It can be beneficial, detrimental, or inconsequential for the fitness of the individual depending on the relative gain to loss in fitness that the interaction produces. 

The selection occurs when specific antagonistic interactions cause the individual to compensate itself with specific traits. 

So mutualism can occur from a change in the outcome in inevitable antagonistic interactions between particular species. 

The richness in mutualism in an environment depends on the richness of antagonistic  interactions that take place.