We　analyze　the　evolutionary　branching　phenomenon　of　intermediate　predator　species　in　a　tritrophic　food　chain　model　by　using　adaptive　dynamics　theory.　Specifically,　we　consider　the　adaptive　diversification　of　an　intermediate　predator　species　that　feeds　on　a　prey　species　and　is　fed　upon　by　a　top　predator　species.　We　assume　that　the　intermediate　predator＇s　ability　to　forage　on　the　prey　can　adaptively　improve,　but　this　comes　at　the　cost　of　decreased　defense　ability　against　the　top　predator.　First,　we　identify　the　general　properties　of　trade-off　relationships　that　lead　to　a　continuously　stable　strategy　or　to　evolutionary　branching　in　the　intermediate　predator　species.　We　find　that　if　there　is　an　accelerating　cost　near　the　singular　strategy,　then　that　strategy　is　continuously　stable.　In　contrast,　if　there　is　a　mildly　decelerating　cost　near　the　singular　strategy,　then　that　strategy　may　be　an　evolutionary　branching　point.　Second,　we　find　that　after　branching　has　occurred,　depending　on　the　specific　shape　and　strength　of　the　trade-off　relationship,　the　intermediate　predator　species　may　reach　an　evolutionarily　stable　dimorphism　or　one　of　the　two　resultant　predator　lineages　goes　extinct.　Crown　Copyright　(C)　2015　Published　by　Elsevier　Ltd.　All　rights　reserved.