It is not necessary that both partners get equal benefits from each other.
Explanation:
Mutualism is a symbiotic relationship between two individuals. In mutualism both the partners get benefitted from each other but it is not necessary that both get equal benefits from each other.
Every mutualistic relationship is different and in all mutualistic relationships, altruistic behavior is not seen by any partner species. Each partner is selfish and help the other partner unintentionally in mutualism. So it is not necessary that each partner gain equal benefit in this relationship.
For paleontologists, however, the Phylogenetic Species Concept is most frequently employed, simply because we just can't test reproductive isolation in fossil taxa.
