ABSTRACT
Mutualism is a reciprocal relationship between two individuals of different species which results in increased fitness for both parties. Symbiotic mutualism occurs when the organisms live together in close physical association, whereas nonsymbiotic mutualisms involve species which do not live together. The majority of mutualisms are facultative, where the partners may coexist without a reliance on each other, and are only mutualists opportunistically. In nitrogen-poor soils, legumes benefit greatly from the nitrogen-fixing activities of the bacteria which form nodules in the plant roots. Air currents can effectively disperse very small seeds, but the only options for large seeds are either dispersal by water currents or by animals. Some mutualisms provide one partner with a defense against predators or competitors. Mutualists reside inside the guts or cells of their animal partners are common. Evolutionary changes in both partners have then resulted in both partners benefiting from the relationship.
