What acts as the primary evolutionary mechanism that stops a mutually beneficial relationship from collapsing?

Answer

The evolutionary cost of specialization and resulting interdependence.

Interdependence created by specialization acts as the glue; if the host loses the ability to perform a critical function handled by the symbiont, selection strongly pressures against separation.

What acts as the primary evolutionary mechanism that stops a mutually beneficial relationship from collapsing?
biologyevolutioninteractioncoevolutionsymbiosis