefits provided to their partner (and costs to themselves), or by increasing their own benefits (and costs to their partner). In either case, theory predicts that cheater fitness will increase and the cheater phenotype may spread to fixation, reducing mutualism to parasitism.

Mutualism has also been suggested to be destabilized if a species extracts a benefit from a mutualistic species without returning benefits to that species, though, by definition, such interactions involving cheater species are not mutualistic. For example, some bees pierce holes in flowers to extract nectar without ever exhibiting behaviors that pollinating insects display. Such nectar-robbing insects are also often referred to as 'cheater' species, as they extract benefits without returning them. As another example, certain close relatives of yucca moths lay their eggs in yucca flowers and developing fruit, but have lost the morphological and behavioral traits that confer benefits on yucca plants. These moths function as seed predators and the interaction is by definition parasitism rather than mutualism.

Despite the strong negative effects that they can inflict, there is no evidence that such parasites and cheaters necessarily lead to the destabilization of mutualisms. In fact, given the ubiquity of mutualism in nature, the implication is either that parasites and cheaters are somehow held in check, or that such cheating rarely arises in nature. There is growing recognition that at least in some mutualisms, certain traits or behaviors of one or both mutualists act to reduce or inhibit such parasites and cheaters. As one example, many flowers exhibit structural features that protect nectar from floral visitors that do not contact the stigmas and stamens while feeding. As another example, plants may be able to curtail the growth of mycorrhizae to reduce the benefits they extract, and thereby prevent their costs from exceeding benefits. Such topics of evolutionary stability, conflicts of interests, and cheating within mutualisms remain in great debate.

See also: Coevolution; Cooperation; Pollination; Seed Dispersal.

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