An interaction between two species that benefits both. Individuals that interact with mutualists experience higher sucess than those that do not. Hence, behaving mutualistically is advantageous to the individual, and it does not require any concern for the well-being of the partner. At one time, mutualisms were thought to be rare curiosities primarily of interest to natural historians. However, it is now believed that every species is involved in one or more mutualisms. Mutualisms are thought to lie at the root of phenomena as diverse as the origin of the eukaryotic cell, the diversification of flowering plants, and the pattern of elevated species diversity in tropical forests.