▁as ▁a ▁distinct ▁species , ▁it ▁is ▁not ▁unlikely ▁that ▁the ▁Lib yan ▁population ▁is ▁un ited ▁with ▁them ▁as ▁a ▁sub species . ▁ ▁On ▁S ardin ia , ▁a ▁population ▁of ▁Greek ▁tort o ises ▁shares ▁the ▁small ▁size , ▁the ▁yellow ▁head ▁spot , ▁and ▁the ▁contrast ing ▁mark ings ▁with ▁the ▁Tun is ian ▁population . ▁Their ▁tax onom ic ▁status ▁is ▁en ig m atic , ▁as ▁is ▁their ▁very ▁existence ▁on ▁the ▁island , ▁separated ▁from ▁North ▁Africa ▁by ▁a ▁considerable ▁stretch ▁of ▁the ▁Mediter rane an ▁which ▁tort o ises ▁are ▁hardly ▁able ▁to ▁cross . ▁There ▁is ▁a ▁distinct ▁local ▁form ▁of ▁the ▁margin ated ▁tort o ise ▁on ▁the ▁island , ▁however , ▁and ▁that ▁seems ▁to ▁have ▁origin ated ▁from ▁a ▁deliber ate ▁introduction ▁by ▁humans , ▁perhaps ▁by ▁Greek ▁or ▁Roman ▁land own ers ▁in ▁the ▁classical ▁anti qu ity . ▁While ▁the ▁local ▁margin ated ▁tort o ises ▁show ▁evidence ▁of ▁pron ounced ▁gen etic ▁dr ift ▁and ▁may ▁thus ▁just ifi ably ▁regarded ▁as ▁a ▁sub species ▁despite ▁the ▁absence ▁of ▁line age ▁sorting , ▁the ▁Greek ▁tort o ises ▁of ▁S ardin ia ▁are ▁probably ▁best ▁considered ▁an ▁introduced ▁population ▁not ▁yet ▁worthy ▁of ▁tax onom ic ▁separation ▁pending ▁further ▁research . ▁ ▁Non eth eless , ▁because ▁the ▁present ▁tax on ▁may ▁well ▁turn ▁out ▁not ▁to ▁be ▁limited ▁to ▁Tun is ia ▁and ▁its ▁immediate ▁surr ound ings , ▁the ▁alternative ▁common ▁name ▁N abe ul ▁tort