, ▁as ▁this ▁functions ▁to ▁prevent ▁fusion ▁between ▁the ▁species . ▁However , ▁Co y ne ▁and ▁Or r ▁found ▁equal ▁levels ▁of ▁post zy got ic ▁isol ation ▁among ▁sym pat ric ▁and ▁al lop at ric ▁species ▁pairs ▁in ▁closely ▁related ▁D ros oph ila . ▁Nevertheless , ▁differential ▁fusion ▁remains ▁a ▁possible , ▁though ▁not ▁complete , ▁contrib utor ▁to ▁species ▁disc rim ination . ▁ ▁Ex amples ▁Sym pat ry ▁has ▁been ▁increasing ly ▁ev iden ced ▁in ▁current ▁research . ▁Because ▁of ▁this , ▁sym pat ric ▁spe ci ation   – ▁which ▁was ▁once ▁highly ▁deb ated ▁among ▁research ers   – ▁is ▁progress ively ▁gain ing ▁cred ibility ▁as ▁a ▁vi able ▁form ▁of ▁spe ci ation . ▁ ▁Or ca : ▁partial ▁sym pat ry ▁Several ▁distinct ▁types ▁of ▁k iller ▁wh ale ▁( O rc inus ▁or ca ), ▁which ▁are ▁character ized ▁by ▁an ▁array ▁of ▁morph ological ▁and ▁behavior al ▁differences , ▁live ▁in ▁sym pat ry ▁throughout ▁the ▁North ▁Atlantic , ▁North ▁Pacific ▁and ▁Ant ar ctic ▁o ce ans . ▁In ▁the ▁North ▁Pacific , ▁three ▁wh ale ▁populations   – ▁called ▁" trans ient ", ▁" res ident ", ▁and ▁" off sh ore "   – ▁demonstrate ▁partial ▁sym pat ry , ▁crossing ▁paths ▁with ▁relative ▁frequency . ▁The ▁results ▁of ▁recent ▁gen etic ▁anal ys es ▁using ▁m t D NA ▁indicate ▁that ▁this ▁is ▁due ▁to ▁secondary ▁contact , ▁in ▁which ▁the ▁three ▁types ▁encountered ▁one