gence ▁in ▁al lop at ry ▁followed ▁by ▁range ▁exp ans ions ▁leading ▁to ▁an ▁area ▁of ▁sym pat ry . ▁Sym pat ric ▁species ▁or ▁tax a ▁in ▁secondary ▁contact ▁may ▁or ▁may ▁not ▁inter bre ed . ▁ ▁Types ▁of ▁populations ▁Four ▁main ▁types ▁of ▁population ▁pairs ▁exist ▁in ▁nature . ▁Sym pat ric ▁populations ▁( or ▁species ) ▁contrast ▁with ▁par apat ric ▁populations , ▁which ▁contact ▁one ▁another ▁in ▁adjacent ▁but ▁not ▁shared ▁ranges ▁and ▁do ▁not ▁inter bre ed ; ▁per ip at ric ▁species , ▁which ▁are ▁separated ▁only ▁by ▁areas ▁in ▁which ▁neither ▁organ ism ▁occurs ; ▁and ▁al lop at ric ▁species , ▁which ▁occur ▁in ▁entirely ▁distinct ▁ranges ▁that ▁are ▁neither ▁adjacent ▁nor ▁over la pping . ▁Al lop at ric ▁populations ▁isolated ▁from ▁one ▁another ▁by ▁ge ographical ▁factors ▁( e . g ., ▁mountain ▁ranges ▁or ▁bodies ▁of ▁water ) ▁may ▁experience ▁gen etic — and , ▁ultimately , ▁phen ot yp ic — changes ▁in ▁response ▁to ▁their ▁varying ▁environments . ▁These ▁may ▁drive ▁al lop at ric ▁spe ci ation , ▁which ▁is ▁arg u ably ▁the ▁dominant ▁mode ▁of ▁spe ci ation . ▁ ▁E vol ving ▁definitions ▁and ▁controvers y ▁The ▁lack ▁of ▁ge ographic ▁isol ation ▁as ▁a ▁definit ive ▁bar rier ▁between ▁sym pat ric ▁species ▁has ▁yield ed ▁controvers y ▁among ▁e colog ists , ▁bi olog ists , ▁and ▁zo olog ists ▁regarding ▁the ▁valid ity ▁of ▁the ▁term . ▁As ▁such , ▁research