Mutualism (from Latin mutuus meaning lent or borrowed) is a form of symbiosis, or close association between organisms of two or more species, in which both participants in the relationship benefit. Because both participants are gaining something from the relationship, this type of symbiosis is often symbolized as (+, +). Mutualism is only one of three recognized categories of symbiotic relationships. The other two are commensalism, in which one participant benefits and the other is neither helped nor harmed, and parasitism, in which one benefits and the other is harmed. Some mutualistic relationships have existed over such long periods of time that the two species have evolved simultaneously to live with each other. In some cases, the association is so close that it is obligatory; one or both of the individuals in the relationship would not be able to survive without the other. For example, in some types of lichens...