▁used ▁syn onym ously ▁but ▁have ▁more ▁precise ▁mean ings ▁are ▁crypt ic ▁species ▁for ▁two ▁or ▁more ▁species ▁hidden ▁under ▁one ▁species ▁name , ▁sib ling ▁species ▁for ▁two ▁crypt ic ▁species ▁that ▁are ▁each ▁other ' s ▁closest ▁relative , ▁and ▁species ▁f lock ▁for ▁a ▁group ▁of ▁closely - related ▁species ▁that ▁live ▁in ▁the ▁same ▁habitat . ▁As ▁inform al ▁tax onom ic ▁ranks , ▁species ▁group , ▁species ▁aggregate , ▁and ▁su pers pecies ▁are ▁also ▁in ▁use . ▁ ▁Two ▁or ▁more ▁tax a ▁that ▁were ▁once ▁considered ▁cons pec ific ▁( of ▁the ▁same ▁species ) ▁may ▁later ▁be ▁sub div ided ▁into ▁in fr as pec ific ▁tax a ▁( ta xa ▁within ▁a ▁species , ▁such ▁as ▁b acter ial ▁stra ins ▁or ▁plant ▁vari eties ), ▁but ▁that ▁is ▁not ▁a ▁species ▁complex . ▁ ▁A ▁species ▁complex ▁is ▁in ▁most ▁cases ▁a ▁mon oph y let ic ▁group ▁with ▁a ▁common ▁ancest or , ▁but ▁there ▁are ▁exceptions . ▁It ▁may ▁represent ▁an ▁early ▁stage ▁after ▁spe ci ation ▁but ▁may ▁also ▁have ▁been ▁separated ▁for ▁a ▁long ▁time ▁period ▁without ▁evol ving ▁morph ological ▁differences . ▁Hy brid ▁spe ci ation ▁can ▁be ▁a ▁component ▁in ▁the ▁evolution ▁of ▁a ▁species ▁complex . ▁ ▁Species ▁complex es ▁exist ▁in ▁all ▁groups ▁of ▁organ isms ▁and ▁are ▁identified ▁by ▁the ▁rig orous ▁study ▁of ▁differences ▁between ▁individual ▁species ▁that ▁uses ▁minute ▁morph ological ▁details , ▁tests ▁of ▁re product ive ▁isol ation