What is Reductive evolution?

Reductive evolution is the process by which microorganisms remove genes from their genome. It can occur when bacteria found in a free-living state enter a restrictive state or are completely absorbed by another organism becoming intracellular. The bacteria will adapt to survive and thrive in the restrictive state by altering and reducing its genome to get rid of the newly redundant pathways that are provided by the host. In an endosymbiont or symbiogenesis relationship where both the guest and host benefit, the host can also undergo reductive evolution to eliminate pathways that are more efficiently provided for by the guest.