ynt op ic ▁in ▁North ▁America . ▁In ▁contrast , ▁the ▁mar bled ▁new t ▁and ▁the ▁northern ▁cr ested ▁new t ▁have ▁a ▁large ▁sym pat ric ▁range ▁in ▁western ▁France , ▁but ▁differ ▁in ▁their ▁habitat ▁prefer ences ▁and ▁only ▁rarely ▁occur ▁s ynt op ically ▁in ▁the ▁same ▁bre eding ▁p onds . ▁ ▁Sym pat ric ▁spe ci ation ▁ ▁The ▁lack ▁of ▁ge ographic ▁constraint ▁in ▁isol ating ▁sym pat ric ▁populations ▁implies ▁that ▁the ▁emer ging ▁species ▁avoid ▁inter bre eding ▁via ▁other ▁mechan isms . ▁Before ▁spe ci ation ▁is ▁complete , ▁two ▁diver ging ▁populations ▁may ▁still ▁produce ▁vi able ▁off spring . ▁As ▁spe ci ation ▁progress es , ▁isol ating ▁mechan isms   – ▁such ▁as ▁gam etic ▁in compat ibility ▁that ▁rend ers ▁fert il ization ▁of ▁the ▁egg ▁impossible   – ▁are ▁selected ▁for ▁in ▁order ▁to ▁increase ▁the ▁re product ive ▁divide ▁between ▁the ▁two ▁populations . ▁ ▁Species ▁disc rim ination ▁Sym pat ric ▁groups ▁frequently ▁show ▁a ▁greater ▁ability ▁to ▁disc rim inate ▁between ▁their ▁own ▁species ▁and ▁other ▁closely ▁related ▁species ▁than ▁do ▁al lop at ric ▁groups . ▁This ▁is ▁shown ▁in ▁the ▁study ▁of ▁hy brid ▁zones . ▁It ▁is ▁also ▁apparent ▁in ▁the ▁differences ▁in ▁levels ▁of ▁pre zy got ic ▁isol ation ▁( by ▁factors ▁that ▁prevent ▁formation ▁of ▁a ▁vi able ▁z yg ote ) ▁in ▁both ▁sym pat ric ▁and ▁al lop at ric ▁populations . ▁There ▁are ▁two ▁main ▁theories