gene flow between population holds a species together genetically
reproductive isolation when bio barriers prevent two species fron interbreeding.
pre zygotic barrier

habit iso
temproal iso
mechanical iso
gametic iso

postzygotic barrier, might not be viable
reduced hybrid viability
reduced hybrid fertility, liger sterile
hybrid breakdown, genetic problems in F2 like deformities or sterilization

reinforcement should be stronger for sympatric than allopatric populations


fusion: weakened reproductive barrier