Mutualism is an interaction between two populations that positively affects the fitness of both populations, where the fitness of a population is defined as the success of a population in propagating its genetic material. An example of a mutualism that directly impacts reproductive success is the relationship between plant and pollinator. By definition a pollinator aids the plant in its reproductive success. In turn, pollinators gain resources, either by gathering nectar or by exploiting a potential plant on which to oviposit, thereby increasing its reproductive success as well. The term diffuse mutualism is sometimes used to describe the most common case of mutualism in which the relationship weakly or indirectly impacts reproductive success. This term is meant to point out that the fitness of one population is weakly dependent on the mutualistic interaction, either because the interaction has little effect or because there are so many other processes during the life cycle that contribute to the fitness of the organism, that the effect of that particular interaction is relatively small. The most common result of a mutualistic interaction is that the traits on which this interaction acts evolve in each population to match each other to maximize the fitness benefits resulting from the interaction. Although this terminology seems to imply that there is an active component, the coevolutionary results of ecological interactions are garnered through mechanisms such as mutation, drift, selection, and recombination.

Obligate Mutualists

An obligate mutualism is a specific type of mutualism in which each population needs to participate in the mutua-listic interaction in order for the population to survive. Therefore, the interaction not only is beneficial to both parties, but also necessary. Although rare, obligate mutualisms are the subject of intense investigation for several reasons. First, the populations often develop very specialized organs or traits in order to efficiently continue the mutualistic interaction; the populations coevolve. Second, because the interaction is necessary, there is very high selection on these traits. Lastly, some investigations have focused on why obligate mutualisms should persist, and in some cases evolve multiple times within the same lineage, when other alternatives require less energy or specialization. Below are two examples and brief descriptions of commonly studied plant-pollinator obligate mutualisms, the fig-fig wasp system and the yucca-yucca moth system, and their coevolutionary consequences.

Fig-fig wasp

Fig trees, plants of the genus Ficus, are a keystone species in many ecosystems and are found on every continent, with the exception of Antartica. There are 755 species of fig identified and an estimated 1300-2600 fig wasp species in existence. Most fig trees are pollinated by the fig wasps, and in these cases, the pollinating fig wasps are completely dependent on the fig for sustenance and reproduction, the ingredients for a strong obligate mutualism that directly impacts reproductive success.

Female fig wasps travel from fig to fig to find a suitable fig in which they can successfully oviposit. These female wasps are called foundresses, and figs can have more than one foundress. By ovipositing in the style, the foundress also pollinates the fig with the pollen she has carried from her originating fig. Once the eggs are inside, the fig matures, and the larvae hatch and eat the seeds of the fig for sustenance. After the larvae mature into wasps, they mate within the fig. The females exit the fig carrying with them pollen from the fig, and the males die, trapped within the fig. Females then repeat the cycle, sometimes traveling more than 10 km in search of suitable figs in which to lay their eggs.

In the case of the monoecious fig trees, wasp larvae eat only a portion of the seeds. The exploitation of fig seeds as a nutritional resource for fig wasp larvae could easily be overexploited in the case of monoecious figs if too many females oviposit in the same fig ovule. The evolution of dioecious fig trees, with only half of the fruit suitable for wasp oviposition, is considered an adaptation to potential fig wasp overexploitation of the seeds. This is one example of coevolution acting on a pair of traits, but another example of an important trait in this mutualism is the style length of the fig and ovipositor length in the fig wasp. It has been found that the distribution of these characters, in their respective populations, is closely matched, and this specificity and matching are considered a result of coevolution between the two species.

Coevolution is also currently being explored for its potential role in the high species diversity of the fig-fig wasp system. There are documented cases in which more than one fig wasp species have an obligate plant-pollinator with the same fig tree. Sympatric speciation may be another coevolutionary consequence of mutualistic interactions and is a scenario currently being investigated by several mathematical models, some ofwhich are discussed later in this article. The speciation and resulting lineage tracking of pollinator trait by the plant or vice versa is referred to as co-speciation. For more information and an in-depth discussion on the mechanisms and results of the fig-fig wasp interactions, see Weiblen's paper, 'How to be a fig wasp', and Cook and Rasplus' article listed in the 'Further reading' section.

Yucca-yucca moth

Yucca are plants of the genera Yucca and Hesperoyucca, which include the well-known Joshua tree, and are cousins to the agave plant. Yucca moths are insects of the genera Tegeticula and Parategeticula, and as of 2003 there were 21 recognized species of yucca moth. There are several differences between the life cycle of this system and that of the fig-fig wasp system. The female yucca moth has specialized mouth parts which she uses to collect and transfer the yucca pollen. The morphology of the mouth is a trait on which selection acts strongly, and coevolution with the yucca host is observed. In addition to ovipositing within the yucca ovule, some species of yucca moth oviposit onto the flower. The yucca moth larvae develop within the fruit, eating the seeds for sustenance and then emerge from the fruit to mate.

Phylogeny in the yucca-yucca moth system is under considerable investigation, but the phylogenetic tree has yet to be definitively resolved. The lineage tracking of plant and pollinator expected by theory does not seem to hold in every case, so the possibility and feasibility of historical host shifts are under discussion. A host shift occurs when the insect starts interacting with a different species as their host. John Thompson, a noted researcher and theorist in the field of coevolution, has studied cousins of the yucca moth, the greya moth, which exhibit a tight mutualism with plant of the genus Lithophragma and have experienced a host shift in some regions to a plant of the genus Heuchera. Understanding how and why this host shift has occurred may help explain why host shifts occur despite the evolution of specialized mutualistic interactions.

Within the yucca moth lineage, there are both pollinating and nonpollinating species. The pollinating species participates in the mutualistic interaction with the yucca tree, and the nonpollinating species parasitizes the mutualistic interaction of the pollinating species system. This cheater or parasite species seems to have evolved from the mutualistic species and lays its eggs into the fertilized flowers and fruit without providing the pollination service. Because the resource is used and seeds are eaten without providing pollination, this interaction is antagonistic (see the next section). Therefore, it may seem surprising that the mutualism is maintained. A coevolutionary response to the cheating has been observed; when over-exploited by yucca moths, that is, when too many moths oviposit into the same fruit, one yucca species aborts the over-exploited fruit. This mechanism gained by coevolution may help to maintain the mutualistic interactions. The yucca-yucca moth interactions described in this section are much more complex than can be briefly discussed here. For more information and an in-depth discussion on the yuccayucca moth system and its stability, see the 'Further reading' section.

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